Statements

Statements

A program consists of a collection of functions (one of which must be int main() {...}) and type and object declarations. A function may contain declarations and statements. Statements have the following forms, where s is a statement, and t is a true/false expression.
s; // Expression or declaration
; // Empty statement
{s; s;} // A block of 0 or more statements is a statement
if (t) s; // If t is true then s
if (t) s; else s; // else is optional
while (t) s; // Loop 0 or more times
for (s1; t; s2) s; // s1; while (t) {s; s2;}
break; // Jump from while, for, do, switch
return x; // Return x to calling function
try {throw x;} // Throw exception, abort if not caught, x has any type
catch (T y) {s;} // if x has type T then y=x, jump to s
catch (...) {s;} // else jump here (optional)
do s; while (t); // (uncommon) s; while (t) s;
continue; // (uncommon) Start next loop of while, for, do
switch (i) { // (uncommon) Test int expression i to const C
case C: s; break; // if (i==C) go here
default: s; // optional, else go here
}
label: goto label; // (rare) Jump to label within a function

A statement may be a declaration or an expression. Objects and types declared in a block are local to that block. (Functions cannot be defined locally). It is normal (but not required) to show statements on separate lines and to indent statements enclosed in a block. If braces are optional, we indent anyway. For instance,
{ // start of block
int a[10], i=0, j; // declaration
a[i+2]=3; // expression
} // end of block, a, i, and j are destroyed
declares the array of int a with elements a[0] through a[9] (whose values are initially undefined), i with initial value 0, and j with an undefined initial value. These names can only be used in scope, which is from the declaration to the closing brace.

The for loop is normally used for iteration. For instance, the following both exit the loop with i set to the index of the first element of a such that a[i] is 0, or to 10 if not found.
for (i=0; i<10; i=i+1) { i=0;
if (a[i]==0) { while (i<10) {
break; if (a[i]==0)
} break;
} i=i+1;
}
The braces in the for loop are optional because they each enclose a single statement. In the while loop, the outer braces are required because they enclose 2 statements. All statements are optional: for (;;) loops forever. The first statement in a for loop may declare a variable local to the loop. for (int i=0; i<10; i=i+1)

It is only possible to break from the innermost loop of a nested loop. continue in a for loop skips the rest of the block but executes the iteration (s2) and test before starting the next loop.

return x; causes the current function to return to the caller, evaluating to x. It is required except in functions returning void, in which case return; returns without a value. The value returned by main() has no effect on program behavior and is normally discarded. However it is available as the $status in a UNIX csh script or ERRORLEVEL in a Windows .BAT file.
int sum(int x, int y) { // Function definition
return x+y;
}
int main() {
int a=sum(1,2); // a=3;
return 0; // By convention, nonzero indicates an error
}

A test of several alternatives usually has the form if (t) s; else if (t) s; else if (t) s; ... else s;. A switch statement is an optimization for the special case where an int expression is tested against a small range of constant values. The following are equivalent:
switch (i) { if (i==1)
case 1: j=1; break; j=1;
case 2: // fall thru else if (i==2 || i==3) // || means "or else"
case 3: j=23; break; j=23;
default: j=0; else
} j=0;

throw x jumps to the first catch statement of the most recently executed try block where the parameter declaration matches the type of x, or a type that x can be converted to, or is .... At most one catch block is executed. If no matching catch block is found, the program aborts (Unexpected exception). throw; with no expression in a catch block throws the exception just caught. Exceptions are generally used when it is inconvenient to detect and handle an error in the same place.
void f() {
throw 3;
}

int main() {
try {
f();
}
catch(int i) { // Execute this block with i = 3
throw; // throw 3 (not caught, so program aborts)
}
catch(...) { // Catch any other type
}
}

Creating Libraries (namespaces)

Creating Libraries (namespaces)



Libraries usually come in the form of a header and an object (.o) file. To use them, #include "header.h" and link the .o file using g++. If the .o was compiled in C rather than C++, then indicate this with extern "C" {} to turn off name mangling. C++ encodes or "mangles" overloaded function names to allow them to be linked, but C does not since it doesn't allow overloading. extern "C" { // Turn off name mangling
#include "header.h" // Written in C
}
When writing your own library, use a unique namespace name to prevent conflicts with other libraries. A namespace may span multiple files. Types, objects, and functions declared in a namespace N must be prefixed with N:: when used outside the namespace, or there must be a using namespace N; in the current scope.
Also, to guard against possible multiple inclusions of the header file, #define some symbol and test for it with #ifndef ... #endif on the first and last lines. Don't have a using namespace std;, since the user may not want std visible.
#ifndef MYLIB_H // mylib.h, or use #if !defined(MYLIB_H)
#define MYLIB_H
#include <string>
// No using statement
namespace mylib {
class B {
public:
std::string f(); // No code
}
}
#endif

// mylib.cpp, becomes mylib.o
#include <string>
#include "mylib.h"
using namespace std; // OK
namespace mylib {
string B::f() {return "hi";}
}
#define could be used to create constants through text substitution, but it is better to use const to allow type checking. #define X Y has the effect of replacing symbol X with arbitrary text Y before compiling, equivalent to the g++ option -DX=Y. Each compiler usually defines a different set of symbols, which can be tested with #if, #ifdef, #ifndef, #elsif, #else, and #endif. #ifdef unix // Defined by most UNIX compilers
// ...
#else
// ...
#endif
Preprocessor statements are one line (no semicolon). They perform text substitutions in the source code prior to compiling. #include <header> // Standard header
#include "header.h" // Include header file from current directory
#define X Y // Replace X with Y in source code
#define f(a,b) a##b // Replace f(1,2) with 12
#define X \ // Continue a # statement on next line
#ifdef X // True if X is #defined
#ifndef X // False if X is #defined
#if !defined(X) // Same
#else // Optional after #if...
#endif // Required

Polymorphism

Polymorphism



Polymorphism is the technique of defining a common interface for a hierarchy of classes. To support this, a derived object is allowed wherever a base object is expected. For example, String s="Hello";
Vector<char> v=s; // Discards derived part of s to convert
Vector<char>* p=&s; // p points to base part of s
try {throw s;} catch(Vector<char> x) {} // Caught with x set to base part of s
s=Vector<char>(5); // Error, can't convert base to derived

// Allow output of Vector<char> using normal notation
ostream& operator << (ostream& out, const Vector<char>& v) {
copy(v.begin(), v.end(), ostream_iterator<char>(out, "")); // Print v to out
return out; // To allow (cout << a) << b;
}
cout << s; // OK, v refers to base part of s
ofstream f("file.txt");
f << s; // OK, ofstream is derived from ostream

A derived class may redefine inherited member functions, overriding any function with the same name, parameters, return type, and const-ness (and hiding other functions with the same name, thus the overriding function should not be overloaded). The function call is resolved at compile time. This is incorrect in case of a base pointer or reference to a derived object. To allow run time resolution, the base member function should be declared virtual. Since the default destructor is not virtual, a virtual destructor should be added to the base class. If empty, no copy constructor or assignment operator is required. Constructors and = are never virtual.
class Shape {
public:
virtual void draw() const;
virtual ~Shape() {}
};
class Circle: public Shape {
public:
void draw() const; // Must use same parameters, return type, and const
};

Shape s; s.draw(); // Shape::draw()
Circle c; c.draw(); // Circle::draw()
Shape& r=c; r.draw(); // Circle::draw() if virtual
Shape* p=&c; p->draw(); // Circle::draw() if virtual
p=new Circle; p->draw(); // Circle::draw() if virtual
delete p; // Circle::~Circle() if virtual

An abstract base class defines an interface for one or more derived classes, which are allowed to instantiate objects. Abstractness can be enforced by using protected (not private) constructors or using pure virtual member functions, which must be overridden in the derived class or else that class is abstract too. A pure virtual member function is declared =0; and has no code defined.
class Shape {
protected:
Shape(); // Optional, but default would be public
public:
virtual void draw() const = 0; // Pure virtual, no definition
virtual ~Shape() {}
};
// Circle as before

Shape s; // Error, protected constructor, no Shape::draw()
Circle c; // OK
Shape& r=c; r.draw(); // OK, Circle::draw()
Shape* p=new Circle(); // OK
Run time type identification
C++ provides for run time type identification, although this usually indicates a poor design. dynamic_cast<T>(x) checks at run time whether a base pointer or reference is to a derived object, and if so, does a conversion. The base class must have at least one virtual function to use run time type checking. #include <typeinfo> // For typeid()
typeid(*p)==typeid(T) // true if p points to a T
dynamic_cast<T*>(p) // Convert base pointer to derived T* or 0.
dynamic_cast<T&>(r) // Convert base reference to derived T& or throw bad_cast()
For example, class B {public: virtual void f(){}};
class D: public B {public: int x;} d; // Bad design, public member in D but not B
B* p=&d; p->x; // Error, no B::x
D* q=p; q->x; // Error, can't convert B* to D*
q=(D*)p; q->x; // OK, but reinterpret_cast, no run time check
q=dynamic_cast<D*>(p); if (q) q->x; // OK

Inheritance

Inheritance



Inheritance is used to write a specialized or enhanced version of another class. For example, an ofstream is a type of ostream. class D: public B defines class D as derived from (subclass of) base class (superclass) B, meaning that D inherits all of B's members, except the constructors, destructor, and assignment operator. The default behavior of these special member functions is to treat the base class as a data member. class String: public Vector<char> {
public:
String(const char* s=""): Vector<char>(strlen(s)) {
copy(s, s+strlen(s), begin()); // Inherits Vector<char>::begin()
}
};
String a="hello"; // Calls Vector<char>::Vector(5);
a.size(); // 5, inherits Vector<char>::size()
a[0]='j'; // "jello", inherits Vector<char>::operator[]
String b=a; // Default copy constructor uses Vector's copy constructor on base part
b=a; // Default = calls Vector's assignment operator on base part
The default destructor String::~String() {} is correct, since in the process of destroying a String, the base is also destroyed, calling Vector<char>::~Vector() {delete data[];}. Since there is no need to write a destructor, there is no need to redefine copying or assignment either.
Although String inherits Vector<char>::data, it is private and inaccessible. A protected member is accessible to derived classes but private elsewhere.
class B {
protected:
int x;
} b; // Declare class B and object b
b.x=1; // Error, x is protected

class D: public B {
void f() {x=1;} // OK
};
By default, a base class is private, making all inherited members private. Private base classes are rare and typically used as implementations rather than specializations (A string is a vector, but a stack is not). class Stack: Vector<int> { // or class Stack: private Vector<int>
public:
bool empty() const {return size()==0;} // OK
} s;
s.size(); // Error, private
s.empty(); // OK, public

A class may have more than one base class (called multiple inheritance). If both bases are in turn derived from a third base, then we derive from this root class using virtual to avoid inheriting its members twice further on. Any indirectly derived class treats the virtual root as a direct base class in the constructor initialization list.
class ios {...}; // good(), binary, ...
class fstreambase: public virtual ios {...}; // open(), close(), ...
class istream: public virtual ios {...}; // get(), operator>>(), ...
class ifstream: public fstreambase, public istream { // Only 1 copy of ios
ifstream(): fstreambase(), istream(), ios() {...} // Normally ios() would be omitted

Classes

Classes

Classes provide data abstraction, the ability to create new types and hide their implementation in order to improve maintainability. A class is a data structure and an associated set of member functions (methods) and related type declarations which can be associated with the class or instances (objects) of the class. A class is divided into a public interface, visible wherever the class or its instances are visible, and a private implementation visible only to member functions of the class.
class T { // Create a new type T
private: // Members are visible only to member functions of T (default)
public: // Members are visible wherever T is visible
// Type, object, and function declarations
};
T::m; // Member m of type T
T x; // Create object x of type T
x.m; // Member m of object x
T* p=&x; p->m; // Member m of object pointed to by p
Typically the data structure is private, and functionality is provided by member functions. Member function definitions should be separated from the declaration and written outside the class definition, or else they are assumed to be inline (which is appropriate for short functions). A member function should be declared const (before the opening brace) if it does not modify any data members. Only const member functions may be called on const objects. class Complex { // Represents imaginary numbers
private:
double re, im; // Data members, represents re + im * sqrt(-1)
public:
void set(double r, double i) {re=r; im=i;} // Inlined member function definition
double real() const {return re;} // const - does not modify data members
double imag() const; // Declaration for non-inlined function
};
double Complex::imag() const {return im;} // Definition for imag()
int main() {
Complex a, b=a; // Objects of type Complex
a.set(3, 4); // Call a member function
b=a; // Assign b.re=a.re; b.im=a.im
b==a; // Error, == is not defined
cout << a.re; // Error, re is private
cout << a.real(); // OK, 3
cout << Complex().real(); // OK, prints an undefined value
Complex().set(5, 6); // Error, non-const member called on const object

A class has two special member functions, a constructor, which is called when the object is created, and a destructor, called when destroyed. The constructor is named class::class, has no return type or value, may be overloaded and have default arguments, and is never const. It is followed by an optional initialization list listing each data member and its initial value. Initialization takes place before the constructor code is executed. Initialization might not be in the order listed. Members not listed are default-initialized by calling their constructors with default arguments. If no constructor is written, the compiler provides one which default-initializes all members. The syntax is:
class::class(parameter list): member(value), member(value) { code...}

The destructor is named class::~class, has no return type or value, no parameters, and is never const. It is usually not needed except to return shared resources by closing files or deleting memory. After the code executes, the data members are destroyed using their respective destructors in the reverse order in which they were constructed.
class Complex {
public:
Complex(double r=0, double i=0): re(r), im(i) {} // Constructor
~Complex() {} // Destructor
// Other members...
};
Complex a(1,2), b(3), c=4, d; // (1,2) (3,0) (4,0) (0,0)

A constructor defines a conversion function for creating temporary objects. A constructor that allows 1 argument allows implicit conversion wherever needed, such as in expressions, parameter passing, assignment, and initialization.
Complex(3, 4).real(); // 3
a = 5; // Implicit a = Complex(5) or a = Complex(5, 0)

void assign_if(bool, Complex&, const Complex&);
assign_if(true, a, 6); // Implicit Complex(6) passed to third parameter
assign_if(true, 6, a); // Error, non-const reference to Complex(6), which is const

Operators may be overloaded as members. The expression aXb for operator X can match either operator X(a, b) (global) or a.operator X(b) (member function), but not both. Unary operators omit b. Operators =, [], and -> can only be overloaded as member functions.
class Complex {
public:
Complex operator + (const Complex& b) const { // const because a+b doesn't change a
return Complex(re+b.re, im+b.im);
}
// ...
};

Complex operator - (const Complex& a, const Complex& b) {
return Complex(a.real()-b.real(), a.imag()-b.imag());
}

Complex a(1, 2), b(3, 4);
a+b; // OK, a.operator+(b) == Complex(4, 6)
a-b; // OK, operator-(a, b) == Complex(-2, -2)
a+10; // OK, Complex(1, 12), implicit a+Complex(10, 0)
10+a; // Error, 10 has no member operator+(Complex)
a-10; // OK, Complex(1, -8)
10-a; // OK, Complex(7, -4)

The member function (+) has the advantage of private access (including to other objects of the same class), but can only do implicit conversions on the right side. The global function (-) is symmetrical, but lacks private access. A friend declaration (in either the private or public section) allows private access to a global function.
class Complex {
friend Complex operator-(const Complex&, const Complex&);
friend class T; // All member functions of class T are friends
// ...
};

A conversion operator allows implicit conversion to another type. It has the form of a member function named operator T() const with implied return type T. It is generally a good idea to allow implicit conversions in only one direction, preferably with constructors, so this member function is usually used to convert to pre-existing types.
class Complex {
public:
operator double() const {return re;}
// ...
}

Complex a(1, 2);
a-10; // Error, double(a)-10 or a-Complex(10) ?
a-Complex(10); // Complex(-9, 2);
double(a)-10; // -9

An explicit constructor does not allow implicit conversions.
class Complex {
explicit Complex(double r=0, double i=0);
// ...
};

Complex a=1; // Error
Complex a(1); // OK
a-10; // OK, double(a)-10 = -9
a-Complex(10); // OK, Complex(-9, 0)

A class or member function may be templated. The type parameter must be passed in the declaration for objects of the class.
template <class T>
class Complex {
T re, im;
public:
T real() const {return re;}
T imag() const {return im;}
Complex(T r=0, T i=0);
friend Complex<T> operator - (const Complex<T>&, const Complex<T>&);
};

template <class T>
Complex<T>::Complex(T r, T i): re(r), im(i) {}

Complex<int> a(1, 2); // Complex of int
Complex<double> b(1.0, 2.0); // Complex of double
a=a-Complex<int>(3, 4); // Complex<int>(-2, -2)
Complex<Complex<double> > c(b, b); // Note space, not >>
c.real().imag(); // 2.0
Templates can have default arguments and int parameters. The argument to an int parameter must be a value known at compile time. template <class T, class U=T, int n=0> class V {};
V<double, string, 3> v1;
V<char> v2; // V<char, char, 0>

Classes define default behavior for copying and assignment, which is to copy/assign each data member. This behavior can be overridden by writing a copy constructor and operator= as members, both taking arguments of the same type, passed by const reference. They are usually required in classes that have destructors, such as the vector<T>-like class below. If we did not overload these, the default behavior would be to copy the data pointer, resulting in two Vectors pointing into the same array. The assignment operator normally returns itself (*this) by reference to allow expressions of the form a=b=c;, but is not required to do so. this means the address of the current object; thus any member m may also be called this->m within a member function.
template <class T>
class Vector {
private:
T* data; // Array of n elements
int n; // size()
public:
typedef T* iterator; // Vector::iterator means T*
typedef const T* const_iterator; // Iterators for const Vector
int size() const {return n;} // Number of elements
T& operator[](int i) {return data[i];} // i'th element
const T& operator[](int i) const {return data[i];} // i'th element of const Vector
iterator begin() {return data;} // First, last+1 elements
iterator end() {return data+size();}
const_iterator begin() const {return data;} // Const versions
const_iterator end() const {return data+size();}
Vector(int i=0): data(new T[i]), n(i) {} // Create with size i
~Vector() {delete[] data;} // Return memory
Vector(const Vector<T>& v): data(new T[v.n]), n(v.n) { // Copy constructor
copy(v.begin(), v.end(), begin());
}
Vector& operator=(const Vector& v) { // Assignment
if (&v != this) { // Assignment to self?
delete[] data; // If not, resize and copy
data=new T[n=v.n];
copy(v.begin(), v.end(), begin());
}
return *this; // Allow a=b=c;
}
template <class P> Vector(P b, P e): data(new T[e-b]), n(e-b) { // Templated member
copy(b, e, data); // Initialize from sequence [b, e)
}
};
A type defined in a class is accessed through class::type Vector<int>::iterator p; // Type is int*
Vector<int>::const_iterator cp; // Type is const int*
Member functions may be overloaded on const. Overloaded member functions need not have the same return types. const member functions should not return non-const references or pointers to data members. Vector<int> v(10); // Uses non-const [], begin(), end()
const Vector<int> cv(10); // Uses const [], begin(), end()
cv=v; // Error, non-const operator= called on cv
v[5]=cv[5]; // OK. assigns to int&
cv[5]=v[5]; // Error, assigns to const int&
p=cv.begin(); // Error, would allow *p=x to write into cv
cp=cv.begin(); // OK because can't assign to *cp

Defining Iterators. Sometimes a container's iterator types must be defined as nested classes overloading the usual pointer operations rather than typedef'ed to pointers. In order to work properly with functions defined in <algorithm>, iterators should define the following 5 public typedefs:
iterator_category: one of the following (defined in <iterator>):
output_iterator_tag (if sequential writing is supported)
input_iterator_tag (if sequential reading is supported)
forward_iterator_tag (if both are supported)
bidirectional_iterator_tag (if the iterator can be decremented)
random_access_iterator_tag (if all pointer operations are supported)
value_type: the type of the elements, for example, T
difference_type: the result of iterator subtraction, usually ptrdiff_t (a signed int type)
pointer: the type returned by operator->(), usually T* or const T*
reference: the type returned by operator*(), usually T& or const T&

Operator -> should be overloaded as a unary function returning a pointer to a class to which -> will be applied, i.e. x->m is interpreted as x.operator->()->m. Nested class members are named Outer::Inner::member. Outer and inner classes cannot access each other's private members. Templated members defined outside the class need their own template declarations.

template <class T> class Vector {
public:

// Reverse iterator for Vector, i.e. ++p goes to the previous element.
class reverse_iterator {
private:
T* p; // Points to current element
public:

// typedefs needed to work with <algorithm> functions
typedef std::random_access_iterator_tag iterator_category; // Defined in <iterator>
typedef T value_type; // Type of element
typedef ptrdiff_t difference_type; // Result of iterator subtraction, usually int
typedef T* pointer; // Type returned by operator ->
typedef T& reference; // Type returned by operator *

reverse_iterator(T* a=0): p(a) {} // Implicit conversion from T* and iterator
iterator base() const {return p;} // Convert to normal iterator

// Forward operators
reverse_iterator& operator++() {--p; return *this;} // prefix
reverse_iterator operator++(int); // postfix, we pretend it's binary
reference operator*() const {return *p;}
pointer operator->() const {return p;} // We pretend it's unary
bool operator==(Vector<T>::reverse_iterator b) const {return p==b.p;}
bool operator!=(Vector<T>::reverse_iterator b) const {return p!=b.p;}
// Also, bidirectional and random operators
};
reverse_iterator rbegin() {return end()-1;}
reverse_iterator rend() {return begin()-1;}
// Other members...
};

// Code for postfix ++
template <class T>
inline Vector<T>::reverse_iterator Vector::reverse_iterator::operator++(int dummy) {
Vector<T>::reverse_iterator result = *this;
++*this;
return result;
};

// Print a Vector in reverse order
int main() {
Vector<int> a(10);
for (Vector<int>::reverse_iterator p=a.rbegin(); p!=a.rend(); ++p)
cout << *p << endl;
vector<T> supplies random reverse_iterator and const_reverse_iterator as above. Const iterators would typedef pointer as const T* and reference as const T&.

A static data member is shared by all instances of a class. It must be initialized in a separate declaration, not in the class definition or in the constructor initialization list. A static member function cannot refer to this or any non-static members (and therefore it makes no sense to make them const). Static members may be referenced either as object.member or class::member.
class Counter {
static int count; // Number of Counters that currently exist (private)
public:
static int get() {return count;}
Counter() {++count;}
~Counter() {--count;}
Counter(const Counter& c) {++count;} // Default would be wrong
Counter& operator=(const Counter& c) {return *this;} // Default would be OK
};
int Counter::count = 0; // Initialize here, OK if private
main() {
Counter a, b, c;
cout << b.get(); // 3
cout << Counter::get(); // 3
}
Inheritance
Inheritance is used to write a specialized or enhanced version of another class. For example, an ofstream is a type of ostream. class D: public B defines class D as derived from (subclass of) base class (superclass) B, meaning that D inherits all of B's members, except the constructors, destructor, and assignment operator. The default behavior of these special member functions is to treat the base class as a data member. class String: public Vector<char> {
public:
String(const char* s=""): Vector<char>(strlen(s)) {
copy(s, s+strlen(s), begin()); // Inherits Vector<char>::begin()
}
};
String a="hello"; // Calls Vector<char>::Vector(5);
a.size(); // 5, inherits Vector<char>::size()
a[0]='j'; // "jello", inherits Vector<char>::operator[]
String b=a; // Default copy constructor uses Vector's copy constructor on base part
b=a; // Default = calls Vector's assignment operator on base part
The default destructor String::~String() {} is correct, since in the process of destroying a String, the base is also destroyed, calling Vector<char>::~Vector() {delete data[];}. Since there is no need to write a destructor, there is no need to redefine copying or assignment either.
Although String inherits Vector<char>::data, it is private and inaccessible. A protected member is accessible to derived classes but private elsewhere.
class B {
protected:
int x;
} b; // Declare class B and object b
b.x=1; // Error, x is protected

class D: public B {
void f() {x=1;} // OK
};
By default, a base class is private, making all inherited members private. Private base classes are rare and typically used as implementations rather than specializations (A string is a vector, but a stack is not). class Stack: Vector<int> { // or class Stack: private Vector<int>
public:
bool empty() const {return size()==0;} // OK
} s;
s.size(); // Error, private
s.empty(); // OK, public

A class may have more than one base class (called multiple inheritance). If both bases are in turn derived from a third base, then we derive from this root class using virtual to avoid inheriting its members twice further on. Any indirectly derived class treats the virtual root as a direct base class in the constructor initialization list.
class ios {...}; // good(), binary, ...
class fstreambase: public virtual ios {...}; // open(), close(), ...
class istream: public virtual ios {...}; // get(), operator>>(), ...
class ifstream: public fstreambase, public istream { // Only 1 copy of ios
ifstream(): fstreambase(), istream(), ios() {...} // Normally ios() would be omitted

cassert Library

<cassert>

Provides a debugging function for testing conditions where all instances can be turned on or off at once. assert(false); prints the asserted expression, source code file name, and line number, then aborts. Compiling with g++ -DNDEBUG effectively removes these statements.
assert(e); // If e is false, print message and abort
#define NDEBUG // (before #include <assert.h>), turn off assert

cstdio Library

<cstdio>



The stdio library is made mostly obsolete by the newer iostream library, but many programs still use it. There are facilities for random access files and greater control over output format, error handling, and temporary files. Mixing both I/O libraries is not recommended. There are no facilities for string I/O.

Global objects stdin, stdout, stderr of type FILE* correspond to cin, cout, cerr. s is type char*, c is char, n is int, f is FILE*.
FILE* f=fopen("filename", "r"); // Open for reading, NULL (0) if error
// Mode may also be "w" (write) "a" append, "a+" random access read/append,
// "rb", "wb", "ab", "a+b" are binary
fclose(f); // Close file f
fprintf(f, "x=%d", 3); // Print "x=3" Other conversions:
"%5d %u %-8ld" // int width 5, unsigned int, long left justified
"%o %x %X %lx" // octal, hex, HEX, long hex
"%f %5.1f" // double: 123.000000, 123.0
"%e %g" // 1.23e2, use either f or g
"%c %s" // char, char*
"%%" // %
sprintf(s, "x=%d", 3); // Print to array of char s
printf("x=%d", 3); // Print to stdout (screen unless redirected)
fprintf(stderr, ... // Print to standard error (not redirected)
getc(f); // Read one char (as an int, 0-255) or EOF (-1) from f
ungetc(c, f); // Put back one c to f
getchar(); // getc(stdin);
putc(c, f) // fprintf(f, "%c", c);
putchar(c); // putc(c, stdout);
fgets(s, n, f); // Read line including '\n' into char s[n] from f. NULL if EOF
gets(s) // fgets(s, INT_MAX, f); no '\n' or bounds check
fread(s, n, 1, f); // Read n bytes from f to s, return number read
fwrite(s, n, 1, f); // Write n bytes of s to f, return number written
fflush(f); // Force buffered writes to f
fseek(f, n, SEEK_SET); // Position binary file f at n
// or SEEK_CUR from current position, or SEEK_END from end
ftell(f); // Position in f, -1L if error
rewind(f); // fseek(f, 0L, SEEK_SET); clearerr(f);
feof(f); // Is f at end of file?
ferror(f); // Error in f?
perror(s); // Print char* s and last I/O error message to stderr
clearerr(f); // Clear error code for f
remove("filename"); // Delete file, return 0 if OK
rename("old", "new"); // Rename file, return 0 if OK
f = tmpfile(); // Create temporary file in mode "wb+"
tmpnam(s); // Put a unique file name in char s[L_tmpnam]

Example: input file name and print its size
char filename[100]; // Cannot be a string
printf("Enter filename\n"); // Prompt
gets(filename, 100, stdin); // Read line ending in "\n\0"
filename[strlen(filename)-1]=0; // Chop off '\n';
FILE* f=fopen(filename, "rb"); // Open for reading in binary mode
if (f) { // Open OK?
fseek(f, 0, SEEK_END); // Position at end
long n=ftell(f); // Get position
printf("%s has %ld bytes\n", filename, n);
fclose(f); // Or would close when program ends
}
else
perror(filename); // fprintf(stderr, "%s: not found\n", filename);
// or permission denied, etc.
printf(), fprintf(), and sprintf() accept a variable number of arguments, one for each "%" in the format string, which must be the appropriate type. The compiler does not check for this. printf("%d %f %s", 2, 2.0, "2"); // OK
printf("%s", 5); // Crash: expected a char* arg, read from address 5
printf("%s"); // Crash
printf("%s", string("hi")); // Crash: use "hi" or string("hi").c_str()

cstring Library

<cstring>

Functions for performing string-like operations on arrays of char marked with a terminating '\0' (such as "quoted literals" or as returned by string::c_str(). Mostly obsoleted by type string.
strcpy(dst, src); // Copy src to dst. Not bounds checked
strcat(dst, src); // Concatenate to dst. Not bounds checked
strcmp(s1, s2); // Compare, <0 if s1<s2, 0 if s1==s2, >0 if s1>s2
strncpy(dst, src, n); // Copy up to n chars, also strncat(), strncmp()
strlen(s); // Length of s not counting \0
strchr(s,c); strrchr(s,c);// Address of first/last char c in s or 0
strstr(s, sub); // Address of first substring in s or 0
// mem... functions are for any pointer types (void*), length n bytes
memcpy(dst, src, n); // Copy n bytes from src to dst
memmove(dst, src, n); // Same, but works correctly if dst overlaps src
memcmp(s1, s2, n); // Compare n bytes as in strcmp
memchr(s, c, n); // Find first byte c in s, return address or 0
memset(s, c, n); // Set n bytes of s to c

ctime Library

<ctime>

Functions for reading the system clock. time_t is an integer type (usually long). tm is a struct.
clock()/CLOCKS_PER_SEC; // Time in seconds since program started
time_t t=time(0); // Absolute time in seconds or -1 if unknown
tm* p=gmtime(&t); // 0 if UCT unavailable, else p->tm_X where X is:
sec, min, hour, mday, mon (0-11), year (-1900), wday, yday, isdst
asctime(p); // "Day Mon dd hh:mm:ss yyyy\n"
asctime(localtime(&t)); // Same format, local time

cmath Library

<cmath>

Functions take double and return double.
sin(x); cos(x); tan(x); // Trig functions, x in radians
asin(x); acos(x); atan(x);// Inverses
atan2(y, x); // atan(y/x)
sinh(x); cosh(x); tanh(x);// Hyperbolic
exp(x); log(x); log10(x); // e to the x, log base e, log base 10
pow(x, y); sqrt(x); // x to the y, square root
ceil(x); floor(x); // Round up or down (as a double)
fabs(x); fmod(x, y); // Absolute value, x mod y

cctype Library

<cctype>


Character tests take a char c and return bool.
isalnum(c); // Is c a letter or digit?
isalpha(c); isdigit(c); // Is c a letter? Digit?
islower(c); isupper(c); // Is c lower case? Upper case?
isgraph(c); isprint(c); // Printing character except/including space?
isspace(c); iscntrl(c); // Is whitespace? Is a control character?
ispunct(c); // Is printing except space, letter, or digit?
isxdigit(c); // Is hexadecimal digit?
c=tolower(c); c=toupper(c); // Convert c to lower/upper case

cstdlib Library

<cstdlib>

Miscellaneous functions. s is type char*, n is int
atoi(s); atol(s); atof(s);// Convert char* s to int, long, double e.g. atof("3.5")
abs(x); labs(x); // Absolute value of numeric x as int, long
rand(); // Pseudo-random int from 0 to RAND_MAX (at least 32767)
srand(n); // Initialize rand(), e.g. srand(time(0));
system(s); // Execute OS command s, e.g. system("ls");
getenv(s); // Environment variable or 0 as char*, e.g. getenv("PATH");
exit(n); // Kill program, return status n, e.g. exit(0);
void* p = malloc(n); // Allocate n bytes or 0 if out of memory. Obsolete, use new.
p = calloc(n, 1); // Allocate and set to 0 or return NULL. Obsolete.
p = realloc(p, n); // Enlarge to n bytes or return NULL. Obsolete.
free(p); // Free memory. Obsolete: use delete

new Library

<new>

The default behavior of new is to throw an exception of type bad_alloc if out of memory. This can be changed by writing a function (taking no parameters and returning void) and passing it to set_new_handler().
void handler() {throw bad_alloc();} // The default
set_new_handler(handler);
new(nothrow) may be used in place of new. If out of memory, it returns 0 rather than throw bad_alloc. int* p = new(nothrow) int[1000000000]; // p may be 0

functional Library

<functional>


Functions in <functional> create function objects, which are objects that behave like functions by overloading operator(). These can be passed to algorithms that take function arguments, e.g.
vector<int> v(10);
sort(v.begin(), v.end(), greater<int>()); // Sort v in reverse order
int x=accumulate(v.begin(), v.end(), 1, multiplies<T>); // Product of elements
The following create function objects that take one or two parameters x and y of type T and return the indicated expression, i.e., equal_to<int>()(3,4) returns false. // Predicates (return bool)
equal_to<T>() // x==y
not_equal_to<T>() // x!=y
greater<T>() // x>y
less<T>() // x<y
greater_equal<T>() // x>=y
less_equal<T>() // x<=y
logical_and<bool>() // x&&y
logical_or<bool>() // x||y
logical_not<bool>() // !x (unary)

// Arithmetic operations (return T)
plus<T>() // x+y
minus<T>() // x-y
multiplies<T>() // x*y
divides<T>() // x/y
modulus<T>() // x%y
negate<T>() // -x (unary)
A binder converts a 2-argument function object into a 1-argument object by binding a fixed value c to the other argument, e.g. bind2nd(less<int>(), 10) returns a function object that takes one argument x and returns true if x<10. bind1st(f, c) // An object computing f(c,y)
bind2nd(f, c) // An object computing f(x,c)

i=find_if(v.begin(), v.end(), bind2nd(equal_to<int>(), 0)); // Find first 0
The following convert ordinary functions and member functions into function objects. All functions must be converted to be passed to bind1st and bind2nd. Member functions must also be converted to be passed to algorithms. ptr_fun(f) // Convert ordinary function f to object
mem_fun(&T::f) // Convert member function of class T
mem_fun_ref(T::f) // Same

i=find_if(v.begin(), v.end(), mem_fun(&string::empty)); // Find ""
transform(v.begin(), v.end(), v.begin(), bind2nd(ptr_fun(pow), 2.0)); // Square elements
not1() and not2() negate a unary or binary function object. not1(f) // Object computing !f(x)
not2(f) // Object computing !f(x,y)

i=find_if(v.begin(), v.end(), not1(bind2nd(equal_to<int>(), 0))); // Find nonzero

iterator Library

<iterator>





An inserter is an output iterator that expands the container it points to by calling push_back(), push_front(), or insert(). The container must support this operation. A stream iterator can be used to do formatted input or output using >> or <<
back_inserter(c); // An iterator that appends to container c
front_inserter(c); // Inserts at front of c
inserter(c, p); // Inserts in front of p
ostream_iterator<T>(out, cp); // Writes to ostream separated by char* cp (default " ")
istream_iterator<T>(in); // An input iterator that reads T objects from istream
The most common use is to copy to an empty vector, deque, or list. vector<int> from(10), to;
copy(from.begin(), from.end(), back_inserter(to));

numeric Library

<numeric>


In the following, plus, minus, and times are optional functions taking 2 arguments x and y that return x+y, x-y, and x*y respectively, e.g. int plus(int x, int y) {return x+y;} x=accumulate(b, e, x, plus); // x + sum over [b,e)
x=inner_product(b, e, b2, x, plus, times); // x + sum [b,e)*[b2,e2)
adjacent_difference(b, e, minus); // for i in (b,e) *i -= i[-1]
partial_sum(b, e, plus); // for i in [b,e) *i += sum [b,i)

algorithm Library

<algorithm>



In the following, b and e are input iterators, and d is an output iterator, unless otherwise specified. Parameters eq and lt are optional, and default to functions that take 2 arguments x and y and return x==y and x<y respectively, e.g. bool eq(x,y) {return x==y;}. x and y are objects of the type pointed to by the iterators. p is a pair of iterators. f is a function or function object as noted.
// Operations on ordinary objects
swap(x1, x2); // Swap values of 2 objects of the same type
min(x1, x2); // Smaller of x1 or x2, must be same type
max(x1, x2); // Larger of x1 or x2, must be same type

// Properties of sequences (input iterators)
equal(b, e, b2, eq); // true if [b,e)==[b2,...)
lexicographical_compare(b, e, b2, e2, lt); // true if [b,e)<[b2,e2)
i=min_element(b, e); // Points to smallest in [b,e)
i=max_element(b, e); // Points to largest
n=count(b, e, x); // Number of occurrences of x in [b,e)
n=count_if(b, e, f); // Number of f(x) true in [b,e)

// Searching, i points to found item or end (e) if not found
i=find(b, e, x); // Find first x in [b,e)
i=find_if(b, e, f); // Find first x where f(x) is true
i=search(b, e, b2, e2, eq);// Find first [b2,e2) in [b,e) (forward)
i=find_end(b, e, b2, e2, eq); // Find last [b2,e2) in [b,e) (forward)
i=search_n(b, e, n, x, eq);// Find n copies of x in [b,e) (forward)
p=mismatch(b, e, b2, eq); // Find first *p.first in [b,e) != *p.second in [b2,.) (forward)
i=adjacent_find(b, e, eq); // Find first of 2 equal elements (forward)

// Modifying elements
i=copy(b, e, d); // Copy [b,e) to [d,i)
fill(d, i, x); // Set all in [d,i) to x (forward)
i=fill_n(d, n, x); // Set n elements in [d,i) to x
generate(d, i, f); // Set [d,i) to f() (e.g. rand) (forward)
i=generate_n(d, n, f); // Set n elements in [d,i) to f()
f=for_each(b, e, f); // Call f(x) for each x in [b,e)
i=transform(b, e, d, f); // For x in [b,e), put f(x) in [d,i)
i=transform(b, e, b2, d, f); // For x in [b,e), y in [b2,.), put f(x,y) in [d,i)
replace(b, e, x, y) // Replace x with y in [b,e)
replace_if(b, e, f, y); // Replace with y in [b,e) where f(x) is true
i=replace_copy(b, e, d, x, y); // Copy [b,e) to [d,i) replacing x with y
i=replace_copy_if(b, e, d, f, y); // Copy replacing with y where f(x) is true

// Rearranging sequence elements
sort(b, e, lt); // Sort [b,e) by < (random)
stable_sort(b, e, lt); // Sort slower, maintaining order of equal elements (random)
partial_sort(b, m, e, lt); // Sort faster but leave [m,e) unsorted (random)
nth_element(b, m, e, lt); // Sort fastest but only *m in proper place (random)
iter_swap(b, e); // swap(*b, *e) (forward)
i=swap_ranges(b, e, b2); // swap [b,e) with [b2,i) (forward)
i=partition(b, e, f); // Moves f(x) true to front, [i,e) is f(x) false (bidirectional)
i=stable_partition(b, e, f); // Maintains order within each partition
i=remove(b, e, x); // Move all x to end in [i,e) (forward)
i=remove_if(b, e, f); // Move f(x) true to front in [b,i) (forward)
i=remove_copy(b, e, d, x); // Copy elements matching x to [d,i)
i=remove_copy_if(b, e, d, f); // Copy elements x if f(x) is false to [d,i)
replace(b, e, x1, x2); // Replace x1 with x2 in [b,e)
i=replace_copy(b, e, d, x1, x2); // Copy [b,e) to [d,i) replacing x1 with x2
reverse(b, e); // Reverse element order in [b,e) (bidirectional)
i=reverse_copy(b, e, d); // Copy [b,e) to [d,i) reversing the order (b,e bidirectional)
rotate(b, m, e); // Move [b,m) behind [m,e) (forward)
i=rotate_copy(b, m, e, d); // Rotate into [d,i)
random_shuffle(b, e, f); // Random permutation, f() defaults to rand()
next_permutation(b, e, lt);// Next greater sequence, true if successful (bidirectional)
prev_permutation(b, e, lt);// Previous permutation, true if successful (bidirectional)

// Operations on sorted sequences
i=unique(b, e, eq); // Move unique list to [b,i), extras at end
i=unique_copy(b, e, d, eq); // Copy one of each in [b,d) to [d,i)
i=binary_search(b, e, x, lt); // Find i in [b,e) (forward)
i=lower_bound(b, e, x, lt); // Find first x in [b,e) or where to insert it (forward)
i=upper_bound(b, e, x, lt); // Find 1 past last x in [b,e) or where to insert it (forward)
p=equal_range(b, e, x, lt); // p.first = lower bound, p.second = upper bound (forward)
includes(b, e, b2, e2, lt); // true if [b,e) is a subset of [b2,e2)
i=merge(b, e, b2, e2, d, lt); // Merge [b,e) and [b2,e2) to [d,i)
inplace_merge(b, m, e, lt); // Merge [b,m) and [m,e) to [b,e) (bidirectional)
i=set_union(b, e, b2, e2, d, lt); // [d,i) = unique elements in either [b,e) or [b2,e2)
i=set_intersection(b, e, b2, e2, d, lt); // [d,i) = unique elements in both
i=set_difference(b, e, b2, e2, d, lt); // [d,i) = unique elements in [b,e) but not [b2,e2)
i=set_symmetric_difference(b, e, b2, e2, d, lt); // [d,i) = elements in one but not both
Algorithms never change the size of a container. When copying, the destination must be large enough to hold the result. int a[5]={3,1,4,1,6};
vector b(5);
copy(a, a+5, v.begin()); // Copy a to v
remove(a, a+5, 1); // {3,4,6,1,1}, returns a+3
sort(a, a+4); // {1,3,4,6,1}

C++ Standard Library Functions

C++ Standard Library Functions

Many C++ standard library functions operate on sequences denoted by iterators or pointers. Iterators are a family of types that include pointers. They are classified by the operators they support.
input: ++p, p++, p=q, p==q, p!=q, *p (read-only)
output: p=q, p==q, p!=q, *p++ = x (alternating write/increment)
forward: input and output and p->m, *p (multiple read-write)
bidirectional: forward and --p, p--, implemented by list, map, multimap, set, multiset.
random: bidirectional and p<q, p>q, p<=q, p>=q, p+i, i+p, p-i, p-q, p[i], implemented by arrays (as pointers), string, vector, deque.

Some algorithms require certain iterator types, but will accept more powerful types. For example, copy(b, e, d) require b and e to be at least input iterators and d to be at least an output iterator. But it will accept forward, bidirectional, or random iterators because these all support input and output operations. sort() requires random iterators and will accept no other type.

The notation [b,e) denotes the sequence of e-b objects from b[0] to e[-1], i.e. b points to the beginning of the sequence and e points one past the end. For most containers, v, the sequence is [v.begin(), v.end()). For an array of n elements, the sequence is [a, a+n).

stdexcept exception Library

<stdexcept>, <exception>

The standard library provides a hierarchy of exception types. Not all of them are used by the library, but any may be thrown.
Type Header Thrown by
exception stdexcept, exception
logic_error stdexcept
length_error stdexcept
domain_error stdexcept
out_of_range stdexcept .at(i) (vector/string/deque index out of bounds)
invalid_argument stdexcept, bitset bitset("xxx") (not '0' or '1')
runtime_error stdexcept
range_error stdexcept
overflow_error stdexcept
underflow_error stdexcept
bad_alloc new new, new[] (out of memory)
bad_cast typeinfo dynamic_cast<T&> (can't convert to derived)
bad_typeid typeinfo typeid(*p) when p==0
bad_exception exception
ios_base::failure ios, iostream, fstream istream::clear(), ostream::clear()
Catching a base class catches all derived classes, thus catch(exception e) catches all of the above types. However, C++ allows throwing exceptions not derived from exception, so this may not catch everything. All exceptions provide the following interface: throw exception(msg) // Throw exception with char* or string msg
throw exception(); // Default msg
catch(exception e) {e.what();} // msg as a char*

New exceptions may be derived from existing types to maintain this interface (see inheritance).
class MyError: public exception {
public:
MyError(const string& msg=""): exception(msg) {}

complex Library

<complex>


A complex supports complex arithmetic. It has real and imaginary parts of type T. Mixed type expressions promote real to complex (e.g. double to complex<double> and lower precision to higher precision (e.g. complex<int> to complex<double>). complex<T> x; // (0,0), T is a numeric type
complex<T> x=r; // (r,0), convert real r to complex
complex<T> x(r, i); // (r,i)
x=polar<T>(rho, theta); // Polar notation: radius, angle in radians
x.real() // r
x.imag() // i
abs(x) // rho = sqrt(r*r+i*i)
arg(x) // tan(theta) = i/r
norm(x) // abs(x)*abs(x)
conj(x) // (r,-i)
x+y // Also - * / == != = += -= *= /= and unary + -
sin(x) // Also sinh, sqrt, tan, tanh, cos, cosh, exp, log, log10, pow(x,y)
cout << x // Prints in format "(r,i)"
cin >> x // Expects "r", "(r)", or "(r,i)"

valarray Library

<valarray>




A valarray is like a fixed sized array or vector that supports arithmetic operations on all the elements at once. For instance, if x and y are valarrays of the same size, then x+y is a valarray containing the sums of the corresponding elements. Likewise, y=sqrt(x) assigns y[i]=sqrt(x[i]) to each element of y. In mixed type expressions, a scalar (element of type T) is promoted to a valarray of the same size by duplicating it, e.g. x+1 adds 1 to all elements of x.
valarray<T> v(n); // n elements of type T, initially T() or 0
valarray<T> v(x, n); // n copies of x (note arguments are backwards)
valarray<T> v(a, n); // Initialize from array a[0]..a[n-1]
valarray<T> v; // size is 0
v.size() // Number of elements, n
v[i] // i'th element, 0 <= i < n, not checked
v+=x, v+=v // Add x or v[i] to all v[i], also = -= *= /= %= ^= &= |= <<= >>=
v+v, v+x, x+v // Also - * / % ^ & | << >> and unary + - ~ !
sqrt(v) // Also all functions in cmath
x=v.sum() // Sum of all elements
v.shift(n) // Move all v[i] to v[i+n], shift in 0
v.cshift(n) // Move v[i] to v[(i+n) % v.size()]
v.resize(n) // Change size to n, but reset all elements to 0
v.resize(n, x) // Change size to n, set all elements to x

bitset Library

<bitset>


A bitset<N> is like a vector<bool> with fixed size N, but without iterators, and supporting logical operators like an N-bit int. Its elements have the values 0 or 1. It is implemented efficiently, with 8 elements per byte. bitset<N> b; // N-bit bitset, N must be a compile time constant
bitset<N> b=x; // Initialize b[0]..b[31] from bits of long x
b[i] // i'th bit, 0 <= i < N or throw out_of_range()
b.size() // N, cannot be changed
b.set(i) // b[i] = 1
b.reset(i) // b[i] = 0
b.flip(i) // b[i] = 1 - b[i]
b.test(i) // true if b[i] == 1
b.set() // Set all bits, also b.reset(), b.flip()
b & b2 // Bitwise AND, also | ^ ~ << >> &= |= ^= <<= >>= == !=
b.count() // Number of bits set to 1
b.any() // true if b.count() > 0
b.none() // true if b.count() == 0
cin >> b // Read bits as '0' and '1' e.g. "10101"
cout << b // Write bits as '0' and '1'
bitset<N> b(s); // Initialize from string s of '0' and '1' or throw invalid_argument()
s=b.template to_string<char>() // Convert to string
x=b.to_ulong() // Convert to unsigned long, throw overflow_error() if bits > 31 set

stack Library

<stack>


Items are popped from the top of a stack in the reverse order in which they were pushed. It does not provide any new functionality beyond a vector, deque, or list, and does not support iterators or indexing. stack<T> s; // Stack with elements of type T
s.size(), s.empty() // As with queue
s.push(x); // Put x on top
x=s.top(); // Last item pushed, may be assigned to
s.pop(); // Remove the top item

queue Library

<queue>


A queue is a container in which elements are inserted at the back and removed from the front. This could also be done with a deque or list, so no new capabilities are provided. A queue does not support iterators or indexing. queue<T> q; // Queue of type T
q.size() // Number of items in q
q.empty() // true if q.size() == 0
q.push(x) // Put x in the back
x=q.back() // The item last pushed, may be assigned to
x=q.front() // The next item to pop, may be assigned to
q.pop() // Remove the front item
A priority_queue is more useful. It sorts the items as they are pushed so that the largest is on top and removed first. priority_queue<T> q; // Element type is T
priority_queue<T, vector<T>, f> q; // Use functoid f(x,y) instead of x < y to sort
q.size(), q.empty() // As before
q.push(x) // Insert x
x=q.top() // Largest item in q, cannot be assigned to
q.pop() // Remove top item

set Library

<set>


A set<K> and multiset<K> are like a map and multimap, but without values. Iterators point to a K rather than a pair. There is no [] operator. set<K> m; // Elements are sorted by < on K
m.insert(k) // Add an element
m.erase(k) // Remove an element
m.find(k)!=m.end() // Test if k is in m

map Library

<map>


A map<K,V> m is a set of key-value pairs with unique, sorted keys of type K and values of type V. m[k] efficiently (O(log n) time) returns the value associated with k (as an lvalue), or creates a default value (0 if V is numeric) if k is used for the first time. A map iterator points to a pair<const K, V>, which has members first of type const K and second of type V. pair<K,V> x(k,v); // Create a pair x containing copies of k and v
x.first // k
x.second // v
x=make_pair(k,v) // x.first=k; x.second=v;

map<K,V> m; // map sorted by < on K
map<K,V,f>() // map sorted by f(x,y) instead of x<y on K
m[k]=v; // Associate v (type V) with unique key k of type K
m[k] // Retrieve v, or associate V() with k if new
m.size() // Number of unique keys
m.empty() // true if m.size() == 0
map<K,V>::iterator // bidirectional, points to a pair<const K, V>
map<K,V>::const_iterator // points to a pair<const K, const V>
m.begin() // Points to first pair (lowest k)
m.end() // Points 1 past last pair
m.find(k) // Points to pair containing k or m.end() if not found
m.erase(k) // Remove key K and its associated value
m.erase(b) // Remove pair pointed to by iterator b
m.erase(b, e) // Remove sequence [b, e)
m.clear() // Make empty: m.erase(m.begin(), m.end())
m==m2 // Compare maps, also !=, <, <=, >, >=
We use m.find(k) rather than m[k] when we wish to look up k without increasing the size of m if k is not found. // Read words, print an alphabetical index of words with their counts
string s;
map<string, int> m;
while (cin >> s)
++m[s];
for (map<string, int>::const_iterator p=m.begin(); p!=m.end(); ++p)
cout << p->first << " " << p->second << endl;
A multimap is a map that allows duplicate keys. It support all map operations except []. Elements are added by inserting a pair<K,V> and retrieved by m.equal_range(k) which returns a pair of iterators defining the sequence of pairs matching k. multimap<K,V,f> m; // f defaults to < on K
m.insert(make_pair(k,v)) // Insert a pair
pair<multimap<K,V,f>::iterator, multimap<K,V,f>::iterator> p
= m.equal_range(k) // Sequence with key k is [p->first, p->second)

f (when used as a template argument) is a functoid (or function object), a class that looks like a function by overloading (). For example:
template <class T> class GreaterThan {
public:
bool operator()(const T& a, const T& b) const {return b < a;}
};

map<string, int, GreaterThan<T> > m; // keys sorted in reverse order

list Library

<list>


A list is like a deque but optimized for insert and erase at any point at the cost of random access. It lacks [] (indexing), and its iterators are bidirectional, not supporting [], +, -, <, >, <=, or >=. list adds v.splice(d, v2, b); // Move *b from list v2 to in front of d in v
v.splice(d, v2); // Move all elements of list v2 to in front of d in v
v.splice(d, v2, b, e); // Move [b,e) in v2 to in front of d at v
v.remove(x); // Remove all elements equal to x
v.remove_if(f); // Remove elements x where f(x) is true
v.sort(); // Sort list
v.sort(f); // Sort list using function bool f(x,y) instead of x < y
v.merge(v2); // Merge sorted list v2 into sorted list v
v.merge(v2, f); // Merge using f(x,y) instead of x < y to sort v
v.unique(); // Remove duplicates from sorted list
v.unique(f); // Use f(x,y) instead of x == y
v.reverse(); // Reverse order of elements
Iterators can only be moved one element at a time using ++ or --, and compared using == or !=. char* cp="ABCDE";
list<char> v(cp, cp+5); // v.size() is 5
for (list<char>::const_iterator p=v.begin(); p!=v.end(); ++p) // Print ABCDE
cout << *p;

deque Library

<deque>


A deque (double ended queue) is just like a vector, but optimized for adding and removing elements at either end in O(1) time. It lacks reserve() and capacity() and adds v.push_front(x) // v.insert(v.begin(), x)
v.pop_front() // v.erase(v.begin())

Vector Library

<vector>


A vector<T> is like an array of T, but supports copying, assignment, and comparison. Its size can be set and changed at run time, and it can efficiently implement a stack (O(1) time to push or pop). It has random iterators like string, which behave like type T* (or const T* if the vector is const). If T is numeric, elements are initialized to 0. It is not possible to have an initialization list such as {1,2,3}. vector<T>() // Empty vector, elements of type T
vector<T>(n) // n elements, default initialized
vector<T>(n, x) // n elements each initialized to x
vector<T> v2=v; // Copy v to v2
v2=v; // Assignment
v2<v // Also >, ==, !=, <=, >= if defined for T
vector<T>(b, e) // Initialize to sequence [b, e)
v.size() // n
vector<T>::size_type // Type of v.size(), usually unsigned int
v.empty() // true if v.size() == 0
v[i] // i'th element, 0 <= i < v.size() (unchecked), may be assigned to
v.at(i) // v[i] with bounds check, throws out_of_range
v.begin(), v.end() // Iterators [b, e)
vector<T>::iterator // Iterator type, also const_iterator
v.back() // v[v.size()-1] (unchecked if empty)
v.push_back(x) // Increase size by 1, copy x to last element
v.pop_back() // Decrease size by 1 (unchecked if empty)
v.front() // v[0] (unchecked)
v.resize(n) // Change size to n >= 0 (unchecked)
v.insert(d, x) // Insert x in front of iterator d, shift, increase size by 1
v.insert(d, n, x) // Insert n copies of x in front of d
v.insert(d, b, e) // Insert copy of [b, e) in front of d
v.erase(d) // Remove *d, shift, decrease size by 1
v.erase(d, e) // Remove subsequence [d, e)
v.clear() // v.erase(v.begin(), v.end())
v.reserve(n) // Anticipate that v will grow to size n >= v.size()
v.capacity() // Reserved size
For insert and erase, d and e must point into v (and d <= e) or the program may crash. Elements from *d to the end are shifted and the size is changed as needed. Saved copies of iterators may become invalid after any change of size or capacity (not checked).
To implement push_back() efficiently, a vector typically doubles the reserved space when it runs out in order to minimize memory reallocation and copying. reserve() allows this strategy to be optimized.
// Read words from input into a stack, print in reverse order
string s;
vector<string> v;
while (cin >> s)
v.push_back(s);
while (!v.empty()) {
cout << v.back() << endl;
v.pop_back();
}

String Library

<string>



A string is like an array of char, but it also supports copying, assignment, and comparison, and its size may be set or changed at run time. '\0' has no special meaning. There is implicit conversion from char* to string in mixed type expressions. string() // Empty string
string(cp) // Convert char* cp to string
string(n, c) // string of n copies of char c
s=s2 // Assign char* or string s2 to string s
s1<s2 // Also ==, !=, >, <=, >=, either s1 or s2 may be char*
s.size() // Length of string s
string::size_type // Type of s.size(), usually unsigned int
s.empty() // True if s.size() == 0
s[i] // i'th char, 0 <= i < s.size() (unchecked), may be assigned to
s.at(i) // s[i] with bounds check, throws out_of_range
s1+s2 // Concatenate strings, either s1 or s2 may be char or char*
s+=s2 // Append string, char, or char* s2 to string s
s.c_str() // string s as a const char* with trailing '\0'
s.substr(i, j) // Substring of string s of length j starting at s[i]
s.substr(i) // Substring from s[i] to the end
s.find(s2) // Index of char, char*, or string s2 in s, or string::npos if not found
s.rfind(s2) // Index of last occurrence of s2 in s
s.find_first_of(s2) // Index of first char in s that occurs in s2
s.find_last_of(s2) // Index of last char in s that occurs in s2
s.find_first_not_of(s2) // Index of first char in s not found in s2
s.find_last_not_of(s2) // Index of last char in s not found in s2
s.replace(i, j, s2) // Replace s.substr(i, j) with s2
s.size() should be converted to int to avoid unsigned comparison. string s(3,'a'); // "aaa"
s += "b"+s; // "aaabaaa"
for (int i=0; i!=int(s.size()); ++i) { // print s one char at a time
cout << s[i];
s.size() > -1; // false! -1 is converted to unsigned
string supports standard container operations with regard to iterators. string iterators are random, supporting all the pointer operators of char*. The notation [b,e) means the sequence such that pointer or iterator b points to the first element and e points one past the last element. s.begin() // Iterator pointing to s[0]
s.end() // Iterator pointing 1 past last char
string::iterator // Iterator type, like char*
string::const_iterator // Type if s is const, like const char*
string(b, e) // string initialized from sequence [b,e)
s.erase(b) // Remove char in s pointed to by b
s.erase(b, e) // Remove substring [b,e) from s
s.replace(b, e, s2) // Replace substring [b,e) with string s2
Conversion from iterator to const_iterator is allowed, but not the other way. const_iterator should be used if the string is not going to be modified. char* cp="ABCDE";
string s(cp, cp+5); // "ABCDE"
string s2(s.begin()+1, s.end()-1); // "BCD"
for (string::const_iterator p=s.begin(); p!=s.end(); ++p) // Print s one char at a time
cout << *p; // or p[0]
As with arrays and pointers, indexing and iterator dereferencing are not checked at run time. Creating a string with a negative or very large size is also trouble. string s(-1, 'x'); // Crash, negative size
string s2(s.end(), s.begin()); // Crash, negative size
s[-1]='x'; // Crash, out of bounds
*s.end()='x'; // Crash, out of bounds
string::iterator p; *p='x'; // Crash, dereferencing uninitialized iterator

fstream Library

<fstream>
Defines types ifstream and ofstream representing input and output files respectively. ifstream is derived from istream, inheriting all its operations (such as >>). In addition, ifstream in(cp); // Open file named cp for reading
ifstream in(cp, ios::in | ios::binary); // Open in binary mode
bool(in); // true if open successful

cp is the file name. It must be a char*, not string (use s.c_str() to convert string s). Input is normally in text mode. In Windows, carriage returns ('\r') are discarded, and an ASCII 26 ('\032') signals end of file. In binary mode and in UNIX, no such translation occurs. The file is closed when the ifstream is destroyed.
{
ifstream f("input.dat", ios::in | ios::binary);
if (!f)
cerr << "File not found\n";
else {
int i=f.get(); // First byte or EOF if empty
}
} // f closed here

ofstream is derived from ostream, inheriting all its operations (such as <<). In addition,
ofstream os(cp); // Open file named cp for writing
ofstream os(cp, ios::out | ios::binary); // Open in binary mode

In text mode in Windows, writing '\n' actually writes "\r\n". The file named cp is overwritten if it exists, or created otherwise. The file is flushed and closed when the ofstream is destroyed.

iomanip library

<iomanip>

Defines manipulators for formatted output of numeric types. They have no effect on strings. setw() applies only to the next object printed, but the others remain in effect until changed.
out << setw(i); // Pad next output to i chars, then back to 0
out << setfill(c); // Pad with c (default ' ')
out << setprecision(i); // Use i significant digits for all float, double

cout << setw(6) << setprecision(3) << setfill('0') << 3.1; // print "003.10"

iostream Library

<iostream>

The header <iostream> defines global object cin of type istream, and global objects cout, cerr, clog of type ostream. cin represents standard input, normally the keyboard, unless redirected to a file or piped on the command line. cout represents standard output, which is normally the screen unless redirected or piped. Writing to cerr or clog both write to the screen even if output is redirected. The difference is that writing a newline ('\n') flushes any buffered output to cerr but not to cout or clog.

In the following, in is an istream (cin), out is an ostream (cout, cerr, clog), i is int, c is char, and cp is char*.
in >> x; // Read 1 word to numeric, string, or char* x, return in
in.get(); // Read 1 char (0-255) or EOF (-1) as an int
in.get(c); // Read 1 char into c, return in
in.unget(); // Put back last char read, return in
in.getline(cp, i); // Read up to i chars into char cp[i] or until '\n', return in
in.getline(cp, i, c); // Read to c instead of '\n', return in
getline(in, s); // Read up to '\n' into string s, return in
in.good(); // true if no error or EOF
bool(in); // in.good();
in.bad(); // true if unexpected char in formatted input
in.clear(); // Allow more input after bad, or throw an ios::failure
in.eof(); // true if end of file
in.fail(); // true if system input error

out << x; // Formatted output, redirected with >
out << endl; // Print '\n' and flush

Input with >> reads a contiguous sequence of non-whitespace characters. If x is numeric and the next word contains invalid characters (such as "1.5" or "foo" for an int), then the first offending character remains unread, in.bad() is set, and no further input can occur until in.clear() is called. Input into a char* array is not bounds checked. Input returns the istream to allow chaining, and has a conversion to bool to test for success. Output also returns the ostream to allow chaining.
// Read and print pairs of strings and ints until something goes wrong
// Input: hi 3 there 5 this is 1 test
// Output: hi 3
there 5

string s; int i;
while (cin >> s >> i)
cout << s << " " << i << endl;
cin.clear();

The get() methods read one character including whitespace. The various getline() functions read up through the next newline character and discard the newline. The methods good(), bad(), eof(), fail(), clear(), and implicit conversion to bool are available in ostream, just as in istream, but are seldom used.

Integrated Development Environment(IDE) For C++

You type a C++ program (typically referred to as source code) using the editor make any necessary correction and save the program on a secondary storage device, such as you hard disk.C++ source code filename often end with the .cpp,.cxx,.cc

Two editors widely used on  Linux systems are  vi and emacs.

For Windows Users
Dev Cplusplus
Microsoft Visual C++

Basic Of C++

Basics

C++ is a compiled language, an upward compatible superset of C and an (incompatible) predecessor to Java. C++ compiles C programs but adds object oriented (OO) features (classes, inheritance, polymorphism), templates (generic functions and classes), function and operator overloading, namespaces (packages), exception handling, a library of standard data structures (string, vector, map, etc.) and formatted text I/O (istream, ostream). Unlike Java, C++ lacks a standard graphical user interface (GUI), network interface, garbage collection, and threads, and allows non-OO programming and unchecked low-level machine operations with pointers. However, C++ executes faster than Java and requires no run-time support.

A C++ program is a collection of function, object, and type declarations. Every program must have a function int main() { ... } where the curly braces enclose a block, a sequence of declarations and statements ending in semicolons which are executed in order. A statement is an expression, block, or control statement that alters the order of execution, such as if, while, for, break, return. Some types (std::string), objects (std::cout), and functions are defined in header files, requiring the line #include <header> before use. Items defined in the standard headers are in the namespace std. The std:: prefix may be dropped after the statement using namespace std;. For instance,
// Comment: prints "Hello world!" and an OS-independent newline
#include <string> // Defines type std::string
#include <iostream> // Defines global object std::cout
using namespace std; // Allow std:: to be dropped
int main() { // Execution starts here
string s="Hello world!\n"; // Declares object s of type string
cout << s; // An expression as a statement, << is the output operator
return 0; // Execution ends here
}

The symbol // denotes a comment to the end of the line. You may also use /* ... */ for multiline comments. Spacing and indentation is used for readability. C++ is mostly free-form, except that the end of line is significant after # and //. C++ is case sensitive.

C++ source code files should be created with a text editor and have the extension .cpp. If the above is called hello.cpp, it may be compiled and run as follows in a UNIX shell window:
g++ hello.cpp -o hello -Wall -O
./hello
The -o option renames the executable file, by default a.out. -Wall turns on all warnings (recommended). -O optimizes (compiles slower but runs faster).
In Windows, the GNU C++ compiler is called DJGPP. To compile and run from an MS-DOS box:
gxx hello.cpp -o hello.exe
hello
The output file must have a .EXE extension (default is A.EXE). There is also a .OBJ file which you can delete.
To use the network or GUI interface in UNIX, you must use the X and socket libraries, which don't work in Windows. In Windows, you must use the Windows API and a compiler that supports them, such as from Microsoft, Borland, or Symantec. GUI/network programming is nonportable and outside the scope of this document.

Links to free and commercial C++ compilers can be found at cplusplus.com.

History of C++

The C++ programming language has a history going back to 1979, when Bjarne Stroustrup was doing work for his Ph.D. thesis. One of the languages Stroustrup had the opportunity to work with was a language called Simula, which as the name implies is a language primarily designed for simulations. The Simula 67 language - which was the variant that Stroustrup worked with - is regarded as the first language to support the object-oriented programming paradigm. Stroustrup found that this paradigm was very useful for software development, however the Simula language was far too slow for practical use.

Shortly thereafter, he began work on "C with Classes", which as the name implies was meant to be a superset of the C language. His goal was to add object-oriented programming into the C language, which was and still is a language well-respected for its portability without sacrificing speed or low-level functionality. His language included classes, basic inheritance, inlining, default function arguments, and strong type checking in addition to all the features of the C language.

The first C with Classes compiler was called Cfront, which was derived from a C compiler called CPre. It was a program designed to translate C with Classes code to ordinary C. A rather interesting point worth noting is that Cfront was written mostly in C with Classes, making it a self-hosting compiler (a compiler that can compile itself). Cfront would later be abandoned in 1993 after it became difficult to integrate new features into it, namely C++ exceptions. Nonetheless, Cfront made a huge impact on the implementations of future compilers and on the Unix operating system.

In 1983, the name of the language was changed from C with Classes to C++. The ++ operator in the C language is an operator for incrementing a variable, which gives some insight into how Stroustrup regarded the language. Many new features were added around this time, the most notable of which are virtual functions, function overloading, references with the & symbol, the const keyword, and single-line comments using two forward slashes (which is a feature taken from the language BCPL).

In 1985, Stroustrup's reference to the language entitled The C++ Programming Language was published. That same year, C++ was implemented as a comercial product. The language was not officially standardized yet, making the book a very important reference. The language was updated again in 1989 to include protected and static members, as well as inheritance from several classes.

In 1990, The Annotated C++ Reference Manual was released. The same year, Borland's Turbo C++ compiler would be released as a commercial product. Turbo C++ added a plethora of additional libraries which would have a considerable impact on C++'s development. Although Turbo C++'s last stable release was in 2006, the compiler is still widely used.

In 1998, the C++ standards committee published the first international standard for C++ ISO/IEC 14882:1998, which would be informally known as C++98. The Annotated C++ Reference Manual was said to be a large influence in the development of the standard. The Standard Template Library, which began its conceptual development in 1979, was also included. In 2003, the committee responded to multiple problems that were reported with their 1998 standard, and revised it accordingly. The changed language was dubbed C++03.

In 2005, the C++ standards committee released a technical report (dubbed TR1) detailing various features they were planning to add to the latest C++ standard. The new standard was informally dubbed C++0x as it was expected to be released sometime before the end of the first decade. Ironically, however, the new standard would not be released until mid-2011. Several technical reports were released up until then, and some compilers began adding experimental support for the new features.

In mid-2011, the new C++ standard (dubbed C++11) was finished. The Boost library project made a considerable impact on the new standard, and some of the new modules were derived directly from the corresponding Boost libraries. Some of the new features included regular expression support (details on regular expressions may be found here), a comprehensive randomization library, a new C++ time library, atomics support, a standard threading library (which up until 2011 both C and C++ were lacking), a new for loop syntax providing functionality similar to foreach loops in certain other languages, the auto keyword, new container classes, better support for unions and array-initialization lists, and variadic templates.