Special member functions

Special member functions

Special member functions [cite book
last = ISO/IEC
authorlink = International Organization for Standardization
title = International Standard ISO/IEC 14882: Programming languages—C++ = Languages de programmation—C++
edition = 1
publisher = ISO/IEC
date = 1998
oclc = 71718919
pages = §12
] in C++ are functions which the compiler will automatically generate if they are used, but not declared explicitly by the programmer.The special member functions are:
* Default constructor (if no other constructor is explicitly declared)
* Copy constructor
* Copy assignment operator
* Destructor

In these cases the compiler generated versions of these functions perform a "memberwise" operation. For example the compiler generated destructor will destroy each sub-object (base class or member) of the object.


The following example depicts two classes: Explicit for which all special member functions are explicitly declared and Implicit for which none are declared.

class Explicit { friend class Implicit; string msg; public: Explicit(void) : msg("") { cout << "Default constructor " << msg << endl; } Explicit(const string& value) : msg(value) { cout << "Non-default constructor " << msg << endl; } Explicit(const Explicit& other) : msg(other.msg) { cout << "Copy constructor " << msg << endl; } Explicit& operator=(const Explicit& other) { cout << "Copy assignment operator " << msg << endl; if (this != &other) { msg = other.msg; } return *this; } ~Explicit(void) { cout << "Destructor " << msg << endl; ;

class Implicit : public Explicit { int i; void* p; Explicit member; public: void Spew(void) { cout << "Implicit(" << msg << ", " << member.msg << ")" << endl; };};

In this case the class Implicit has not explicitly defined the destructor and the compiler will create a destructor equivalently to this:// Sub-objects are destroyed in the opposite order to their constructionImplicit::~Implicit(){ member.~Explicit(); // destroy member (void)p; // do nothing for p, void* has no destructor (void)i; // do nothing for i, int has no destructor ~Explicit(); // call the base class's destructor



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