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Special member functions[1] 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:
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 compiler generated functions will be public
, non-virtual[2] and the copy constructor and assignment operators will receive const&
parameters (and not be of the alternative legal forms).
The following example depicts two classes: Explicit for which all special member functions are explicitly declared and Implicit for which none are declared.
#include <iostream> class Explicit { friend class Implicit; string msg; public: Explicit() : msg("") { std::cout << "Default constructor " << msg << '\n'; } Explicit(const string& value) : msg(value) { std::cout << "Non-default constructor " << msg << '\n'; } Explicit(const Explicit& other) : msg(other.msg) { std::cout << "Copy constructor " << msg << '\n'; } Explicit& operator=(const Explicit& other) { std::cout << "Copy assignment operator " << msg << '\n'; if (this != &other) { msg = other.msg; } return *this; } ~Explicit() { std::cout << "Destructor " << msg << '\n'; } }; class Implicit : public Explicit { int i; void* p; Explicit member; public: void Spew() { std::cout << "Implicit(" << msg << ", " << member.msg << ")\n"; } };
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 construction Implicit::~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 }
In C++98 before the introduction of move semantics the special member functions[3] were: