In theC++programming language,special member functions^[1] arefunctions which thecompiler will automatically generate if they are used, but notdeclared explicitly by the programmer.The automatically generated special member functions are:

• Default constructor if no other constructor is explicitly declared.
• Copy constructor if no move constructor and move assignment operator are explicitly declared.

If a destructor is declared generation of a copy constructor is deprecated (C++11, proposal N3242^[2]).

• Move constructor if no copy constructor, copy assignment operator, move assignment operator and destructor are explicitly declared.
• Copy assignment operator if no move constructor and move assignment operator are explicitly declared.

If a destructor is declared, generation of a copy assignment operator is deprecated.

• Move assignment operator if no copy constructor, copy assignment operator, move constructor and destructor are explicitly declared.
• Destructor

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

The compiler generated functions will bepublic, non-virtual^[3] and the copy constructor and assignment operators will receiveconst& parameters (and not be of thealternative legal forms).^[4]

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

#include<iostream>#include<string>#include<utility>classExplicit{public:Explicit(){std::cout<<"Default constructor "<<message_<<'\n';}explicitExplicit(std::stringmessage):message_(std::move(message)){std::cout<<"Non-default constructor "<<message_<<'\n';}Explicit(constExplicit&other){std::cout<<"Copy constructor "<<message_<<'\n';*this=other;// invoke copy assignment operator}Explicit&operator=(constExplicit&other){std::cout<<"Copy assignment operator "<<message_<<'\n';if(this!=&other){message_=other.message_;}return*this;}Explicit(Explicit&&other)noexcept{std::cout<<"Move constructor "<<message_<<'\n';*this=std::move(other);// invoke move assignment operator}Explicit&operator=(Explicit&&other)noexcept{std::cout<<"Move assignment operator "<<message_<<'\n';if(this!=&other){message_=std::move(other.message_);}return*this;}~Explicit(){std::cout<<"Destructor "<<message_<<'\n';}private:friendclassImplicit;std::stringmessage_;};classImplicit:publicExplicit{public:voidSpew(){std::cout<<"Implicit("<<message_<<", "<<member_.message_<<")\n";}private:Explicitmember_;};

Here are the signatures of the special member functions:

In C++03 before the introduction ofmove semantics (in C++11) the special member functions^[5] were:

• Default constructor (if no other constructor is explicitly declared)
• Copy constructor
• Copy assignment operator
• Destructor

• C++

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