Special member functions

In the C++ programming language, special member functions ^[1] are functions which the compiler will automatically generate if they are used, but not declared 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 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 ^[3] and the copy constructor and assignment operators will receive const& parameters (and not be of the alternative legal forms). ^[4]

Example

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>#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_;};

Signatures

Here are the signatures of the special member functions:

Function	syntax for class MyClass
Default constructor	MyClass();
Copy constructor	MyClass(const MyClass& other);
Move constructor	MyClass(MyClass&& other) noexcept;
Copy assignment operator	MyClass& operator=(const MyClass& other);
Move assignment operator	MyClass& operator=(MyClass&& other) noexcept;
Destructor	virtual ~MyClass();

C++03

In C++03 before the introduction of move 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

References

1. ISO/IEC (2011). ISO/IEC 14882:2011 (3 ed.). ISO/IEC. pp. §12.
2. "Enforcing the Rule of Zero".
3. Except for the destructor if a base class already has a virtual destructor.
4. Similarly, the move constructor/assignment operators will receive && parameters instead of the alternatives.
5. ISO/IEC (1998). International Standard ISO/IEC 14882: Programming languages—C++ = Languages de programmation—C++ (1 ed.). ISO/IEC. pp. §12. OCLC   71718919.