Special member functions
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:
- Default constructor (if no other constructor is explicitly declared)
- Copy constructor if no move constructor or move assignment operator is explicitly declared.
- If a destructor is declared generation of a copy constructor is deprecated.
- Move constructor if no copy constructor, move assignment operator or destructor is explicitly declared.
- Copy assignment operator if no move constructor or move assignment operator is 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 or destructor is 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[2] and the copy constructor and assignment operators will receive const&
parameters (and not be of the alternative legal forms).
Example
The following example depicts two classes: Explicit for which all C++98 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"; } };
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 | ~MyClass(); |
C++98
In C++98 before the introduction of move semantics the special member functions[3] were:
- Default constructor (if no other constructor is explicitly declared)
- Copy constructor
- Copy assignment operator
- Destructor