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 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).
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> class Explicit { public: Explicit { std::cout << "Default constructor " << message_ << '\n'; } explicit Explicit(std::string message) : message_(std::move(message)) { std::cout << "Non-default constructor " << message_ << '\n'; } Explicit(const Explicit& other) { std::cout << "Copy constructor " << message_ << '\n'; *this = other; // invoke copy assignment operator } Explicit& operator=(const Explicit& 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: friend class Implicit; std::string message_; }; class Implicit : public Explicit { public: void Spew { std::cout << "Implicit(" << message_ << ", " << member_.message_ << ")\n"; } private: Explicit member_; }; 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 noexcept; |
C++03
In C++03 before the introduction of move semantics the special member functions[4] were:
- Default constructor (if no other constructor is explicitly declared)
- Copy constructor
- Copy assignment operator
- Destructor
References
- ^ ISO/IEC (2011). ISO/IEC 14882:2011 (3 ed.). ISO/IEC. pp. §12.
- ^ http://accu.org/index.php/journals/1896
- ^ Except for the destructor if a base class already has a virtual destructor.
- ^ ISO/IEC (1998). International Standard ISO/IEC 14882: Programming languages—C++ = Languages de programmation—C++ (1 ed.). ISO/IEC. pp. §12. OCLC 71718919.