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 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.