Special member functions explained

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: for which all special member functions are explicitly declared and for which none are declared.

1. include
2. include
3. include

class Explicit ;

class Implicit : public Explicit ;

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

Notes and References

1. Book: ISO/IEC . International Organization for Standardization . ISO/IEC 14882:2011 . 3 . ISO/IEC . 2011 . §12 .
2. Web site: 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. Book: ISO/IEC . International Organization for Standardization . International Standard ISO/IEC 14882: Programming languages—C++ = Languages de programmation—C++ . 1 . ISO/IEC . 1998 . 71718919 . §12 .