item-17-understand-special-member-function-generation

Item 17. Understand special member function generation

Test Codes

Notes

In official C++ parlance, the special member functions are the ones that C++ is willing to generate on its own.

C++98's rule

• C++98 has four such functions:
  • the default constructor
  • the destructor
  • the copy constructor
  • the copy assignment operator
• These functions are generated only if they're needed, i.e., if some code uses them without their being expressly declared in the class.
  • A default constructor is generated only if the class declares no constructor at all.
• Generated special member functions are implicitly public and inline, and they're nonvirtual unless the function in question is a destructor in a derived class inheriting from a base class with a virtual destructor.
• Rule of Three
  • if you declare any of a copy constructor, copy assignment operator, or destructor, you should declare all three.

In C++11

• As of C++11, the special member functions club has two more inductees:
  • the move constructor
  • the move assignment operator
• As is the case with the copy operations, the move operations aren't generated if you declare them yourself. However, the precise conditions under which they're generated differ a bit from those for copy operations.
  • The two copy operations are independent: declaring one doesn't prevent compilers from generating the other.
  • The two move operations are NOT independent: if you declare either, that prevents compilers from generating the other.
  • Futhermore, move operations won't be generated for any class that explicitly declares a copy operation.
  • Declaring a move operation(construction or assignment) in a class causes compilers to disable the copy operations.
  • The Rule of Three motivates the fact that C++11 does not generate move operations for a class with a user-declared destructor.
• So move operations are generated for classes(when needed) only if these things are true:
  • No copy operations are declared in the class.
  • No move operations are declared in the class.
  • No destructor is declared in the class.
• At some point, analogous rules may be extended to the copy operations, because C++11 deprecates the automatic generation of copy operations for classes declaring copy operations or a destructor.
• C++11 allows =default to explicitly have default behavior. This approach is often useful in polymorphic base classes.

class Base {
public:
    virtual ~Base() = default;  // let compiler generate it but virtual

    Base(Base&&) = default;
    Base& operator=(Base&&) = default;

    Base(const Base&) = default;
    Base& operator=(const Base&) = default;
}

• C++11 rules summary
  • Default constructor
    • same rules as C++98.
    • Generate only if the class contains no user-declared constructors.
  • Destructor
    • Essentially same rules as C++98;
    • sole difference is that destructors are noexcept by default(see Item 14).
    • As in C++98, virtual only if a base class destructor is virtual.
  • Copy constructor
    • Same runtime behavior as C++98: memberwise copy construction of non-static data members.
    • Generated only if the class lacks a user-declared copy construtor.
    • Deleted if the class declares a move operation.
    • Generation of this function in a class with a user-declared copy assignment operator or destructor is deprecated.
  • Copy assignment operator
    • Same runtime behavior as C++98: memberwise copy assignment of non-static data members.
    • Generated only if the class lacks a user-declared copy assignment operator.
    • Deleted if the class declares a move operation.
    • Generation of this function in a class with a user-declared copy constructor or destructor is deprecated.
  • Move constructor and move assignment operator
    • Each performs memberwise moving of non-static data members.
    • Generated only if the class contains no user-declared copy operations, move operations, or destructor.
• Note that there's nothing in the rules about the existence of a member function template preventing compilers from generating the special member functionss.
  • For example, compilers will still generate copy and move operations for Widget, even though these templates could be instantiated to produce the signatures for the copy constructor and copy assignment operator.(That would be the case when T is Widget.)

class Widget {
public:
    // compilers will still generate copy and move operations in this case, 
    // even though these templates could be instantiated to produce 
    //    the signatures for the copy constructor and copy assignment operators. 
    // (that would be the case when T is Widget.)

    template<typename T>
    Widget(const T&);

    template<typename T>
    Widget& operator=(const T&);
}

References

• (Chinese) Understanding C++11 - default_delete_control
• Rule of three (C++ programming)
• Special member functions