Special member functions

Special member functions 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
• Copy assignment operator
• 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 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.

<source lang="cpp"> class Explicit {

   friend class Implicit;
   string msg;
   public:
    Explicit(void) : msg("")
    {
        cout << "Default constructor " << msg << endl;
    }
    Explicit(const string& value) : msg(value)
    {
        cout << "Non-default constructor " << msg << endl;
    }
    Explicit(const Explicit& other) : msg(other.msg)
    {
        cout << "Copy constructor " << msg << endl;
    }
    Explicit& operator=(const Explicit& other)
    {
        cout << "Copy assignment operator " << msg << endl;
        if (this != &other)
        {
            msg = other.msg;
        }
        return *this;
    }
    ~Explicit(void)
    {
        cout << "Destructor " << msg << endl;
    }

};

class Implicit : public Explicit {

   int i;
   void* p;
   Explicit member;
   public:
    void Spew(void) { cout << "Implicit(" << msg << ", " << member.msg << ")" << endl; };

}; </source>

In this case the class Implicit has not explicitly defined the destructor and the compiler will create a destructor equivalently to this: <source lang="cpp"> // Sub-objects are destroyed in the opposite order to their construction Implicit::~Implicit() {

   member.~Explicit(); // destroy member
   (void)p;           // do nothing for p, void* has no destructor
   (void)i;           // do nothing for i, int has no destructor
   ~Explicit();        // call the base class's destructor

} </source>