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.

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; };
};

In this case the class Implicit has not explicitly defined the destructor and the compiler will create a destructor equivalently to this:

// 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

}