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Introduction To Constructors And Destructors

There are several special member functions that determine how the objects of a class are created, initialized, copied, and destroyed. Constructors and destructors are the most important of these. They have many of the characteristics of normal member functions—you declare and define them within the class, or declare them within the class and define them outside—but they have some unique features:

- They do not have return value declarations (not even void).
- They cannot be inherited, though a derived class can call the base class’s constructors and destructors.
- Constructors, like most C++ functions, can have default arguments or use member initialization lists.
- Destructors can be virtual, but constructors cannot. (See Virtual destructors.)
- You can’t take their addresses.

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    int main (void)
    {
    .
    .
    .
    void *ptr = base::base;    // illegal
    .
    .
    .
    }

- Constructors and destructors can be generated by the compiler if they haven’t been explicitly defined; they are also invoked on many occasions without explicit calls in your program. Any constructor or destructor generated by the compiler will be public.
- You cannot call constructors the way you call a normal function. Destructors can be called if you use their fully qualified name.

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    {
    .
    .
    .
    X *p;
    .
    .
    .
    p–>X::~X();                // legal call of destructor
    X::X();                    // illegal call of constructor
    .
    .
    .
    }

- The compiler automatically calls constructors and destructors when defining and destroying objects.
- Constructors and destructors can make implicit calls to operator new and operator delete if allocation is required for an object.
- An object with a constructor or destructor cannot be used as a member of a union.
- If no constructor has been defined for some class X to accept a given type, no attempt is made to find other constructors or conversion functions to convert the assigned value into a type acceptable to a constructor for class X. Note that this rule applies only to any constructor with one parameter and no initializers that use the “=” syntax.

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    class X { /* ... */ X(int); };
    class Y { /* ... */ Y(X); };
    Y a = 1;                   // illegal: Y(X(1)) not tried

If class X has one or more constructors, one of them is invoked each time you define an object x of class X. The constructor creates x and initializes it. Destructors reverse the process by destroying the class objects created by constructors.

Constructors are also invoked when local or temporary objects of a class are created; destructors are invoked when these objects go out of scope.

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