Introduction To Constructors And Destructors
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There are several special C++ 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 cannot take their addresses:
int main (void)
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void *ptr = base::base; // illegal
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}
- Constructors and destructors can be generated by the compiler if they have not 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;
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p->X::~X(); // legal call of destructor
X::X(); // illegal call of constructor
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}
- 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.
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.