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Home » Tip Bank » C++
Language: C++
Expertise: Beginner
Jun 15, 1999

The Order of Destructors' Call

An object's destructor invokes the destructors of its direct base classes and member objects. The invocation occurs in the reverse order of the construction of the subobjects. Note, however, that all destructors are called with their qualified name, ignoring any possible virtual overriding destructors in derived classes. For example:

 
  struct A  {
    virtual ~A() { cout << "destroying A" << endl; }
  };

  struct B: public A  {
    ~B() { cout << "destroying B" << endl;} 
  };

  int main()
  {
    B b;
  };

This program displays:

 
  destroying B
  destroying A 

This is because the compiler inserts additional code into the user-defined destructor of class B. The augmented destructor looks like this:

 
  B::~B()   {
      //user-written code
    cout<<"destroying B"<<endl; 
      //pseudo C++ code inserted by the compiler comes after the user-written code:
    this->A::~A();   // (1) destructor called with its qualified name
  }

In other words, although the destructor of class A is virtual, the qualified call inserted into the destructor of B is resolved statically (a function call with a qualified name bypasses the virtual mechanism). Therefore, A's destructor is called rather than B's when the statement marked (1) is executed.

Danny Kalev
 
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