CODEWITHSUNDEEP
c++polymorphism
triple fault
triple fault

Reputation: 14138

Deleting polymorphic objects and memory leaks

Suppose I have a class A and a class B which inherits from A. Then I do something like: 

A* a = new B();
delete a;

Why a memory leak happens only when there is a dynamic memory allocation within B? How C++ knows to remove the "B part" when there is dynamic memory allocations within B but fails where it is?

[Update:]

How does the following code does not result in a memory leak: [I suspect it to be undefined behaviour, but I don't really understand anything :(]

#include <iostream>
using std::cout;
using std::endl;


class A {
public:
    virtual void f() {
        cout << "1" << endl;
    }
    ~A() {
        cout<< "A A'tor" << endl;
    }
};

class B : public A {
private:

public:
    B(){}
    ~B(){
        cout<< "B D'tor" << endl;
    }
    void f() {
        cout << "2" << endl;
    }

};

int main() {
    A* a = new B();
    //a->f();
    delete a;
    return 0;
}

Upvotes: 1

Views: 916

Answers (2)

d3coy
d3coy

Reputation: 395

This is because the destructor isn't virtual. If there is ever a chance in your program to delete derived class objects via base class pointers, you should define a virtual destructor in the base class. This will allow B's destructor to get called first, allowing you to free any dynamic memory B may have allocated. A's destructor will get called after B's.

Another good rule of thumb is to train yourself to think about making your destructor virtual anytime you make a method virtual. Since the motivation to make a method virtual means you will be calling methods on base class pointers, it follows that you will likely be deleting objects through base class pointers.

Upvotes: 0

user966379
user966379

Reputation: 2993

While compiler sees statement "delete a;"
It knows only static type of "a" is pointer to A, if there is no virtual destructor in class A. As a result wrong destructor gets called leading to memory leak

If there is a virtual destructor in class A then compiler comes to know that Dynamic type of "a" is pointer to B and there will a vtable from where it will get B's destructor address at run time

Upvotes: 1

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