problem_19.md

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Problem 19 - I'm leaking!

2017-10-26

class X {
        int *a;
    public:
        X(int n): a{new int[n]} {}
        ~X() { delete[] a; }
};

class Y: public X {
        int *b;
    public:
        Y(int n, int m): X{n}, b{new int[m]} {}
        ~Y() { delete[] b; }    // Note: Y's dtor will call X's dtor (step 3)
};

X *px = new Y{3, 4};
delete px;  // Leaks

This calls X's destructor, but not Y's

Solution: make the destructor virtual

class X {
        ...
    public:
        ...
        virtual ~X() { delete[] a; }   
};

Now there is no more leak.

Always make the destructor virtual in classes that are meant to be superclasses, even if the destructor does nothing.

• You never know what the subclass' destructor might do, so you need to make sure its destructor gets called

If a class is not meant to be a superclass, then no need to incur the cost of virtual methods needlessly/

• Leave the destructor non-virtual

class X final { // Cannot be subclassed
    ...
};

Like override, final is another contextual keyword (right before the brace).

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