C++ RAII and polymorphism compatibility

Question

There is no finally block in C++ because of RAII. Now if I have a pointer object and exception happens in one of the method, how this object will be deleted?. Here is a sample code I have written.

class A
{
public:
    A()
    {
        cout<<"I am inside A\n";
    }

    virtual void mymethod()
    {
        throw 0;
    }

    virtual ~A()
    {
        cout<<"A destroyed\n";
    }
};

class B : public A
{
public :
    //A a;

    B()
    {
        cout<<"I am inside B \n";
    }

    virtual void mymethod()
    {
        throw 0;
    }

    ~B()
    {
        cout<<"B destroyed\n";
    }
};

int _tmain(int argc, _TCHAR* argv[])
{
    try
    {
        A *b = new B();

        b->mymethod();

        delete b;
    }

    catch (int i)
    {
        cout<<"exception";
    }
    return 0;
}

Now in this how to delete the pointer object (b).

What about `unique_ptr`? Also `A` destructor should be virtual. — sbabbi, Mar 14 '16 at 11:27
There's no need to write "new B()" when you can write = std::make_unique(); and let it clean itself up (no need for delete b then either). — Robinson, Mar 14 '16 at 11:31
Well in this example there is no need for `new` either. Just `B b;` — sbabbi, Mar 14 '16 at 11:32

score 3 · Answer 1 · answered Mar 14 '16 at 11:36

3

It won't. You're not using RAII. Use a smart pointer.

answered Mar 14 '16 at 11:36

Lightness Races in Orbit

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alain · Answer 2 · 2016-03-14T11:41:44.480

The point of RAII is to use variables with automatic storage duration to achieve exception-safety. You could use a smart pointer if you need an object of dynamic storage duration, or you could simply create an object with automatic storage duration to begin with:

try
{
    B bobj;
    A *b = &bobj;

    b->mymethod();

}

catch (int i)
{
    cout<<"exception";
}

When the stack is unwound because of an exception, all variables with automatic storage duration are properly destroyed, so the smart pointer can deallocate the object in its destructor, and in this example here the object bobj would also be destroyed as it should.

score 0 · Accepted Answer · edited May 23 '17 at 11:45

0

First of, all polymorphic types should have virtual destructors.
Second, when you use owning raw pointers, you basically disable RAII.
The solution is to use a smart-pointer:

If you want a unique asset, use unique_ptr. This wont be copyable.
If you want a shared asset, use shared_ptr. This will do shallow copy.
If you want value-semantics, use clone_ptr. This will do deep copy.
It's from the next standard, but you can find it here

edited May 23 '17 at 11:45

Community

1
1

answered Mar 14 '16 at 11:34

sp2danny

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That proposal looks very preliminary. – Yakk - Adam Nevraumont Mar 14 '16 at 12:42

C++ RAII and polymorphism compatibility

3 Answers3