Re: Polymorphism without virtual in C++

Victor Bazarov <>
Wed, 06 Aug 2008 13:17:24 -0400
feel wrote:

I am sure it's an old question. But I just find a interesting design
about this: Polymorphism without virtual function in a C++ class.

My question is, "why?"

My solution is for some special case, trust me, very special.
1 single root class tree
2 the leaf(lowest level) classes are sealed which means we should not
inherite class from them.
3 PImpl idiom. There is only one data mumber in root class and there
is no any other data mumber in child class and virtual funtions.

In my solution, the destructor of root class is not virtual, but we
can use base class pointer to point derived class object.

My question is: is this design follow the C++ standard? I tested it in
VS2005. it's ok. How about GCC?I remember that this non-virtual
destructor behavor is undefine in C++ standard.

Correct. It's undefined behaviour to delete the derived class object
through the base class pointer in the absence of a virtual destructor.

Here is a simple code example:

class base
        delete[] p;

Drop the semicolons after all the function bodies. They are superfluous
(although not an error).

    int *p;

    base():p(new int[10])

    base(int *pp) : p(pp)

Need a comment here that 'base' takes ownership of the pointer passed to
it. Also, probably want to declare this c-tor "explicit".


class base1 : public base

class my : public base1
    my ()
        p = new int[10];

Memory leak here.


int _tmain(int argc, _TCHAR* argv[])

There is no standard function '_tmain'. There is no standard type
'_TCHAR'. You must have forgotten to include the proper header[s].
Don't check your code with VC++ without turning off extensions.


    base1 *o = new my;

    delete o;

Ka-boom! Undefined behaviour.

     return 0;

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