Re: struct inheritance

From:

James Kanze <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Sat, 14 Feb 2009 02:19:03 -0800 (PST)

Message-ID:

<e0f8949f-7a70-417b-866d-e1c28dc27351@m40g2000yqh.googlegroups.com>

On Feb 14, 4:03 am, Kai-Uwe Bux <jkherci...@gmx.net> wrote:

Neill wrote:

Can somebody explain why prog doesn't point to struct NumExp
please?

Huh? How did you tell?

struct Exp_ {};
typedef Exp_* Exp;
struct NumExp : public Exp_
{
    int num;
    NumExp(int n)
    {
        num = n;
    }
};
int main()
{
    Exp prog = new NumExp(10);
    return 0;
}

What did this program do (in terms of observable behavior)
that you did not expect?

If you run this program in a debugger and the debugger does
not show the correct dynamic type of *prog, then there could
be a problem with your debugger.

Given a pointer to an Exp_, I don't know how the debugger can
possibly know that the object in question is actually a NumExp.
The types aren't polymorphic (which probably means that the
conversion of the pointer was an error).

If the intent is to use the types polymorphically, Exp_ must
have at least one virtual function. If the intent is to new
objects of the derived type dynamically, and assign the
resulting pointer to a pointer to Exp_, Exp_ almost certainly
needs a virtual destructor.

--
James Kanze (GABI Software) email:james.kanze@gmail.com
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