Re: Simple class inheritance question

From:

legalize+jeeves@mail.xmission.com (Richard)

Newsgroups:

comp.lang.c++

Date:

Tue, 5 Apr 2011 06:04:10 +0000 (UTC)

Message-ID:

<inebcq$k4j$1@news.xmission.com>

[Please do not mail me a copy of your followup]

Leigh Johnston <leigh@i42.co.uk> spake the secret code
<gZmdnWyqqdT10gfQnZ2dnUVZ7sWdnZ2d@giganews.com> thusly:

On 05/04/2011 00:05, Richard wrote:

[Please do not mail me a copy of your followup]

Leigh Johnston<leigh@i42.co.uk> spake the secret code
<EPCdnY9S9YGa2QfQnZ2dnUVZ8tadnZ2d@giganews.com> thusly:

On 04/04/2011 23:11, crea wrote:

I know its simple, but I cannot find the answer from books because its a
special case.
Here:
///////////////////////
class A
{
public:
A* a;
};

class B : public A
{
public:
int var;
};

A a1, a2;

a1.a =&a2;
((B*)a1.a)->var = 77;
////////////////
I compiled and it works.me

Just because it compiles and seems to work does not mean that it is
correct; it is incorrect (a bug) as it invokes UB.

How/why can a1.a set B classes member variable's value even though there is
not even a one object created from B? How is it possible.

It is not possible; what you have written is buggy code.

So all objects are from class A and there is no object created from B and we
are still using B's members. I thought that this var-variable is not even
allocated from the memory because there is no B object created??

You are illegally overwriting stack space which doesn't seem to manifest
any ill effects at the moment but may in the future.

IIRC, the error would manifest itself as a runtime crash if you had
used dynamic_cast<B *>(a1.a) instead of a C-style cast. The C-style
cast has no safeguards and basically says "just jam it in there, I
know what I'm doing (even when I don't)".

Not quite; a dynamic_cast downcast requires a polymorphic type.

By that I infer you mean that A must have at least one virtual method?

Yes, that seems reasonable for dynamic_cast<> to work properly.

All this "just jam it in there" behavior with C-style casts is why I
consider them a code smell in C++ code.
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