virtual functions

[innovaltec]

gurus,
i am trying to understand virtual functions.
Why does the line "A s = B()" in the code below not
compile (error C2259: 'A' : can not instiante abstract class)
but the line A* t = new B() works perfectly ok.
I would prefer not to use pointers as memory leaks
tend to occur.
I am using visual studio 2005.

class A
{
public:
virtual void func() = 0;
};

class B : public A
{
public:
virtual void func();
};

void B::func()
{

}

int main()
{
A* t = new B(); // compiles perfectly ok
A s = B(); // error C2259
return 1;
}

thank you

[TheRogue]

Its because you are trying to instantiate an object of type A.
A is an abstract class, so cannot be instantiated.

[GCDEF]

int main()
{
A* t = new B(); // compiles perfectly ok
A s = B(); // error C2259
return 1;
}

thank you

Syntactically that line doesn't even makes sense. Aside from the fact that you can't instantiate an abstract class, what do you think that line should do?

[monarch_dodra]

Syntactically that line doesn't even makes sense. Aside from the fact that you can't instantiate an abstract class, what do you think that line should do?

Create a temporary B, construct an A from that B (if A has a copy constructor). Since B is a A, the temporary B is a valid argument to a const A&. Finally, once the A is built, the temporary B is destroyed.

I'll grant you it's a stupid way of doing it, but it doesn't look like invalid C++ to me. Then again, you've proven me wrong more than once, so I'd be happy if you tought me something new.

[GCDEF]

Create a temporary B, construct an A from that B (if A has a copy constructor). Since B is a A, the temporary B is a valid argument to a const A&. Finally, once the A is built, the temporary B is destroyed.

I'll grant you it's a stupid way of doing it, but it doesn't look like invalid C++ to me. Then again, you've proven me wrong more than once, so I'd be happy if you tought me something new.

I was talking about this line.

A s = B(); // error C2259

It's invalid because A is an abstract class.

[monarch_dodra]

I was talking about this line.

A s = B(); // error C2259

It's invalid because A is an abstract class.

I guess I read too much into your "Aside from"...