Is this construction validate in C++?

[mscava]

class A
{
public:
...
protected:
...
private:
class B : public A { ... };
};

Because compiler gives me errors like:
- invalid use of undefined type `class A'
- forward declaration of `class A'

Thanks for replies

[Matt Pearson]

ms****@gmail.com wrote:

class A
{
public:
...
protected:
...
private:
class B : public A { ... };
};

Because compiler gives me errors like:
- invalid use of undefined type `class A'
- forward declaration of `class A'

Thanks for replies

To inherit, you must have a complete type. A is not complete until the
end of its declaration block. A forward declaration should work:

class A {
.....
class B;
};

class A::B : public A {};

[Victor Bazarov]

ms****@gmail.com wrote:

class A
{
public:
...
protected:
...
private:
class B : public A { ... };
};

Because compiler gives me errors like:
- invalid use of undefined type `class A'
- forward declaration of `class A'

Thanks for replies

No, you can only derive from a complete type. At the point
where you define 'B', 'A' has not been completely defined
yet.

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[mscava]

Thank you very much Matt...

[kwikius]

ms****@gmail.com wrote:

class A
{
public:
...
protected:
...
private:
class B : public A { ... };
};

OTOH you can do:

template <typename T = void>
class A
{
public:
int value;
class B : public A<B{ public: int other_value;};
};
int main()
{
A<x;
x.value = 1;
A<>::B y;
y.value = 2;
y.other_value = 3;
}

HTH

regards
Andy Little