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]

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]

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

 

[mscava]

Thank you very much Matt...

 

[kwikius]

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