c++ inherit class template with this class as the type

Question

I want my class template A to be able to have a member, that is only defined in B, while B inherits A.

Here is the code that would theoretically work, but doesn't compile.

template< typename T >
class A
{
public:
    typename T::member mSomething;
};

class B : public A< B >
{
public:
    struct member
    {
        int val;
    };
};

int main( int argc, char** argv )
{
    B* b = new B;
    A<B>* a = b;

    b->mSomething.val = 5;

    if( a->mSomething.val == 5 )
    {
        std::cout << "doing good";
    }

    return 0;
}

This code does not compile on MSVCP 2010 with the error" 'member' : is not a member of 'B' "

How could I get around this?

Answer 1

CRTP base classes can't access types defined in the derived class. When A<B> is instantiated, B is an incomplete type and has no members, so you can't access the contents.

You must define the type outside B .

Answer 2

For anyone wondering, this is the solution that I ended up using:

// Base class in file 1
template< typename T, typename TT >
struct A
{
    TT foo;
};

// derived in file 2, one class for the member outside the definition
// and one main class.
struct C
{
    int val;
};

struct B : public A< B, C >
{

};

// Usage from wherever you please.
int main( int argc, char** argv )
{
    B* b = new B;
    A<B, C>* a = b;

    b->foo.val = 5;

    if( a->foo.val == 5 )
    {
        std::cout << "doing good";
    }

    return 0;
}