Question

class A
{
 class B;
 B::data myData; //Error: incomplete type not allowed.

    class B
    {
    public:
        struct data
        {
        int number;
        };
    };
};

In the code above, how could I declare a member variable of type data in class A?

Answer 1

B must be defined before you use it in the declaration of A::myData:

class A
{
    class B
    {
    public:
        struct data
        {
            int number;
        };
    };

     B::data myData;
};