Question

I have a class, say A

template <typename T> class A
{
} ;

and a class derived from A<T>, (retaining type genericity)

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

A situation has arisen where i need to instantiate a B<T> inside a method declared in A<T>. Uh oh.

template <typename T> class A
{
    void go()
    {
        B<T> * newB = new B<T>() ; // oh boy, not working..
    }
} ;

What should I do and how do I work around this?

Answer 1

You need to break the cyclic dependency between the two classes. Trivial in this case: just define your go() function out of line:

template <typename T> class A
{
public:
    void go();
} ;

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

template <typename T>
void A<T>::go()
{
    B<T> * newB = new B<T>() ;
}

I prefer out of line definitions, even when inlining functions, anyway because it avoids cluttering the interface with unnecessary detail. I also prefer to not have cyclic dependencies (certainly not between base and derived) but it can’t always be avoided.