How to forward declare a class which is in a namespace

Question

I am trying to use forward declarations in header files to reduce #includes used and hence reduce dependencies where users include my header file.

However, I am unab

Answer 1

Apart to forward-declare the class from within its namespace (as @billz says), remember to either use (prepend) that namespace when referring to the forward-declared class, or add a using clause:

// B.h
namespace Y { class A; } // full declaration of
// class A elsewhere

namespace X {
    using Y::A;   // <------------- [!]
    class B {
        A* a; // Y::A
    };
}

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Ref: Namespaces and Forward Class Declarations