Compiler Error C2504

'class' : base class undefined

The base class is declared but never defined. Possible causes:

1. Missing include file.

2. External base class not declared with extern.

The following sample generates C2504:

// C2504.cpp
// compile with: /c
class A;
class B : public A {};   // C2504

class C {};
class D : public C {};   // OK