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Compiler Error C2990

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'class' : non-class type as already been declared as a class type

The non generic or template class redefines a generic or template class. Check header files for conflicts.

The following sample generates C2990:

// C2990.cpp
// compile with: /c
template <class T>
class C{};
class C{};   // C2990

C2990 can also occur when using generics:

// C2990b.cpp
// compile with: /clr /c
generic <class T>
ref struct GC;

ref struct GC {};   // C2990

C2990 can also occur due to a breaking change in the Visual C++ compiler for Visual C++ 2005; the compiler now requires that multiple declarations for the same type be identical with respect to template specification. For more information, see Breaking Changes in the Visual C++ 2005 Compiler.

The following sample generates C2990:

// C2990c.cpp
// compile with: /c
template<class T>
class A;

template<class T>
struct A2 {
   friend class A;   // C2990
};

// OK
template<class T>
struct B {
   template<class T>
   friend class A;
};