Compiler Error C2990

       代码在之前的编译器上可以编译通过，但今天在VS2008出现如下C2990报错。后网上搜索到相关资料解决了问题。特将此文档收藏下来，以便日后查询

 

'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:

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// C2990.cpp

// compile with: /c

template <class T>

class C{};

class C{};   // C2990

C2990 can also occur when using generics:

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// 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.

The following sample generates C2990:

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// 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;

};