C/C++ Compiler (cl.hlp) (Table of Contents; Topic list)
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C2504
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     Compiler error C2504
 
     'class' : base class undefined
 
     The specified base class was declared but never defined.
 
     This error can be caused by a missing include file or an external
     base class that was not declared with the extern specifier.
 
     The following is an example of this error:
 
          class A;                 // error, A is undefined
          class A {};              // OK, A is defined
          class B : public A {};   // the error is detected here
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