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Compiler Error C2504

'class' : base class undefined

The base class is declared but never defined.

Possible cause

  • Missing include file.
  • External base class not declared with extern.

The following sample generates C2504:

// C2504.cpp
class A;
class B : public A
{   // C2504, define A before using it as a base class
};

int main()
{
}
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