Compiler Error C2079

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The latest version of this topic can be found at Compiler Error C2079.

identifier' uses undefined class/struct/union 'name'

The specified identifier is an undefined class, structure, or union.

This error can be caused by initializing an anonymous union.

The following sample generates C2079:

// C2079.cpp  
// compile with: /EHsc  
#include <iostream>  
int main() {  
   std::ifstream g;   // C2079  
}  

Possible resolution:

// C2079b.cpp  
// compile with: /EHsc  
#include <fstream>  
int main( ) {  
   std::ifstream g;  
}  

C2079 can also occur if you attempt to declare an object on the stack of a type whose forward declaration is only in scope.

// C2079c.cpp  
class A;  
  
class B {  
   A a;   // C2079  
};  
  
class A {};  

Possible resolution:

// C2079d.cpp  
// compile with: /c  
class A;  
class C {};  
  
class B {  
   A * a;  
   C c;  
};  
  
class A {};