compiler-error-c2504.md

title	ms.custom	ms.date	ms.reviewer	ms.suite	ms.technology	ms.tgt_pltfrm	ms.topic	f1_keywords	dev_langs	helpviewer_keywords	ms.assetid	caps.latest.revision	author	ms.author	manager
Compiler Error C2504 | Microsoft Docs		11/04/2016			cpp-tools		error-reference	C2504	C++	C2504	c9e002a6-a4ee-4ba7-970e-edf4adb83692	9	corob-msft	corob	ghogen

Compiler Error C2504

'class' : base class undefined

The base class is declared but never defined. Possible causes:

1. Missing include file.

2. External base class not declared with extern.

The following sample generates C2504:

// C2504.cpp  
// compile with: /c  
class A;  
class B : public A {};   // C2504  

// OK

class C{};  
class D : public C {};