Knowledge Base

PRB: Error C2787 When Building a Project Using ATL 3.0

Article ID: 192561

Article Last Modified on 12/10/2003

APPLIES TO

Microsoft ActiveX Template Library 3.0, when used with:
- Microsoft Visual C++ 6.0 Enterprise Edition
- Microsoft Visual C++ 6.0 Professional Edition
- Microsoft Visual C++ 6.0 Standard Edition

This article was previously published under Q192561

SYMPTOMS

When building a project with ATL 3.0 you may get a compiler error c2787 (no GUID has been associated with this object).

CAUSE

The newer versions of MIDL use the compiler facility [__declspec(uuid())] to tag a GUID to an interface in the header files it generates. ATL version 3.0 makes use of this fact in the implementation of COM_INTERFACE_ENRY macro; it now uses the keyword __uuidof() to extract the interface ID from the MIDL-generated header. ATL 2.1 uses the token pasting operator (IID_##IFoo) to get the interface ID. This action requires including the MIDL-generated header and also the "project_i.c", which MIDL generated.

If you use an older MIDL-generated header file [which does not use __declspec(uuid())] with ATL 3.0, you will get this compiler error, because __uudiof() can't extract the interface ID.

RESOLUTION

Define the preprocessor symbol _ATL_NO_UUIDOF in the C/C++ tab of the Project Settings dialog box. This forces ATL to use the token pasting operator (##) rather than __uuidof() when forming the interface ID.

STATUS

This behavior is by design.

MORE INFORMATION

Sample Code

   COM_INTERFACE_ENTRY in ATL 2.1
   
      #define COM_INTERFACE_ENTRY(x)\ 
         {&IID_##x), \ 
         offsetofclass(x, _ComMapClass), \ 
         _ATL_SIMPLEMAPENTRY},
   
   COM_INTERFACE_ENTRY in ATL 3.0
   
      #ifndef _ATL_NO_UUIDOF
      #define _ATL_IIDOF(x) __uuidof(x)
      #else
      #define _ATL_IIDOF(x) IID_##x
      #endif
   
      #define COM_INTERFACE_ENTRY(x)\ 
         {&_ATL_IIDOF(x), \ 
         offsetofclass(x, _ComMapClass), \ 
         _ATL_SIMPLEMAPENTRY},

Newer MIDL headers files use the macros DECLSPEC_UUID() or MIDL_INTERFACE(), which expand to __declspec(uuid()). MIDL versions 3.01.75 and later use the __declspec directive.

REFERENCES

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