INFO: C4097: Typedef-Name X Used as Synonym for Class-Name Z

ID: Q113417

The information in this article applies to:

The Microsoft C/C++ Compiler (CL.EXE)
- Microsoft C/C++ for MS-DOS, versions 7.0, 7.0a
- Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5, 1.51, 1.52
- Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 2.1, 4.0, 5.0, 6.0

SUMMARY

When a typedef class name is used as a base class, the following warning will appear if compiled with /Ze (enable Microsoft extensions), which is on by default:

warning C4097:Non-standard extension: typedef-name 'identifier1' used as synonym for class-name 'identifier2'

If compiled with /Za (disable Microsoft extensions) using Visual C++ for Windows or Visual C++ 32-bit Edition 1.0, the following error appears:

error C2505: 'identifier' : is not a legal base class

This error message is correct in accordance with the C++ language specification. In C++ grammar, the "base-list" is specified as being a list of "base-specifiers". A base-specifier reduces to a "complete-class-name", which does not include "typedef-names". Consequently, the compiler is perfectly correct in generating this warning/error for the sample below.

This error is not generated using Visual C++ 32-bit Edition 2.X. Using Visual C++ 32-bit Edition, versions 2.x and above, the compiler generates the following warning with both the /Ze and /Za switches:

C4097: typedef-name 'identifier1' used as synonym for class-name 'identifier2'

MORE INFORMATION

For more information on C++ grammar, see the "Grammar Summary" in the "Annotated C++ Reference Manual" (ARM) by Bjarne Stroustrup and Margaret Ellis (Appendix A) or Appendix C of the C++ "Language Reference" that comes with Microsoft C/C++ 7.0 and Microsoft Visual C++ for Windows, versions 1.0 and 1.5. The same Grammar Summary is in Appendix A of the "C++ Language Reference" manual for Visual C++ 32-bit Edition, versions 1.1 and above, and in the online documentation under the "Grammar Summary" heading of the "C++ Langauge Reference" included with Visual C++ 32-bit Edition, version 4.0.

Sample Code


/* Compile options needed: /Ze /c /W4
*/ 

 class C
 {
   int _w;
 };

 typedef C D;

 class F : public D     // warning C4097
 { };

 class E : private virtual D // warning C4097
 { };

 class G : protected D // warning C4097
 { };

Additional query words: 8.00 8.00c 9.00 9.10

Keywords : kbCompiler kbCPPonly kbVC100 kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600
Version : MS- DOS:7.0;WIN3X:1.0,1.5,1.51,1.52;WINNT:1.0,2.0,2.1,4.0,5.0
Platform : MS-DOS NT WINDOWS
Issue type : kbinfo