INFO: Casting Difference Between C and C++ May Cause C2105

• Microsoft C/C++ for MS-DOS, version 7.0
• Microsoft Visual C++ for Windows, versions 1.0, 1.5
• Microsoft Visual C++, 32-bit Edition, versions 1.0, 2.0, 2.1, 4.0, 5.0

SUMMARY

In C++, the result of a cast to a reference type is an lvalue; casts to any other type are not lvalues. This is explicitly stated in the "C++ Annotated Reference Manual," by Ellis and Stroustrup, on page 69. In a .C file, explicit casting to any data type results in an lvalue only if Microsoft language extensions are enabled. The same error is generated in a .C file if Microsoft extensions are disabled.

This difference can cause problems in code that compiles without error using a C compiler. The sample code below illustrates this with a simple increment of a casted result. If it is compiled as a C source file, no errors are generated (although using warning level 4 generates "C4213: nonstandard extension used : cast on l-value"). However, if it is compiled as a C++ source file, the following error is generated:

   error C2105: '++' needs lvalue

   ++(long*&)p;

Sample Code

   /* Compile options needed:  none
   */

   void main()
   {
      char *p;

      ((long *)p)++;   // If the file is saved as a .CPP, this will
   }                   // cause an error.

Keywords          : CLngIss CPPLngIss kbcode
Version           : MS-DOS:7.0;WINDOWS:1.0,1.5;WINDOWS:1.0,2.0,2.1,4.0,5.0
Platform          : MS-DOS NT WINDOWS
Issue type        : kbinfo