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Microsoft KB Archive/100539

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Knowledge Base

Article ID: 100539

Article Last Modified on 7/5/2005


  • Microsoft C/C++ Professional Development System 7.0
  • Microsoft Visual C++ 1.0 Professional Edition
  • Microsoft Visual C++ 1.5 Professional Edition
  • Microsoft Visual C++ 1.0 Professional Edition

This article was previously published under Q100539


When Microsoft C/C++ compiles a source code file that has the .CPP or .CXX filename extension, the compiler does not generate any C4051 warning messages. However, when Microsoft C/C++ compiles a source code file that contains similar code but that has the .C filename extension, the compiler correctly generates C4051 warning messages as follows:

C4051: type conversion; possible loss of data


Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This problem was corrected in Visual C++ 32-bit Edition version 2.0.


According to the online help, the C4051 warning indicates that two data items in an expression have different base types. In the process of converting one of the items, it was truncated. To avoid this warning, cast the data item to an appropriate data type.

The following code example demonstrates this problem.

Sample Code

 * Compile options needed: /W4

void main()
   int i;

   i = 66.66 * 55.55;

Additional query words: 1.00 1.50 7.00 8.00 8.00c

Keywords: kbbug kbfix kbcpponly kbcompiler KB100539