22. Do not use #pragma warning(default:X)

The fragment is taken from TortoiseGIT project. The error is detected by the following PVS-Studio diagnostic: V665 Possibly, the usage of '#pragma warning(default: X)' is incorrect in this context. The '#pragma warning(push/pop)' should be used instead.

#pragma warning(disable:4996)
LONG result = regKey.QueryValue(buf, _T(""), &buf_size);
#pragma warning(default:4996)

Explanation

Programmers often assume that warnings disabled with the "pragma warning(disable: X)" directive earlier will start working again after using the "pragma warning(default : X)" directive. But it is not so. The 'pragma warning(default : X)' directive sets the 'X' warning to the DEFAULT state, which is quite not the same thing.

Suppose that a file is compiled with the /Wall switch used. The C4061 warning must be generated in this case. If you add the "#pragma warning(default : 4061)" directive, this warning will not be displayed, as it is turned off by default.

Correct code

#pragma warning(push)
#pragma warning(disable:4996)
LONG result = regKey.QueryValue(buf, _T(""), &buf_size);
#pragma warning(pop)

Recommendation

The correct way to return the previous state of a warning is to use directives "#pragma warning(push[ ,n ])" and "#pragma warning(pop)". See the Visual C++ documentation for descriptions of these directives: Pragma Directives. Warnings.

Library developers should pay special attention to the V665 warning. Careless warning customization may cause a whole lot of trouble on the library users' side.

A good article on this topic: So, You Want to Suppress This Warning in Visual C++