Question

I want to declare a debug flag is on or off in these both ways:

#define inDebugMode true

or

const bool inDebugMode = true;

The compiler in Visual Studio 2010 always gives a warning:

warning C4127: conditional expression is constant

Why is that? How can I declare it correctly?

Answer 1

Without seeing the code, I suspect you have the following construct:

if (inDebugMode)
{
}

which will always be true, hence the warning.

Recommend using the preprocessor instead of if:

#define inDebugMode 1

#if inDebugMode
#endif

This will remove the warning and prevent the debugging code being compiled when unrequired. Note you can also specify the value of a macro via the compiler switch /D:

cl.exe /DinDebugMode=1 ...

but you need to ensure you rebuild all sources if you choose the command line option, not just the changed sources.