Is it possible to create a null function that will not produce warnings?

Question

I have a logger in a c++ application that uses defines as follows:

#define FINEST(...) Logger::Log(FINEST, _FILE, __LINE, __func, __VA_ARGS_)

However what I would like to do is to be able to switch off these logs since they have a serious performance impact on my system. And, it's not sufficient to simply have my Logger not write to the system log. I really need to get rid of the code produced by the logs.

In order to do this, I changed the define to:

#define FINEST(...)

Which works, but this produces a whole bunch of warning in my code since variables are unused now. So what I would like to have is a sort of NULL FUNCTION that would not actually exist, but would not produce warnings for the unused variables.

So, said another way, I would like it to compile with no warnings (i.e. the compiler thinks that the variables are used for a function) but the function does not actually exist in the application (i.e. produces no performance hit).

Is this possible?

Thanks!

Answer 1

Have you considered turning off that particular warning of your compiler?
It's probably not a good idea, but if you need a quick and dirty solution...

Answer 2

template<bool implemented>
void Logger::Log( ... );

template<>
void Logger::Log<true>( ... )
{
  // with implementation
}

template<>
void Logger::Log<false>( ... )
{
  // without implementation
}

// USE:
#define FINEST(...) Logger::Log<true>(FINEST, _FILE, __LINE, __func, __VA_ARGS_)
// or
#define FINEST(...) Logger::Log<false>(FINEST, _FILE, __LINE, __func, __VA_ARGS_)

// EVEN BETTER (w/o macro):
// flag to switch on/off logging
const bool with_log = true; // or =false

// use   
Logger::Log<with_log>( /* place arguments here */ );

Answer 3

You could define an empty function with unnamed parameters:

void nullFunc(int, int, int, const char*, ...) {
}

Then redefine your macro to call this function:

#define FINEST(...) nullFunc(FINEST, _FILE, __LINE, __func, __VA_ARGS_)

Answer 4

On comeau and GCC, the warning disappears if you wrap the variables into a sizeof:

#define FINEST(...) ((void)sizeof(__VA_ARGS__))

To avoid warnings on GCC about that the left or right side of the comma operator has no effect (if it expands to sizeof(a, b) for example), you can introduce a debug-nullify type that has its own comma operator:

struct debug_nullify { };

// doesn't need a definition
template<typename T>
debug_nullify operator,(debug_nullify, T const &);

#define FINEST(...) ((void)sizeof(debug_nullify(), __VA_ARGS__))

The benefit of sizeof is that argument expressions are not evaluated.

Answer 5

I would reccomend removing the declarations in release too:

int somefunc(int foo, int bar, void* baz)
    {
    DEBUGVAR(Logger,log);
    int othervar;
    /* stuff */
    MAYBELOG(log,"something happened: %i",othervar);
    /* other stuff */
    return 0;
    }

in define:

#if DEBUGLEVEL>0
#define DEBUGVAR(type,...) type __VA_ARGS__
#define MAYBELOG(logger,str,...) logstuff(logger,__FLIE__,__LINE__,str,__VA_ARG__)
#else
#define DEBUGVAR(type,...)
#define MAYBELOG(logger,str,...)
#endif