Using macros with a logger

[Mybowlcut]

Hey.

I want to do this:

void f()
{
    LOG("Called f().");
}

But I only want the LOG macro to work if USE_LOG is defined... in other words, if USE_LOG isn't defined, the LOG macro won't do anything. I'm trying to write the macro for this but I can't get it right:

#ifndef LOG(string)
#define LOG(string)
#ifdef USE_LOG
Logger::Get().Log(string)
#endif
#endif

This gives me a warning:

warning C4067: unexpected tokens following preprocessor directive - expected a newline	c:\...\logger.h

Also I'm just not sure if it will do what I think it will do... if USE_LOG isn't defined and the call "disappears" everywhere it is used in code, will it leave behind semicolons everywhere? If so, is this a bad thing? Will the compiler interpret these as statements and spend time processing them?

Cheers.

[Plasmator]

It's simply:

#ifdef USE_LOG
    #define LOG(string) Logger::Get().Log(string)
#else
    #define LOG(string)
#endif

[Mybowlcut]

It's simply:

#ifdef USE_LOG
    #define LOG(string) Logger::Get().Log(string)
#else
    #define LOG(string)
#endif

Cheers.

For the else case, what would that expand to?

[potatoCode]

Hello MyBowlCut,

That's a very good question.
I know just a little to say that parameterized macro expands its argument as text,
which in C++ could be anything from a number to invalid expression depending on where it's used. But then again, I don't know what it would really expands to.

[Mybowlcut]

Hello Mybowlcut,

That's a very good question. I don't know what it would really expands to.

Fixed.

[potatoCode]

Fixed.

[Mybowlcut]

If anyone knows what this expands to, I'm curious to know.

[treuss]

Quite simple: The preprocessor will replace ``LOG("Called f().")´´ with an empty string, thus your code will look like:

void f()
{
    ;
}

to the compiler. The single semicolon will not translate to any code on compilation.

[treuss]

Oh, and btw, you might want to lookup the __func__ macro which will automatically expand to the function name.

[Mybowlcut]

Cheers treuss.

[Speedo]

Oh, and btw, you might want to lookup the __func__ macro which will automatically expand to the function name.

Note though that __func__ is not a standard macro, so it may vary depending on platform. In MSVC it is __FUNCTION__.