Re: A silly macro technique

From:
"Alf P. Steinbach" <alfps@start.no>
Newsgroups:
comp.lang.c++
Date:
Sat, 06 Mar 2010 20:29:06 +0100
Message-ID:
<hmuae7$thr$1@news.eternal-september.org>
* Alf P. Steinbach:

Replace "debugger api" with "whatever" in this

<code>
#define USE_DEBUGGER_API // Or not

extern void useDebuggerApi();

#define CALL_IF_EXPANDED( doIt, f ) \
    struct S_##f { \
        struct Size2 { char x[2]; }; \
        char foo( ... ) { return 0; } \
        Size2 foo##doIt( int ) { return Size2(); } \
    }; \
    (sizeof( S_##f().foo(0) ) > 1 ? f() : (void)0)

#define CALL_IF( doIt, f ) CALL_IF_EXPANDED( doIt, f )

int main()
{
    CALL_IF( USE_DEBUGGER_API, useDebuggerApi );
}
</code>

And why is it silly?

Well, if only routine is called in any particular build, then there
could just be a macro FUNCNAME, say, defined as the name of the routine
to call.

But hey!

Actually I found a use for it.

I think (not sure yet).


I would be interested in improvements.

For those who wonder (and I guess that would be every reader, yes?) the use case
is where a possibly large number of macro symbols can potentially be defined,
and where it's impractical to define them all, so the code should cope well with
any symbol not being defined.

The remaining problem with this is as noted in the comment:

<code>
// f is called if preprocessor symbol 'symbol' is defined.
// If 'symbol' is defined it must be defined as nothing or as something
// that's valid at the end of a C++ name, e.g. TEST_BLAH defined as 1.
//
// Quirk: f will be called if e.g. TEST_BLAH is defined as TEST_BLAH.

#define TESTING_CALL_IFDEF_( symbol, literal_symbol, f, args ) \
     do { \
         struct S_##f { \
             struct Size2 { char x[2]; }; \
             char foo##literal_symbol( ... ) { return 0; } \
             Size2 foo_##symbol( int ) { return Size2(); } \
         }; \
         if( \
             sizeof( S_##f().foo##literal_symbol(0) ) == 1 \
             ) \
         { f args; } \
     } while( false )

#define TESTING_CALL_IFDEF( symbol, f, args ) \
     TESTING_CALL_IFDEF_( symbol, _##symbol, f, args )
</code>

Cheers,

- Alf

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