Re: A silly macro technique

From:

Kaz Kylheku <kkylheku@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Tue, 23 Feb 2010 19:26:09 +0000 (UTC)

Message-ID:

<20100223105848.249@gmail.com>

On 2010-02-23, Alf P. Steinbach <alfps@start.no> wrote:

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?

It's also silly because USE_DEBUGGER_API could just be a preprocessor
symbol that is always defined, and expands to 0 or 1.

Then CALL_IF is simply

  #define CALL_IF(FLAG, FUNC) ((FLAG) ? FUNC() : (void) 0)

And it doesn't save you typing:

  if (flag) func();

  CALL_IF(flag, func);

Oops!