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| Macros With A Variable Number Of Arguments... Submitted by

A friend just asked me if it would be possible to use the ellipsis (...) in a macro's parameter list, just like it is used in the printf() function:

    int printf(const char* fmt, ...);

The answer is NO. You can't do that with a macro. There's a nice workaround that I've used now and then, though. It uses the fact that, when parsing the parameters passed into a macro, the C preprocessor parses nested sets of parenthesis, and will not use the commas found inside of those nested parenthesis to separate the parameters. So, for example, my solution for a simple printf() wrapper would be something like this:

    bool g_DoLog = false;
    #define LogPrintf(a) if (!g_DoLog) {} else printf a

Now, you can use the macro like this:

    g_DoLog = true;
    ..
    LogPrintf(("Hello %s\n", "World"));

Note that the macro above has complete function semantics: each parameter is only evaluated once, it requires the semicolon and it is a single statement. The double parenthesis are necessary in the general case., but they can be obviated in this example by doing it like this:

    #define LogPrintf if (!g_DoLog) {} else printf
    ..
    LogPrintf("Hello %s\n", "World");

This requires the multiple parameters to go at the end, however, which may not be always possible.
You can also have other non-variable parameters in the macro:

    int g_LogLevel = 0;
    #define LogPrintf(l,a) if (g_LogLevel < (unsigned int)(l)) {} else printf a
    ..
    LogPrintf(3, ("Hello %s\n", "World"));

The double parenthesis are necessary in this case. Still, this can also be done as follows:

    unsigned int g_LogLevel = 0;
    #define LogPrintf(l) if (g_LogLevel <= (unsigned int)(l)) {} else printf
    ..
    LogPrintf(3)("Hello %s\n", "World");

The possibilities are actually quite endless.
Salutaciones,
JCAB

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