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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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