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Extracting the number of digits in a macro constant at compile time

I need to do some preprocessor magic. Suppose I have a global constant

#define MAX_VALUE 99999

I need to do to extract the length of this constant in its decimal representation at compile time. In other words, I don't want to have another constant

#define MAX_VALUE_STRLEN 5

pollution of the global namespace and I don't want to add space to the code that needs to be changed if MAX_VALUE changes. If I have a numeric letter then I can do something like

#define INTLEN(x) (sizeof(#x)/sizeof((#x)[0]) - 1)

and INTLEN(99999)

will then expand to 5

at compile time. Unfortunately, I cannot do something like

INTLEN(MAX_VALUE),

because the preprocessor expands INTLEN first, so I get

 (sizeof("MAX_VALUE")/sizeof(("MAX_VALUE")[0]) - 1)

Is there a preprocessor trick that achieves what I want? Another harder issue that I must safely ignore is that it can be made general enough, if someone decides to add an annotation like, say, 99999L to a constant, that I can still get the correct value?

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1 answer


String with #

and two levels of macro expansion, then chop off the final one NUL

:



#define MAX_VALUE 99999

#define STRINGIFY(x) #x

#define LENGTH(x) (sizeof(STRINGIFY(x)) - 1)

#include <stdio.h>    

int main()
{
    size_t n = LENGTH(MAX_VALUE);
    printf("length = %zu\n", n);
    return 0;
}
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