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I faced a problem - I need to use a macro value both as string and as integer.
#define RECORDS_PER_PAGE 10 /*... */ #define REQUEST_RECORDS \ "SELECT Fields FROM Table WHERE Conditions" \ " OFFSET %d * " #RECORDS_PER_PAGE \ " LIMIT " #RECORDS_PER_PAGE ";" char result_buffer[RECORDS_PER_PAGE][MAX_RECORD_LEN]; /* ...and some more uses of RECORDS_PER_PAGE, elsewhere... */ This fails with a message about "stray #", and even if it worked, I guess I'd get the macro names stringified, not the values. Of course I can feed the values to the final method ( "LIMIT %d ", page*RECORDS_PER_PAGE ) but it's neither pretty nor efficient. It's times like this when I wish the preprocessor didn't treat strings in a special way and would process their content just like normal code. For now, I cludged it with #define RECORDS_PER_PAGE_TXT "10" but understandably, I'm not happy about it.
How to get it right?
446
There is no way you can test whether a defined macro has a value. You can only test whether a specific value matches the value of a macro. Any macro name that you define should always have a value, or it should never have a value.
The double-number-sign or token-pasting operator (##), which is sometimes called the merging or combining operator, is used in both object-like and function-like macros. It permits separate tokens to be joined into a single token, and therefore, can't be the first or last token in the macro definition.
The number-sign or "stringizing" operator (#) converts macro parameters to string literals without expanding the parameter definition. It's used only with macros that take arguments.
To define a macro that uses arguments, you insert parameters between the pair of parentheses in the macro definition that make the macro function-like. The parameters must be valid C identifiers, separated by commas and optionally whitespace.
The xstr macro defined below will stringify after doing macro-expansion.
#define xstr(a) str(a) #define str(a) #a #define RECORDS_PER_PAGE 10 #define REQUEST_RECORDS \ "SELECT Fields FROM Table WHERE Conditions" \ " OFFSET %d * " xstr(RECORDS_PER_PAGE) \ " LIMIT " xstr(RECORDS_PER_PAGE) ";" If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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