Is there a way a macro can use a value of a define passed to it, rather than the define text itself?
This is an odd example which I expected would be possible with the preprocessor.
A C file called test.c which includes itself twice to define two different functions which are called from main.
#ifndef IS_INDIRECT
#define IS_INDIRECT
/* int */
#define NUMTYPE int
#define PREFIX int_
#include "test.c"
#undef NUMTYPE
#undef PREFIX
/* short */
#define NUMTYPE float
#define PREFIX float_
#include "test.c"
#undef NUMTYPE
#undef PREFIX
#include <stdio.h>
int main(int argc, const char **argv)
{
printf("test int %d\n", int_squared(4));
printf("test float %f\n", float_squared(2.5));
return 0;
}
#else
/* function body */
#define fn(prefix, id) prefix ## id
NUMTYPE fn(PREFIX, squared)(NUMTYPE val)
{
return val * val;
}
#endif
Gives the following error:
In file included from test.c:18:0:
test.c:37:12: error: conflicting types for 'PREFIXsquared'
NUMTYPE fn(PREFIX, squared)(NUMTYPE val)
^
test.c:35:24: note: in definition of macro 'fn'
#define fn(prefix, id) prefix ## id
^
In file included from test.c:9:0:
test.c:37:12: note: previous definition of 'PREFIXsquared' was here
NUMTYPE fn(PREFIX, squared)(NUMTYPE val)
^
test.c:35:24: note: in definition of macro 'fn'
#define fn(prefix, id) prefix ## id
I would like to have the macro expand PREFIX to the value it's defined as, so I get int_squared not PREFIXsquared
Is it something like that you're looking for?
#define xxx(x,y) x##y
#define CONCAT(x, y) xxx(x, y)
#define function(type, operation, prm) type CONCAT(operation, type) (type prm)
function (int, square_, value) // int square_int (int value)
{
return value * value;
}
The indirect use of ## allows to define a macro that uses concatenation (functionin our example). CONCAT is expanded to xxx when the macro is defined,
and resolved to x##y when the macro is invoked.
EDIT: added thanks to various contributors:
## is called the token-pasting operator, or sometimes the token-concatenation operatorFrankly they made such a mess of C that I would no longer dare using the language in a professional environment. I'm so glad my IT days are over.
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