ok, let's try to clarify.
the ## operator is treated by the preprocessor, which means it operates before the compilation. it is named the token-pasting operator: its purpose is to take whatever text is in left and right and make a complete token out of that. remember that the token is formed at the preprocessing: the token does not have a type at all, it is only text pasted in the source code, in the same way #define works.
jr_2005 gave an example, which was indeed using strings (note how the arguments to join() are surrounded by ""):
#define join(x, y) sprintf (str, x##y)
char str [256];
join ("Prec_", "Every_2nd_Loop_Sec_W");
but the example works also for variable names:
#define int_min -32768 // define some values
#define int_max 32768
#define float_min -1e99
#define float_max 1e99
// the next line defines a macro which will allow to link the type name to the values defined
#define min( type ) type##_min
#define max( type ) type##_max
int main()
{
int value;
// how it works: when you type "min(int)" in your source code, the preprocessor will perform
// a replacement: copy the source text of the argument, paste it and append the _min
value = min( int );
// so the above line is replaced by the preprocessor by this code:
value = int_min;
// int_min is a #define, so ultimately the preprocessor replaces it by its defined value
value = -32768;
// this last line is what is effectively passed to the compiler.
printf( "the minimum value for type int is:%i\n", min( int ) );
}
token pasting is done before compiling, thus cannot be used at runtime. the following example will not work at all:
#define int_min -32768 // define some values
#define int_max 32768
#define min( type ) type##_min
int main()
{
char line[256];
printf( "enter a type name: " );
gets( line );
printf( "the minimum value for type %s is:%i\n", line, min( line ) );
// this does not work because min(line) is replaced before compiling by line_min, which is not defined.
}
