a macro substitution involving concatenation and line number

Question

I would like to have a macro that produce something like L17, where 17 is the line number when the macro is invoked. However the following only produce L__LINE__

#define STOP L##__LINE__

Wonder if there is a way to make the __LINE__ evaluate before concatenation.

is there any guarantee on the order of macro expansion in the standard? It doesn't seem to work for me. — Alessandro Teruzzi, May 26 '16 at 13:51
@AlessandroTeruzzi see http://stackoverflow.com/a/37463034/719662 for fixed version — , May 26 '16 at 15:30

score 3 · Accepted Answer · answered May 26 '16 at 13:58

3

You need a double concat macro wrapper:

#define CONCAT0(x,y) x ## y
#define CONCAT(x,y) CONCAT0(x,y)
#define STOP CONCAT(L,__LINE__)
int main() {
    int STOP = 42;
    L5 = 41;
}

answered May 26 '16 at 13:58

Thanks, that works! – packetie May 26 '16 at 15:00

a macro substitution involving concatenation and line number

1 Answers1