Question

I read little gnu online docs about macro and little confused about this sentence:

Also multi-character operators such as += can be formed by token pasting

So I wander if macro can perform compound assignments,How?

Answer 1

#define PASTE(a,b) a##b
int foo;
foo PASTE(+,=) 2;

The only time I’ve ever seen this be useful was in the inner loop of a bytecode interpreter, which tends to look like a gigantic case statement with very repetitive code in each arm:

case OP_add: {
    if (sp < 2) goto underflow;
    double x = stack[--sp];
    stack[sp-1] += x;
} break;

Repeat for each supported binary arithmetic operator. With the ability to token-paste, you can generate them all from a macro:

#define ARITH_BINOP(name, oper) case OP_##name: { \
    if (sp < 2) goto underflow; \
    double x = stack[--sp]; \
    stack[sp-1] oper##= x; \
} break /* deliberate absence of semicolon */

and then, in the switch, you can just write

ARITH_BINOP(add, +);
ARITH_BINOP(sub, -);
ARITH_BINOP(mul, *);
/* etc */