6.5.6 Token Pasting

In K&R C, there were at least two ways to combine two tokens. Both invocations in the following produced a single identifier x1 out of the two tokens x and 1.

#define self(a) a
#define glue(a,b) a/**/b ?
self(x)1
glue(x,1)

Again, ISO C could not sanction either approach. In ISO C, both the above invocations would produce the two separate tokens x and 1. The second of the above two methods can be rewritten for ISO C by using the ## macro substitution operator:

#define glue(a,b) a ## b
glue(x, 1)

# and ## should be used as macro substitution operators only when __STDC__ is defined. Since ## is an actual operator, the invocation can be much freer with respect to white space in both the definition and invocation.

The compiler issues a warning diagnostic for an undefined ## operation (C standard, section 3.4.3), where undefined is a ## result that, when preprocessed, consists of multiple tokens rather than one single token (C standard, section 6.10.3.3(3)). The result of an undefined ## operation is now defined as the first individual token generated by preprocessing the string created by concatenating the ## operands.

There is no direct approach to effect the first of the two old-style pasting schemes, but since it put the burden of the pasting at the invocation, it was used less frequently than the other form.