K&R C had at least two ways to combine two tokens. Both invocations in the following code 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 invocations would produce the two separate tokens x and 1. The second of the 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. Because ## 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.
No direct approach reproduces the first of the two old-style pasting schemes but because it put the burden of the pasting at the invocation, it was used less frequently than the other form .