What are the applications of the ## preprocessor o

Question

As mentioned in many of my previous questions, I'm working through K&R, and am currently into the preprocessor. One of the more interesting things — something I never knew before from any of my prior attempts to learn C — is the ## preprocessor operator. According to K&R:

The preprocessor operator ## provides a way to concatenate actual arguments during macro expansion. If a parameter in the replacement text is adjacent to a ##, the parameter is replaced by the actual argument, the ## and surrounding white space are removed, and the result is re-scanned. For example, the macro paste concatenates its two arguments:

#define paste(front, back) front ## back

so paste(name, 1) creates the token name1.

How and why would someone use this in the real world? What are practical examples of its use, and are there gotchas to consider?

Answer 1

Here's a gotcha that I ran into when upgrading to a new version of a compiler:

Unnecessary use of the token-pasting operator (##) is non-portable and may generate undesired whitespace, warnings, or errors.

When the result of the token-pasting operator is not a valid preprocessor token, the token-pasting operator is unnecessary and possibly harmful.

For example, one might try to build string literals at compile time using the token-pasting operator:

#define STRINGIFY(x) #x
#define PLUS(a, b) STRINGIFY(a##+##b)
#define NS(a, b) STRINGIFY(a##::##b)
printf("%s %s\n", PLUS(1,2), NS(std,vector));

On some compilers, this will output the expected result:

1+2 std::vector

On other compilers, this will include undesired whitespace:

1 + 2 std :: vector

Fairly modern versions of GCC (>=3.3 or so) will fail to compile this code:

foo.cpp:16:1: pasting "1" and "+" does not give a valid preprocessing token
foo.cpp:16:1: pasting "+" and "2" does not give a valid preprocessing token
foo.cpp:16:1: pasting "std" and "::" does not give a valid preprocessing token
foo.cpp:16:1: pasting "::" and "vector" does not give a valid preprocessing token

The solution is to omit the token-pasting operator when concatenating preprocessor tokens to C/C++ operators:

#define STRINGIFY(x) #x
#define PLUS(a, b) STRINGIFY(a+b)
#define NS(a, b) STRINGIFY(a::b)
printf("%s %s\n", PLUS(1,2), NS(std,vector));

The GCC CPP documentation chapter on concatenation has more useful information on the token-pasting operator.