Creating C macro with ## and __LINE__ (token concatenation with positioning macro)

Question

I want to create a C macro that creates a function with a name based on the line number. I thought I could do something like (the real function would have statements within the braces):

#define UNIQUE static void Unique_##__LINE__(void) {}

Which I hoped would expand to something like:

static void Unique_23(void) {}

That doesn't work. With token concatenation, the positioning macros are treated literally, ending up expanding to:

static void Unique___LINE__(void) {}

Is this possible to do?

Answer 1

The problem is that when you have a macro replacement, the preprocessor will only expand the macros recursively if neither the stringizing operator # nor the token-pasting operator ## are applied to it. So, you have to use some extra layers of indirection, you can use the token-pasting operator with a recursively expanded argument:

#define TOKENPASTE(x, y) x ## y
#define TOKENPASTE2(x, y) TOKENPASTE(x, y)
#define UNIQUE static void TOKENPASTE2(Unique_, __LINE__)(void) {}

Then, __LINE__ gets expanded to the line number during the expansion of UNIQUE (since it's not involved with either # or ##), and then the token pasting happens during the expansion of TOKENPASTE.

It should also be noted that there is also the __COUNTER__ macro, which expands to a new integer each time it is evaluated, in case you need to have multiple instantiations of the UNIQUE macro on the same line. Note: __COUNTER__ is supported by MS Visual Studio, GCC (since V4.3), and Clang, but is not standard C.