Hello,
gcc 4.4.1
I am maintaining someone's code and I have come across something that I don't understand.
#define RES_API(name, func) name##_##func
Can anyone explain?
Many thanks,
+5
A:
The ## is a concatenation operator. Using RES_API(name1, func1) in your code would be replaced with name1_func1. More information here.
Carl Norum
2009-12-14 07:28:14
+1
A:
Instead of doing OBJ_DoSomething, with this macro you can do RES_API(OBJ, DoSomething). Personally I think its silly.
popester
2009-12-14 07:28:24
A huge amount of the preprocessor stuff is 'silly', until you suddenly find a reason why you can't do without it :)
KevinDTimm
2009-12-14 08:19:11
+3
A:
The ## operator concatenates two tokens. In your case, name is appended with an underscore, and that is appended with func.
So RES_API(aName, aFunc) results in aName_aFunc.
By itself, it seems rather annoying. I could see a use when mixing C and C++ code, as C libraries tend to prefix their functions, while C++ libraries would place them in a namespace.
Given an alternate definition, such as:
#define RES_API(name, func) name##::##func
You suddenly have a generic way to switch between a C interface, or C++.
GMan
2009-12-14 07:29:14
Do you really need the token pasting operators in the C++ code? I would expect the compiler to object that '::' is not a valid identifier.
Jonathan Leffler
2009-12-14 08:00:32
+3
A:
I know you've already got your answer, but there is some great info on the C-FAQ which explains allot of the C Preprocessor magic.
Robert S. Barnes
2009-12-14 07:54:15
Thanks that was a great link. There was some other stuff in there that I needed for some else like doing the varargs functions. Great.
robUK
2009-12-14 07:59:44