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I'm trying to figure out how I can concatenate a #define'd int to a #define'd string using the C Preprocessor. My compiler is GCC 4.1 on CentOS 5. The solution should also work for MinGW.

I'd like to append a version number onto a string, but the only way I can get it to work is to make a copy of the version number defines as strings.

The closest thing I could find was a method of quoting macro arguments, but it doesn't work for #defines

This is does not work.

#define MAJOR_VER 2
#define MINOR_VER 6
#define MY_FILE "/home/user/.myapp" #MAJOR_VER #MINOR_VER

It doesn't work without the #s either because the values are numbers and it would expand to "/home/user/.myapp" 2 6, which isn't valid C.

This does work, but I don't like having copies of the version defines because I do need them as numbers as well.

#define MAJOR_VER 2
#define MINOR_VER 6
#define MAJOR_VER_STR "2"
#define MINOR_VER_STR "6"
#define MY_FILE "/home/user/.myapp" MAJOR_VER_STRING MINOR_VER_STRING
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1

3 Answers 3

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243

Classical C preprocessor question....

#define STR_HELPER(x) #x
#define STR(x) STR_HELPER(x)

#define MAJOR_VER 2
#define MINOR_VER 6
#define MY_FILE "/home/user/.myapp" STR(MAJOR_VER) STR(MINOR_VER)

The extra level of indirection will allow the preprocessor to expand the macros before they are converted to strings.

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5 Comments

STR() in this case will give a Narrow string. Is there a variation to convert this to a wide string?
I couldn't say how many times I googled it and copied from this exact answer, but it's going to be in double digits
The first "STR_HELPER" is required because '#' only works with a macro argument. It took me sometime to figure that out..
@clarkttfu, sort of -- yes, # only works with macro arguments. However, the STR_HELPER macro is needed to avoid turning the macro MAJOR_VER into the string "MAJOR_VAR", wheres we want to the result to be "2".
My brain hurts that this can't be done with a single macro... As a lot of other people, used this dozens of times
16

A working way is to write MY_FILE as a parametric macro:

#define MY_FILE(x,y) "/home..." #x #y

EDIT: As noted by "Lindydancer", this solution doesn't expand macros in arguments. A more general solution is using two-step macros:

#define MY_FILE_(x,y) "/home..." #x #y
#define MY_FILE(x,y) MY_FILE_(x,y)
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Comments

0

You can do that with BOOST_PP_STRINGIZE:

#define MAJOR_VER 2
#define MINOR_VER 6
#define MY_FILE "/home/user/.myapp" BOOST_PP_STRINGIZE(MAJOR_VER) BOOST_PP_STRINGIZE(MINOR_VER)
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5 Comments

Makes me smirk how people throw Boost at everything.
@Frerich: Taking your argument to extreme, people should write their own compilers first in raw machine code, rather than throwing g++ at everything... No point to reinvent the wheel. Good programmers write code, great ones reuse.
@jonescb: just open the boost header and see for yourself.
Yep, I tried it. It did work, but using a Boost header in a C program seems kind of odd to me.
Oh, my bad, did not notice C tag.

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