c pre processor string concatenate

This works:

#define APP_NUMBER 22
#define STR(x) #x
#define ICON_FILE_NAME(num)  "../icons/" STR(num) ".ico"
#define ICON_FILE ICON_FILE_NAME(APP_NUMBER)

#include <stdio.h>
int main(void) {
    printf("ICON_FILE = \"%s\"\n", ICON_FILE);
    return 0;
}

The output is:

ICON_FILE = "../icons/22.ico"

But the ICON_FILE macro expands to "../icons/" "22" ".ico", which is valid syntax for C (adjacent string literals are concatenated), but probably not for a .rc file, which explains the "can't open icon file" message you're getting.

It looks like you're trying to use token-pasting to generate a string literal. The problem with that is that a partial string literal, containing an unmatched " character, cannot be a valid preprocessing token. For example, you can't pass a single " as an argument to a macro.

Consider writing a program (script, whatever) that generates the appropriate #define directives for you.

Windows resource files don't understand the C style literal string concatenation for most elements - string table may be the only exception.

The trick when working with pre-processor macros is then to not use strings as input starting point, the pre-processor doesn't know how to remove quotes.

It would also be helpful to concatenate only once - consider addition of search paths "-I../icons/" rather than adding paths to the resource names.

Pulled the following from boost, example using windows msvc which is using an additional level of indirection than what I have seen in most places.

#    define BOOST_PP_CAT(a, b) BOOST_PP_CAT_OO((a, b))
#    define BOOST_PP_CAT_OO(par) BOOST_PP_CAT_I ## par
#    define BOOST_PP_CAT_I(a, b) a ## b

#    define BOOST_PP_STRINGIZE(text) BOOST_PP_STRINGIZE_A((text))
#    define BOOST_PP_STRINGIZE_A(arg) BOOST_PP_STRINGIZE_I arg
#    define BOOST_PP_STRINGIZE_I(text) #text

In examples seems you could just perform BOOST_PP_CAT(../icons/, BOOST_PP_CAT(num,.ico))

There is an issue passing to stringize only the outer CAT is applied (On windows at least). So BOOST_PP_STRINGIZE(BOOST_PP_CAT(../icons/, BOOST_PP_CAT(num,.ico))) doesn't work.

Addition for concatenation of 3 items

#    define BOOST_PP_CAT2(a, b, c) BOOST_PP_CAT_OO2((a, b, c))
#    define BOOST_PP_CAT_OO2(par) BOOST_PP_CAT_I2 ## par
#    define BOOST_PP_CAT_I2(a, b, c) a ## b ## c

In my testing VS2013 an input of

• .. is being converted to "..." which makes working with relative path difficult
• \ needs to be escaped \\ to work in macro args, but is being converted to "\\" - using / for path works better

Working against the string table can be easier to see output than working with icons

STRINGTABLE
BEGIN
  123  BOOST_PP_STRINGIZE(BOOST_PP_CAT(APP_NUMBER, .ico))
  124  BOOST_PP_STRINGIZE(BOOST_PP_CAT2(../icons/, APP_NUMBER,.ico)))
END

I haven't yet worked out resolution of .. being converted to ... I had the following work using the additional search path,

1000        ICON        BOOST_PP_STRINGIZE(BOOST_PP_CAT(APP_NUMBER, .ico))
1001        ICON        BOOST_PP_STRINGIZE(BOOST_PP_CAT2(icons/, APP_NUMBER, .ico))

I would honestly avoid doing this using preprocessor. Debugging preprocessor errors is a pain and I've seen cases where multiple layers of preprocessor replacements and concatenations were used, resulting in disaster when trying to find issues. Just my 2 cents.

I don't think you will be able to do it with C pre-processor. However, you could do it with m4 pre-processor.

linux_prompt> cat icon.m4
define(APP_NUMBER, 22)

1000 ICON "../icons/APP_NUMBER.ico"

linux_prompt> m4 icon.m4

1000 ICON "../icons/22.icon"

You have to make sure that the rest of the file expands correctly. Do 'man m4' for more info.