C Macros to create strings

问题:

Alternative Titles (to aid search)

• Convert a preprocessor token to a string
• How to make a char string from a C macro's value?

Original Question

I would like to use C #define to build literal strings at compile time.

The string are domains that change for debug, release etc.

I would like to some some thing like this:

#ifdef __TESTING
    #define IV_DOMAIN domain.org            //in house testing
#elif __LIVE_TESTING
    #define IV_DOMAIN test.domain.com       //live testing servers
#else
    #define IV_DOMAIN domain.com            //production
#endif

// Sub-Domain
#define IV_SECURE "secure.IV_DOMAIN"             //secure.domain.org etc
#define IV_MOBILE "m.IV_DOMAIN"

But the preprocessor doesn't evaluate anything within ""

1. Is there a way around this?
2. Is this even a good idea?

回答1:

In C, string literals are concatenated automatically. For example,

const char * s1 = "foo" "bar";
const char * s2 = "foobar";

s1 and s2 are the same string.

So, for your problem, the answer (without token pasting) is

#ifdef __TESTING
    #define IV_DOMAIN "domain.org"
#elif __LIVE_TESTING
    #define IV_DOMAIN "test.domain.com"
#else
    #define IV_DOMAIN "domain.com"
#endif

#define IV_SECURE "secure." IV_DOMAIN
#define IV_MOBILE "m." IV_DOMAIN

回答2:

There are a couple ways to do this:

1. if you're dealing with only string literals, you can simply use simply use strings - placing one string literal after another causes the compiler to concatenate them.

2. if there may be other things than string literals involved (ie., you are creating new identifiers from the macros) use the '##" preprocessor token pasting operator. You'd probably also need to use the '#' 'stringizing operator to make your macros into literal strings.

An example of #1:

#ifdef __TESTING
    #define IV_DOMAIN "domain.org"                        //in house testing
#elif __LIVE_TESTING
    #define IV_DOMAIN "test.domain.com"           //live testing servers
#else
    #define IV_DOMAIN "domain.com"                        //production
#endif

// Sub-Domain
#define IV_SECURE "secure." IV_DOMAIN          //secure.domain.org etc
#define IV_MOBILE "m." IV_DOMAIN

And as far as the token pasting operator goes, I don't think that most of the answers that suggested using the token pasting preprocessor operator have actually tried it - it can be tricky to use.

Using the answer that is often suggested will result in a compiler error when you try to use the IV_SECURE macro, because:

#define IV_SECURE "secure."##IV_DOMAIN

expands to:

"secure"domain.org

You might want to try to use the '#`' 'stringizing' operator:

#define IV_SECURE "secure." #IV_DOMAIN

But that won't work because it only works on macro arguments - not just any old macro.

one thing to be aware of when you're using the token-paste ('##') or stringizing ('#') preprocessing operators is that you have to use an extra level of indirection for them to work properly in all cases.

If you don't do this and the items passed to the token-pasting operator are macros themselves, you'll get results that are probably not what you want:

#include <stdio.h>

#define STRINGIFY2( x) #x
#define STRINGIFY(x) STRINGIFY2(x)
#define PASTE2( a, b) a##b
#define PASTE( a, b) PASTE2( a, b)

#define BAD_PASTE(x,y) x##y
#define BAD_STRINGIFY(x) #x

#define SOME_MACRO function_name

int main() 
{
    printf( "buggy results:\n");
    printf( "%s\n", STRINGIFY( BAD_PASTE( SOME_MACRO, __LINE__)));
    printf( "%s\n", BAD_STRINGIFY( BAD_PASTE( SOME_MACRO, __LINE__)));
    printf( "%s\n", BAD_STRINGIFY( PASTE( SOME_MACRO, __LINE__)));

    printf( "\n" "desired result:\n");
    printf( "%s\n", STRINGIFY( PASTE( SOME_MACRO, __LINE__)));
}

The output:

buggy results:
SOME_MACRO__LINE__
BAD_PASTE( SOME_MACRO, __LINE__)
PASTE( SOME_MACRO, __LINE__)

desired result:
function_name21

So using your original IV_DOMAIN defines and the utilty macros from above, you could do this to get what you want:

// Sub-Domain
#define IV_SECURE "secure." STRINGIFY( IV_DOMAIN)   //secure.domain.org etc
#define IV_MOBILE "m." STRINGIFY( IV_DOMAIN)

回答3:

Strings that are next together are combined by the C compiler.

#define DOMAIN "example.com"
#define SUBDOMAIN "test." DOMAIN
const char *asCString = SUBDOMAIN;
NSString *asNSString = @SUBDOMAIN;

回答4:

I see lots of good and correct answers to your first question, but none to your second, so here's this: I think this is a terrible idea. Why should you have to rebuild your software (particularly the release version) just to change the server name? Also, how will you know which version of your software points at which server? You'll have to build in a mechanism to check at runtime. If it's at all practical on your platform, I recommend you load the domains/URLs from a config file. Only the smallest of embedded platforms may not be "practical" for that purpose :)

回答5:

Try using the ## operator

#define IV_SECURE secure.##IV_DOMAIN

回答6:

What you need are the # and ## operators, and automatic string concatenation.

The # preprocessing operator turns the macro parameter into a string. The ## operator pastes two tokens (such as macro parameters) together.

The possibility that comes to mind to me is

#define IV_DOMAIN domain.org
#define IV_SECURE(DOMAIN) "secure." #DOMAIN

which should change IV_SECURE to

#define IV_SECURE "secure." "domain.org"

which will automatically concatenate to "secure.domain.org" (assuming the phases of translation are the same in C as C++).

ANOTHER EDIT: Please, please read the comments, which show how I've managed to get confused. Bear in mind that I am thoroughly experienced in C, although perhaps a touch rusty. I would delete this answer, but I thought I'd leave it as an example of how easy it is to get confused by the C preprocessor.

回答7:

As others have noted, use token pasting. You should also be aware that macro names like

__TESTING

are reserved in C (don't know about Objective C) for the implementation - you are not allowed to use them in your own code. The reserved names are anything containing double underscores and anything begining with an underscore and an uppercase letter.