Is it possible to use __LINE__ in the auto-generated variable name in C?

问题

To avoid duplication, I want to use __LINE__ in the auto-generated variable name.

#define ROUTE(path, impl) \
char * k##impl##__LINE__##_route = "{"#path":\""#impl"\"}";

But it always be treated as a normal string __LINE__.

Even if I define it as the following, I can not get what I want:

#define ROUTE(path, impl) ROUTE_(path, impl, __LINE__)

#define ROUTE_(path, impl, line) \
char * k##impl##line##_route = "{"#path":\""#impl"\"}";

回答1:

You need one more level of nesting:

#define ROUTE(path, impl) ROUTE_(path, impl, __LINE__)

#define ROUTE_(path, impl, line) ROUTE_1(path, impl, line)

#define ROUTE_1(path, impl, line) \
    char * k##impl##line##_route = "{"#path":\""#impl"\"}";

回答2:

This works:

#define CAT_(A,B) A##B
#define CAT(A,B) CAT_(A,B)


#define ROUTE(path, impl) \
    char * CAT(CAT(k##impl,__LINE__),_route) = "{"#path":\""#impl"\"}";

回答3:

If you're not using the variable try "compound literals"

#include <stdio.h>
#include <time.h>

int main(void) {
    // using preprocessor 'trick'
    struct tm uniquenamewith__LINE__inthename = {0};
    uniquenamewith__LINE__inthename.tm_year = 2019 - 1900;
    uniquenamewith__LINE__inthename.tm_mon = 12 - 1;
    uniquenamewith__LINE__inthename.tm_mday = 18;
    time_t foo = mktime(&uniquenamewith__LINE__inthename);

    // using compound literal
    time_t bar = mktime(&(struct tm){.tm_year=2019-1900, .tm_mon=12-1, .tm_mday=18});

    printf("foo is %lu, bar is %lu\n", (unsigned long)foo, (unsigned long)bar);
    return 0;
}

See code running on ideone

来源：https://stackoverflow.com/questions/59394700/is-it-possible-to-use-line-in-the-auto-generated-variable-name-in-c

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