Count lines between two code locations in C preprocessor

问题

I want to use the C preprocessor to count the amount of lines between two code locations. The basic idea is something like this:

#define START __LINE__
static char* string_list[] = {
    "some string",
    "another string",
    ...
    "last string"
};
#define END __LINE__
#if END - START > 42
    #error Too many entries
#endif

Of course this doesn't work because in this case START and END are merely redefinitions of the __LINE__ macro.
I was playing around a bit with the # and ## operators, but I could not get the preprocessor to evaluate START and END while the preprocessor is running.

My question is: is this possible at all?

Checking the size of the array during runtime is not an option.
Thanks for any hints or ideas in advance!

回答1:

You shouldn't use those macros for that purpose: the code will become completely unmaintainable if you introduce an extra line somewhere. And what if there are too few lines?

Instead of macros, use a static assert:

static_assert(sizeof(string_list) / sizeof(*string_list) == SOME_CONSTANT,
               "Wrong number of entries in string list.");

If you aren't using C11 which has support for static_assert, you can write such an assert yourself, for example like this:

#define COMPILE_TIME_ASSERT(expr) {typedef uint8_t COMP_TIME_ASSERT[(expr) ? 1 : 0];}

回答2:

[Acknowledge @Lundin who points out that you can use a typedef]

This is one way

typedef uint8_t START[__LINE__]; /*put this on one line*/

typedef uint8_t END[__LINE__]; /*put this on another line*/

sizeof(END) - sizeof(START) is a compile-time expression giving the line offset.

Put in macros and rename START and END to taste.

回答3:

If you declare the array thus:

static const char* string_list[42] = {...

The compiler will tell you if you provide too many initialisers. You can provide a sentinel entry to mark the end if necessary:

static char* string_list[43] = { ..., NULL } ;

And perhaps write a function to determine the length at runtime if you need to know that.

来源：https://stackoverflow.com/questions/24551721/count-lines-between-two-code-locations-in-c-preprocessor

标签

c-preprocessor