字符串文字到 char 数组文字

Question

I am writing some code which involves writing char arrays like so:

char arr[] = { CONST1, CONST2, 'h', 'e', 'l', 'l', 'o' };

( CONST1 and CONST2 are enum values) Is there any way I can express this string of char literals, as a single string literal (via preprocessor macros or otherwise)? eg

char arr = {CONST1, CONST2, "hello"};

Wrapping it in either single and double quotes produce compiler errors as expected.

Answer 1

Here's your macro, use it wisely:

#define ENUM )(enum,
#define STR )(str,

#define MAKE_ARRAY(name, seq) \
    char name[END( MAKE_ARRAY_SIZEOF_LOOP_A(start, seq) )]; \
    char *MAKE_ARRAY_ptr = name; \
    END( MAKE_ARRAY_WRITE_LOOP_A(start, seq) ) \
    *MAKE_ARRAY_ptr = '\0';
    
#define CAT(x,y) CAT_(x,y)
#define CAT_(x,y) x##y

#define END(...) END_(__VA_ARGS__)
#define END_(...) __VA_ARGS__##_END

#define MAKE_ARRAY_ptr CAT(_MAKE_ARRAY_ptr_,__LINE__)

#define MAKE_ARRAY_SIZEOF_start(x) 1
#define MAKE_ARRAY_SIZEOF_enum(x) +1
#define MAKE_ARRAY_SIZEOF_str(x) +sizeof(x)-1
#define MAKE_ARRAY_SIZEOF_LOOP_BODY(x,y) CAT(MAKE_ARRAY_SIZEOF_,x)(y)
#define MAKE_ARRAY_SIZEOF_LOOP_A(...) MAKE_ARRAY_SIZEOF_LOOP_BODY(__VA_ARGS__) MAKE_ARRAY_SIZEOF_LOOP_B
#define MAKE_ARRAY_SIZEOF_LOOP_B(...) MAKE_ARRAY_SIZEOF_LOOP_BODY(__VA_ARGS__) MAKE_ARRAY_SIZEOF_LOOP_A
#define MAKE_ARRAY_SIZEOF_LOOP_A_END
#define MAKE_ARRAY_SIZEOF_LOOP_B_END

#define MAKE_ARRAY_WRITE_start(x)
#define MAKE_ARRAY_WRITE_enum(x) *MAKE_ARRAY_ptr++ = (char)x;
#define MAKE_ARRAY_WRITE_str(x) for (size_t i = 0; i < sizeof(x)-1; i++) *MAKE_ARRAY_ptr++ = x[i];
#define MAKE_ARRAY_WRITE_LOOP_BODY(x,y) CAT(MAKE_ARRAY_WRITE_,x)(y)
#define MAKE_ARRAY_WRITE_LOOP_A(...) MAKE_ARRAY_WRITE_LOOP_BODY(__VA_ARGS__) MAKE_ARRAY_WRITE_LOOP_B
#define MAKE_ARRAY_WRITE_LOOP_B(...) MAKE_ARRAY_WRITE_LOOP_BODY(__VA_ARGS__) MAKE_ARRAY_WRITE_LOOP_A
#define MAKE_ARRAY_WRITE_LOOP_A_END
#define MAKE_ARRAY_WRITE_LOOP_B_END

Usage:

enum E {e1, e2};

int main()
{
    MAKE_ARRAY(foo, ENUM e1 ENUM e2 STR "abc")
}

This expands to:

char foo[1 +1 +1 +sizeof("abc")-1];
char *_MAKE_ARRAY_ptr_42 = foo;
*_MAKE_ARRAY_ptr_42++ = (char)e1;
*_MAKE_ARRAY_ptr_42++ = (char)e2;
for (size_t i = 0; i < sizeof("abc")-1; i++)
    *_MAKE_ARRAY_ptr_42++ = "abc"[i];
*_MAKE_ARRAY_ptr_42 = '\0';

Here:

• _MAKE_ARRAY_ptr_42 is a helper pointer. The number at the end comes from __LINE__ , so you can't use two MAKE_ARRAY s at a single line. Replace it with a global variable if you want to.
• In char foo[1 +1 +1 +sizeof("abc")-1]; :
  • 1 at the beginning is always there to make room for the null-terminator.
  • Two +1 s come from ENUM <name> .
  • +sizeof("abc")-1 comes from STR "abc" .
• *_MAKE_ARRAY_ptr_42 = '\0'; at the end adds a null-terminator.