Best way to pass a string array to a function imposing const-ness in all levels down

Question

In C we cannot assign a char** to const char *const * because of this problem. So, what's the base way to declare a function accepting an array of strings?

My input parameters may be of any types, including:

char **
const char **
...

`char **` cannot be assigned to `const char **` but `const char **` can be assigned to `const char * const *`... — Antti Haapala -- Слава Україні, Nov 18 '19 at 14:06
@AnttiHaapala Yes, I know, but my input parameter is of different types, including `char **` and `const char **`, so I need a function to accept both. — frogatto, Nov 18 '19 at 14:08
Best way would be to just accept `const char * const *` and then those using `char **` need an explicit cast, or you could hide the cast behind a `_Generic` selection... — Antti Haapala -- Слава Україні, Nov 18 '19 at 14:10

score 1 · Accepted Answer · answered Nov 18 '19 at 14:36

1

I end up using the Antti Haapala's suggestion (thank you Antti Haapala!). Using const char * const * as the argument type and explicitly casting incompatible types.

#define STR_ARR(v) _Generic((v),             \ 
    char**: (const char *const *)(v),        \
    char *const *: (const char *const *)(v), \
    default: (v))

void foo(const char * const * strings);

Usage:

char **strs1;
foo(STR_ARR(strs1));

const char **strs2;
foo(STR_ARR(strs2));

answered Nov 18 '19 at 14:36

frogatto

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You shouldn't allow a default clause. And I'd use a wrapper macro around the specific function instead, then in API documentation use the macro name instead of the actual function name. – Lundin Nov 18 '19 at 14:45
@Lundin I don't think the default clause causes a problem as the other types will be checked by the compiler. I only whitelisted the required types. – frogatto Nov 18 '19 at 14:49
There's a scary amount of C programmers that follow the rule "when in doubt cast to void pointer". So `foo((void*)my_wildly_incompatible_misaligned_pointer);` is a likely scenario. – Lundin Nov 18 '19 at 14:53

score 1 · Answer 2 · answered Nov 18 '19 at 14:39

The best way is to have the caller sort their code out. Otherwise you can do a type-safe wrapper macro along the lines of this:

#include <stdio.h>

void str_print_ro (const char* str_arr[], size_t n)
{
  for(size_t i=0; i<n; i++)
  {
    puts(str_arr[i]);
  }
}

#define str_print(arr,n) _Generic((arr), \
  const char** : str_print_ro,           \
  char**       : str_print_ro) ((const char**)(arr), n) 

int main (void)
{
  char* arr[] = {"hello", "world"};
  str_print(arr, sizeof arr/sizeof *arr);  
}

It probably doesn't make much sense to const-qualify the pointers themselves. If you for some reason need that too, then const char* const str_arr[] should sort it, and that one can be converted to from char** and const char** both.

Best way to pass a string array to a function imposing const-ness in all levels down

2 Answers2