将 const 与 typedef 和函数指针一起使用的问题

Question

Here is a MRE

#include <string.h>

typedef char* mytype;

typedef int(*cmp)(const mytype, const mytype);

void foo(cmp f) {}

int main(void) {
    foo(strcmp);
}

When I compile I get:

$ gcc mre.c -Wall -Wextra
mre.c: In function ‘foo’:
mre.c:7:14: warning: unused parameter ‘f’ [-Wunused-parameter]
    7 | void foo(cmp f) {}
      |          ~~~~^
mre.c: In function ‘main’:
mre.c:10:9: warning: passing argument 1 of ‘foo’ from incompatible pointer type [-Wincompatible-pointer-types]
   10 |     foo(strcmp);
      |         ^~~~~~
      |         |
      |         int (*)(const char *, const char *)
mre.c:7:14: note: expected ‘cmp’ {aka ‘int (*)(char * const,  char * const)’} but argument is of type ‘int (*)(const char *, const char *)’
    7 | void foo(cmp f) {}
      |          ~~~~^

The first warning is irrelevant. But what do I do about the second? I tried changing the typedef to:

typedef int(*cmp)(mytype const,mytype const);

But that gave the exact same result. When I changed to:

typedef int(*cmp)(const char*, const char*);

it worked, but that's not preferable for obvious reasons. Another thing that worked but also is not preferable is this:

typedef const char* mytype;

typedef int(*cmp)(mytype, mytype);

So what have I missed here?

The problem I'm trying to solve is that I want to create a generic structure where the data is of type mytype . The user of the structure is expected to implement a compare function, but I want it to work with strcmp in case mytype is of type char* .

Note that mytype not necessarily needs to be a pointer. It depends on what data type the user specifies. Also, I know that typedefing pointers is bad practice in general, but I consider this a special case because I want to typedef whatever the type is, and it may be a pointer.

The structure looks like this:

struct node {
    struct node *next;
    mytype data;
};

I managed to solve it with #define mytype char* but that feels very ugly, and I would prefer if there was another way. I want it to be portable, so I don't want to use gcc extensions.

Answer 1

typedef char* mytype;

mytype is a pointer to char . const mytype is a const pointer to char not pointer to const char.

Unfortunately in C you cant use typedef pointers and the use this type to declare pointer to const object.

you need to

typedef int(*cmp)(const char *, const char *);

void foo(cmp f) {}

int main(void) {
    foo(strcmp);
}

or

    foo((cmp)strcmp);

BTW it a very bad practice to hide pointers behind the typedefs.