Does Swift not work with function pointers?

Question

I'm trying to use a C library in Swift, and I'm having trouble calling any function that takes a function pointer as one of it's arguments. For example, part of the lua.h file that I'm trying to use in Swift looks like this:

LUA_API void  (lua_setuservalue) (lua_State *L, int idx);


typedef int (*lua_CFunction) (lua_State *L);

LUA_API void  (lua_callk) (lua_State *L, int nargs, int nresults, int ctx,
                       lua_CFunction k);

I use the bridging header to get access to the library, and from my Swift code I can call lua_setuservalue without any trouble. But if I try to call lua_callk I get "use of unresolved identifier 'lua_callk'". If I remove the function pointer from the declaration for lua_callk, I no longer get this error. Any help is quite appreciated.

Answer 1

This answer refers to an earlier version of the Swift language and may no longer be reliable.

While C function pointers are not available in Swift, you can still use swift closures which are passed to C functions as blocks.

Doing so requires a few "shim" routines in C to take the block and wrap it in a C function. The following demonstrates how it works.

Swift:

func foo(myInt: CInt) -> CInt {
    return myInt
}

var closure: (CInt) -> CInt = foo;

my_c_function(closure)

C:

void my_c_function(int (^closure)(int))
{
    int x = closure(10);
    printf("x is %d\n", x);
}

Of course what you choose to do with the closure, and how you store and recall it for use is up to you. But this should give you a start.