CODEWITHSUNDEEP
objective-cios
kran
kran

Reputation: 259

What's the purpose of *(id*)obj?

typedef struct _wax_instance_userdata {
    id instance;
    BOOL isClass;
    Class isSuper; 
    BOOL actAsSuper; 
} wax_instance_userdata;

https://github.com/probablycorey/wax/blob/master/lib/wax_helpers.m#L497

void* afunc(){ // the function is too long
    void *value = nil;
    // ...
    wax_instance_userdata *instanceUserdata = (wax_instance_userdata *)luaL_checkudata(L, stackIndex, WAX_INSTANCE_METATABLE_NAME);

    instance = instanceUserdata->instance;

    *(id *)value = instance;
    return value;
}

https://github.com/probablycorey/wax/blob/master/lib/wax.m#L243

id* ret = afunc(); //same without this * .
id lastValue = *(id*)ret;
//now I can use lastValue;

Why need do this ? I can't understand the *(id*) also the id* ret = afunc() ,when delete this star , it also works well.

Upvotes: 3

Views: 175

Answers (2)

probablyCorey
probablyCorey

Reputation: 2498

afunc is referencing the function (void *)wax_copyToObjc(...). The intent of this function is to translate a Lua object into a C or Objective-C value. Because it could be a primitive type or an objective-c instance it doesn't know what it is going to return. So always returns a pointer to void (meaning a pointer to something unknown). In the case of an id, it will return a pointer to an id.

It might be easier to explain what is happening with an int, it will alloc space for the int and copy the its value:

value = calloc(sizeof(int), 1)
*(int *)value = lua_tointeger(L, stackIndex)

(int *)value is translates to "value is a pointer to an int" adding the * in front of it like *(int *)value translates to "copy the int to the alloc'd memory that value points to."

In your example:

id *ret = afunc(); // returns a pointer to an id
id lastValue = *(id*)ret; // dereferences the pointer to id so it is just an id

Upvotes: 1

ksol
ksol

Reputation: 12235

This looks weird. Since the issues came from what looks like C Code, I suggest to not use id. Although it's supposed to be the same as void*, devs tend to use the first in ObjC code, and the latter in C code. You'd gain in clarity.

Now, I don't see why void* value = instance; and void* ret = afunc(); should not do as expected.

Upvotes: 0

Related Questions