解引用指向void的指针

Question

I am working on an implementation of a hashmap in C, and the function for CMapPut is defined as follows:

void CMapPut(CMap *cm, const char *key, const void *elemAddr) 

My question is how do I retrieve the actual value of the element passed into the map? That is, when the client passes in the variables, he passes in the address of the value. In this case it appears to be a void * though, and of course you can't dereference a void * . Any tips?

Answer 1

You should be able to cast the pointer to the type you need:

typedef struct {
    ...
} CMyType;

...

CMyType myinstance;
CMapPut(cm, "key", &myinstance);

I guess you'll store the pointer as const void* . I guess you have a function like CMapGet , you use to retrieve your mapped objects like this:

void *CMapGet(CMap *cm, const char *key);

...

CMyType* myinstance_ptr = CMapGet(cm, key);

Answer 2

Cast your void * to your appropiate type, as mentioned by others. Second, you don't have to const your attribute elemAddr . It's no use, since you probably want to return a non-const pointer anyway.