Why do I get “increment of pointer to unknown structure”?

Question

I'm trying to bulid a structure named "cache", which holds a container (an array of type List which is already implemented) and an iterator. when I'm trying to make the iterator point at the next List in the container I get "increment of pointer to unknown structure" and "arithmetic on pointer to an incomplete type". I would love an explanation as to what I'm doing wrong:

List is typedefed as follows:

typedef struct List_t *List;

so List is a pointer to a List_t structure.

Cache is defined as follows:

typedef struct cache_t* Cache;

struct cache_t {
    List* container;
    int cache_size;
    List iterator;
};

so iterator is a List which is a pointer to a List_t in an array (container). What I'm trying to do is:

cache->iterator ++; //cache is an object of type Cache

which, as far as I know, should move iterator by sizeof(List) to the next List in the array. so why am I getting errors?

thanks!

Answer 1

The pointer is not incremented by sizeof(List) but by sizeof(struct List_t) . The definition of struct List_t is not there in your source, and is thus an incomplete type. This is the reason for the error.