错误：在非结构或联合中请求成员“ id”

Question

I'm writing a function which compares the id of two vertices.

/* Returns whether aim and vertex have the same id */

bool id_eq(void *aim, void *vertex) {
    if(*(Vertex)aim.id ==*(Vertex)vertex.id){
        return true;
}
    return false;
}

aim and vertex are two pointers of struct vertex_t.

typedef struct graph_t* Graph;
typedef struct vertex_t* Vertex;

/* A vertex is an id, a label and a list of incoming and outgoing edges */
struct vertex_t {
    int id;
    char *label;

    /* A list of vertices representing incoming edges */
    List in;
    /* A List of vertices representing outgoing edges */
    List out;
};

But when I compiled it, an error occurred as 'ERROR:request for member 'id' in something not a structure or union'. Could someone please tell me where I went wrong???

Answer 1

The problem you're having is the casts

if( (Vertex)aim.id == (Vertex)vertex.id)

The function receives two void pointers

bool id_eq(void *aim, void *vertex);

Remember when working with structures the -> or . will be taken into account before the cast so this should be solved as following

if( ((Vertex)aim)->id ==((Vertex)vertex)->id)

I'd recommend the return as following, but that's personal taste:

bool id_eq(void *aim, void *vertex) {

    return ((Vertex)aim)->id  == ((Vertex)vertex)->id ? true:false;
}

Edit:

The "? true : false" is not necessary as the == will already result in a bool but it makes the code a little clearer.

Answer 2

Change

if(*(Vertex)aim.id ==*(Vertex)vertex.id){

to

if((Vertex)aim->id == (Vertex)vertex->id){

or

if((*(Vertex)aim).id == (*(Vertex)vertex).id){

(Vertex) casts aim to "a pointer to a struct vertex_t ". Then you can get the id of the struct vertex_t it points to by using the -> operator.

Also note that * (deference) has a lower procedure than . and -> .