Question

I have read the topic about “pointer”, but i still have some question.

// graph.cpp

struct Edge {
    int from;
    int to;
    unsigned int id;
    Edge(): from(0), to(0), id(0) {};
};

struct Vertex {
    int label;
    vector<Edge> edge;
};

class Graph: public vector<Vertex> {
    int gid;
    unsigned int edge_size;
};

if I declare a iterator in another file

bool get_forward_root (Graph &g, Vertex &v, vector<Edge*> &result) {
    for(vector<Edge>::iterator it = v.edge.begin(); it != v.edge.end(); it++) {
        if(v.label <= g[it->to].label)
        result.push_back(&(*it));
    }
}

In my understanding, it can be viewed as pointer, since v.edge.begin() is the first Edge object in vector<Edge>, but what is &(*it) ?

Question 2. What is the difference between g, &g, *g ?

In my understanding:

• &g is the memory address.
• *g is a Graph pointer point to a graph object, so we can use Graph *g = new Graph();
• g is a Graph object

the difference between *g and g is how we use, for example the two conditions are the same:

condition 1:

Graph *g = new Graph();
g->gid = 0;

condition 2:

Graph g;
g.gid = 0;

Question 3.

what is below meaning?

Graph &g

and why we use g[it->to].label not &g[it->to].label
Thank very much:)

Answer 1

Question 1: what is &(*it)

it acts like a pointer, but it’s not a pointer. If it were a pointer, &*it would be the same as it. In the general case, &(*it) is the address (a real pointer) of the object that the iterator it points to. We can assume here that the & operator was not overloaded.

Question 2: What is the difference between g, &g, *g?

g is g. &g is the address of g. *g is the object g points to (if g is a pointer). Your 2 conditions (I don’t understand why you call them conditions) do pretty much the same thing, yes.

Question 3: what is Graph &g?

It’s called a reference. When defined, it should be immediately initialized. Think of a reference as another name of an object. (Better, read a book, see below).

All of your questions will be thoroughly answered in any decent C++ beginner book. I especially recommend Lippman’s C++ primer for this purpose. Find other good titles here.