Pointing to vectors

#include <iostream>
#include <vector>

using namespace std;

int main () 
{
    vector <int> qwerty;
    qwerty.push_back(5);

    vector <int>* p = &qwerty;

    cout << p[0];  //error: no match for 'operator<<' in 'std::cout << * p'

}

I'm g开发者_如何学Pythonenerally unclear on how to use pointers with vectors, so I'm pretty mystified as to why this is not working. To my mind, this should print 5 to screen.

---

// either
cout << (*p)[0];
// or
cout << p->operator[](0);

---

Your 'cout' line is equivalent to:

cout << qwerty;

because p is a pointer to qwerty, which you then dereference with [0].

qwerty is a vector of type int, which can't be printed directly.

If you look at http://www.cplusplus.com/reference/stl/vector/ , you can see there is a class method for [] overload, so qwerty[0] would return an int.

So cout << qwerty[0]; would work.

---

To better understand what does "p[0]" mean, you can try the following statement:

cout << p[0][0]; 

this statement will enable you to print out "5" on the console. Because p[0] return the object reference "qwerty", then since vector object support index operation, you can use (p[0])[0] to get the first element in the vector.