Re: Type visibility

I have made two modules:

1) main.cpp
 Contains a main function and struct P.
 Includes head.h and std::vector

#include "head.h"
#include<vector>
#include<iostream>
struct P {
  int x;
  int y;
};

int main() {
  typedef std::vector<P> container;
  container v;
  P p;
  p.x = 22;
  p.y = 33;
  v.push_back(p);

  A<container> a(v);
  int res = a.getValue();
  std::cout << res << std::endl;
 return 0;
}

2) head.h
 A header file that contains the class A.

template <typename T >
class A {
  public:
    T t;

  A(T t_){
    t = t_;
  }
  int getValue() {
    int res = (*t.begin()).x;
    return res;

  }
};

In class A getValue() returns the x field of the struct defined in
main.cpp.
But how does class A know about the type 'P' which is only created in
main?