print vector of objects within an object - c++

Question

I'm trying to print an object Order (actually a vector of Orders). Order has some data members, including a vector with other objects, Purchase.
I can print the vector<Purchase> to cout on its own, and I can print vector<Objects> if I ignore the vector<Purchase> member. But the tricky part is to print vector<Objects> with vector<Purchase> included.
Here is my code:
#include <iostream>
#include <string>
#include <fstream>
#include <vector>
#include <algorithm>
#include <sstream>
using namespace std;
struct Purchase {
string name;
double unit_price;
int count;
};
struct Order {
string name;
string adress;
double data;
vector<Purchase> vp;
};
template<typename Iter> //this is my general print-vector function
ostream& print(Iter it1, Iter it2, ostream& os, string s) {
while (it1 != it2) {
os << *it1 << s;
++it1;
}
return os << "\n";
}
ostream& operator<<(ostream& os, Purchase p) {
return os << "(" << p.name << ", " << p.unit_price << ", " << p.count << ")";
}
ostream& operator<<(ostream& os, Order o) {
vector<Purchase> vpo = o.vp;
ostringstream oss;
oss << print(vpo.begin(), vpo.end(), oss, ", "); //This is what I would like to do, but the compiler doesn't like this conversion (from ostream& to ostringstream)
os << o.name << "\n" << o.adress << "\n" << o.data << "\n"
<< oss << "\n";
return os;
}
int main() {
ifstream infile("infile.txt");
vector<Order> vo;
read_order(infile, vo); //a function that reads a txt-file into my vector vo
print(vo.begin(), vo.end(), cout, "");
return 0;
}
As you can see, I had the idea to use ostringstreams as a temporary variable, that I would store the vector<Purchase> before I pass it on to the ostream& os. But this is a no go. What would be a good solution to this problem?
I am fairly new to C++ and are just learning the different uses of streams, so please bear with me if this is a stupid question.

Answer 1

Looks like you have two minor typos.
First, remove the indicated portion:
oss << print(vpo.begin(), vpo.end(), oss, ", ")
// ↑↑↑↑↑↑↑
Then, later in that same function, you cannot stream a stringstream, but you can stream the string serving as its underlying buffer, so use std::stringstream::str():
os << o.name << "\n" << o.adress << "\n" << o.data << "\n"
<< oss.str() << "\n";
// ↑↑↑↑↑↑
With those fixes in place, and the missing read_order function abstracted away, your program compiles.

Answer 2

The easiest way is to write an overload of operator<< that takes a const reference to a std::vector<Purchase> and then just stream the vector into the ostream:
std::ostream& operator<<(std::ostream& os, const std::vector<Purchase>& v);