Help about STL copy algorithm

I defined a class `test' and also overloaded the << operator for it. It came out that I can apply cout to objects of test to print the contents to the console, but I got stuck when trying to print them to the console using copy. It'll be a compile-time error when using copy!

Is there anything wrong with my code? Do I need to overload more operators?

Code:

#include <algorithm>
#include <iostream>
#include <iterator>
#include <string>
#include <vector>

using namespace std;

class test {
        friend ostream& operator <<(ostream &out, test &t);
public:
        test(int n) { a = n; }
private:
        int a;
};

int main()
{
        vector<test> vec;
        vec.push_back(1);
        vec.push_back(2);
        for ( vector<test>::iterator it = vec.begin(); it != vec.end(); ++it )
        {
                cout << *it << ' ';
        }
// compile-time error when using copy
//        copy(vec.begin(), vec.end(), ostream_iterator<test>(cout, " "));
        cout << endl;
}

ostream& operator <<(ostream &out, test &t)
{
        return (out << t.a);
}

it works for me. No compile time errors at all.

that's because I commented out the errorous code

Code:

//        copy(vec.begin(), vec.end(), ostream_iterator<test>(cout, " "));

it'll be a compile-time error if you uncomment this line.

Works if I make your operator << take const 'test' references

As a general rule, if you are not going to change something, declare it const. The standard libraries apparently require your code to be const-correct.