Capped back insert iterator (C++)

cbi.cc

#include <utility>      // std::forward
#include <iterator>     // std::iterator, std::output_iterator_tag

template <class Container>
struct capped_back_insert_iterator final: std::iterator<std::output_iterator_tag, void, void, void, void>
{
    capped_back_insert_iterator(Container & con, int cap)
        : con{con}, capacity{cap}, size{0} {}

    capped_back_insert_iterator & operator * () { return *this; }
    capped_back_insert_iterator & operator ++ () { return *this; }
    capped_back_insert_iterator & operator ++ (int) { return *this; }

    template <class T>
    capped_back_insert_iterator & operator = (T && x)
    {
        if (size < capacity) {
            size++;
            con.push_back(std::forward<T>(x));
        }
        return *this;
    }

private:
    Container & con;
    int capacity;
    int size;
};

template <class Container>
auto capped_back_inserter(Container & con, int cap)
{
    return capped_back_insert_iterator<Container>{con, cap};
}

//////////////////// test ////////////////////
#include <vector>
#include <algorithm>
#include <iostream>

int main()
{
    std::vector<int> a{1, 2, 3, 4, 5, 6, 7, 8};
    std::vector<int> b;
    std::copy(begin(a), end(a), capped_back_inserter(b, 3));
    for (auto x: b) std::cerr << x << "\n";
}