How to declare a template function that takes an output_iterator of T?

Question

I want to put objects into a container (which container is not fixed).

For that I want to use the concept std::output_iterator.

How would I define a function that takes for example a std::insert_iterator<std::vector<T>> and a std::insert_iterator<std::list<T>>?

the concept std::output_iterator takes two template arguments : I and T. So i am unsure how I would declare such a function.

I could do it like oldschool <algorithm> and declare it like this:

template<typename OutIter>
void foo(OutIter writer);

but thats not that expressive imo.

---

UPDATE: Here is my attempt based in @RemyLebeau's answer:

#include <iterator>
#include <vector>
template<typename I, typename T, std::output_iterator<I,T> OutIter>
void foo(OutIter writer) {
    writer++ = T();
}

int main() {

    std::vector<int> ints;
    auto inserter = std::insert_iterator(ints,ints.end());
    foo(inserter);
}

https://godbolt.org/z/afe9rz3c4

Answer 1

@Remy Lebeau has the right idea, but not the right execution. The first template argument to a concept is the type being constrained and does not have to be specified when used to restrain another template arg.

Their answer with correct code:

#include <iterator>
#include <vector>

template<typename T, std::output_iterator<T> OutIter> // note: no template argument I required
void foo(OutIter writer) {
    writer++ = T();
}

int main() {

    std::vector<int> ints;
    auto inserter = std::insert_iterator(ints,ints.end());
    foo<int>(inserter); // note you have to specify int when calling foo
    return 0;
}

This is not entirely your use-case, since you don't want to specify the type for foo's iterator when calling the function. But using the output_iterator concept, foo can specify the iterator type itself:

#include <iterator>
#include <vector>

template<std::output_iterator<int> OutIter> // Explicitly requests an iterator that ints can be written to
void foo(OutIter writer) {
    writer++ = int{};
}

int main() {

    std::vector<int> ints;
    auto inserter = std::insert_iterator(ints,ints.end());
    foo(inserter);
    return 0;
}