C++ generic insert into std container?

Question

If I have the following program:

#include <vector>
#include <set>

template<class T, class U>
void AddToContainer(T& container, U value)
{
  container.push_back(value);
}

int main(char**, int)
{
   std::vector<int> v;
   AddToContainer(v, 1);

   std::set<int> s;
   AddToContainer(s, 1);

   return 0;
}

How can I make the adding to the container generic? Since std::set hasn't got a push_back but only insert, this will fail to compile.

Answer 1

You could use expression SFINAE with a dummy parameter to check if push_back() works:

template <class C, class V>
auto append(C& container, V&& value, int)
    -> decltype(container.push_back(std::forward<V>(value)), void())
{
    container.push_back(std::forward<V>(value));
}

template <class C, class V>
void append(C& container, V&& value, ...)
{
    container.insert(std::forward<V>(value));
}

which your function will just forward to:

template <class C, class V>
void AddToContainer(C& container, V&& value) {
    append(container, std::forward<V>(value), 0);
}

If push_back() is a valid expression, the first overload will be preferred since int is a better match for 0 than ... If push_back() isn't a valid expression, then there's only one viable overload.

---

Whether this is actually a good idea or not is a separate question.

Answer 2

I believe that all* C++ containers (though not the container adapters like priority_queue) have a version of insert that looks like this:

iterator insert(iterator location, T&& value)

For the sequence collections, the location is the actual location; for associative collections (like map and unordered_map), the iterator is a "hint" parameter (for instance, to help a map insert an element quickly if you already know precisely where it belongs in sorted order). However, providing an invalid hint doesn't cause any invalid behavior, so a valid generic insert for C++ collections would be:

template<C, T>
void insert(C& collection, T&& value) {
    collection.insert(collection.end(), std::forward<T>(value));
}

* It looks like forward_list is the only one that doesn't have this method, which makes sense.