c++ - std::map<>::insert using non-copyable objects and uniform initialization

Question

Have a look at the following code:

#include <utility>
#include <map>

// non-copyable but movable
struct non_copyable {
    non_copyable() = default;

    non_copyable(non_copyable&&) = default;
    non_copyable& operator=(non_copyable&&) = default;

    // you shall not copy
    non_copyable(const non_copyable&) = delete;
    non_copyable& operator=(const non_copyable&) = delete;
};

int main() {
    std::map<int, non_copyable> map;
    //map.insert({ 1, non_copyable() });  < FAILS
    map.insert(std::make_pair(1, non_copyable()));
    // ^ same and works
}

Compiling this snippet fails when uncommenting the marked line on g++ 4.7. The error produced indicates that non_copyable can't be copied, but I expected it to be moved.

Why does inserting a std::pair constructed using uniform initialization fail but not one constructed using std::make_pair? Aren't both supposed to produce rvalues which can be successfully moved into the map?

Answer 1

[This is a complete rewrite. My earlier answer had nothing to do with the problem.]

The map has two relevant insert overloads:

• insert(const value_type& value), and

• <template typename P> insert(P&& value).

When you use the simple list-initializer map.insert({1, non_copyable()});, all possible overloads are considered. But only the first one (the one taking const value_type&) is found, since the other doesn't make sense (there's no way to magically guess that you meant to create a pair). The first over­load doesn't work of course since your element isn't copyable.

You can make the second overload work by creating the pair explicitly, either with make_pair, as you already described, or by naming the value type explicitly:

typedef std::map<int, non_copyable> map_type;

map_type m;
m.insert(map_type::value_type({1, non_copyable()}));

Now the list-initializer knows to look for map_type::value_type constructors, finds the relevant mova­ble one, and the result is an rvalue pair which binds to the P&&-overload of the insert function.

(Another option is to use emplace() with piecewise_construct and forward_as_tuple, though that would get a lot more verbose.)

I suppose the moral here is that list-initializers look for viable overloads – but they have to know what to look for!