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?

Manilla

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!

Bask

Yes, that's basically what I wrote before deleting my answer. I have a doubt though: why is an initializer_list<> created here? std::pair<int, non_copyable> does not seem to have a constructor that takes one. I thought the uniform initialization syntax would just pick the regular constructor of pair<>. – Trescott

Also, initializer_list<> elements must be all of the same type, no? – Scilicet

+1 to both comments. There shouldn't even be an initializer_list in my example. It's more like a call to std::pair's constructor using uniform initialization. – Manilla

I suppose what's surprising is that map does not have an insert overload for value_type &&, while vector does. – Bask

@KerrekSB C++17 added one, BTW – Turbofan

@n.caillou: Thanks, four-years-from-the-future person :-) – Bask

I nevertheless appear to be programming six years ago, since I ran into this very problem! – Turbofan

Is there any solution for C++03? I am programming 10 years ago :'-( – Coarctate

@Fabian: You mean you have a non-copyable type in C++03? I don't think a std::map will work with that directly, though you could build your own "fork" that has a dedicated "emplace-like" insertion function for your specific data type, if you really need this feature. Or you could use more indirection and store pointers, but that wouldn't quite be comparable. – Bask

Answer 2

Besides other answers of providing move (assignment) constructor, you could also store the non-copyable object through pointer, especially unique_ptr. unique_ptr will handle resource movement for you.

Carinacarinate

This is confusing answer. unique_ptr will not handle resource movement of the object, but rather you are storing only the pointer to the object itself, therefore no resources of the object are moved - only the unique_ptr itself is moved. – Smithereens