Can I list-initialize a vector of move-only type?

Question

If I pass the following code through my GCC 4.7 snapshot, it tries to copy the unique_ptrs into the vector.

#include 
#include <         


        

Answer 1

Edit: Since @Johannes doesn't seem to want to post the best solution as an answer, I'll just do it.

#include 
#include 
#include 

int main(){
  using move_only = std::unique_ptr;
  move_only init[] = { move_only(), move_only(), move_only() };
  std::vector v{std::make_move_iterator(std::begin(init)),
      std::make_move_iterator(std::end(init))};
}

The iterators returned by std::make_move_iterator will move the pointed-to element when being dereferenced.

---

Original answer: We're gonna utilize a little helper type here:

#include 
#include 

template
struct rref_wrapper
{ // CAUTION - very volatile, use with care
  explicit rref_wrapper(T&& v)
    : _val(std::move(v)) {}

  explicit operator T() const{
    return T{ std::move(_val) };
  }

private:
  T&& _val;
};

// only usable on temporaries
template
typename std::enable_if<
  !std::is_lvalue_reference::value,
  rref_wrapper
>::type rref(T&& v){
  return rref_wrapper(std::move(v));
}

// lvalue reference can go away
template
void rref(T&) = delete;

Sadly, the straight-forward code here won't work:

std::vector v{ rref(move_only()), rref(move_only()), rref(move_only()) };

Since the standard, for whatever reason, doesn't define a converting copy constructor like this:

// in class initializer_list
template
initializer_list(initializer_list const& other);

The initializer_list> created by the brace-init-list ({...}) won't convert to the initializer_list that the vector takes. So we need a two-step initialization here:

std::initializer_list> il{ rref(move_only()),
                                                   rref(move_only()),
                                                   rref(move_only()) };
std::vector v(il.begin(), il.end());