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

As mentioned in other answers, the behaviour of std::initializer_list is to hold objects by value and not allow moving out, so this is not possible. Here is one possible workaround, using a function call where the initializers are given as variadic arguments:

#include 
#include 

struct Foo
{
    std::unique_ptr u;
    int x;
    Foo(int x = 0): x(x) {}
};

template        // recursion-ender
void multi_emplace(std::vector &vec) {}

template
void multi_emplace(std::vector &vec, T1&& t1, Types&&... args)
{
    vec.emplace_back( std::move(t1) );
    multi_emplace(vec, args...);
}

int main()
{
    std::vector foos;
    multi_emplace(foos, 1, 2, 3, 4, 5);
    multi_emplace(foos, Foo{}, Foo{});
}

Unfortunately multi_emplace(foos, {}); fails as it cannot deduce the type for {}, so for objects to be default-constructed you have to repeat the class name. (or use vector::resize)