On 15 Sep 2014, at 9:39, 何子杰Hzj_jie <hzj_jie@hotmail.com> wrote:
hi, all,
So I proposal a movable_initializer_list, it accepts two ways to construct, with an initializer_list<T*> or initializer_list<T>, the prototype is,
The implementation of T* overloads is something like
movable_initializer_list<std::initializer_list<T*> x)
{
for(auto it : x)
{
assert((*it) != nullptr);
v.push_back(std::move(**it));
delete *it;
} }
Should be for (auto p : x) { v.push_back(std::move(*p)); delete p; } because of how for-range works. Seems an incomplete manual change from your published source. However, this is highly problematic because: - There is absolutely no way to make this exception-safe. The intended usage is obviously: SomeType st = { new Foo(123), new Foo(456), new Foo(789) }; But if any constructor here throws, a lot of memory is leaked, and destructors are not called. - If efficiency is your goal, I somehow doubt that an additional dynamic allocation per element for the initial list, then building a vector element by element, is the best way to achieve it. - The interface to this is just horrible. Looking at your test case, there is an absurd amount of boilerplate. Here’s what we really want: SomeType st{ Foo(123), Foo(456), Foo(789) }; Here’s what your thing forces me to do: auto pointers = { new Foo(123), new Foo(456), new Foo(789) }; SomeType st(pointers); // Absolute best case: no ambiguity in overload resolution - actually rather doubtful. If you do it that way, you might as well do it this way: Foo[] foos = [ Foo(123), Foo(456), Foo(789) }; // No dynamic allocation SomeType st(make_move_iterator(begin(foos)), make_move_iterator(end(foos))); More efficient, and you could make helpers that combine the make_move_iterator and begin/end calls to reduce boilerplate.
While the implementation of T overloads is something like
moveable_initializer_list<std::initializer_list<T> x) {
for(auto it : x) v.push_back(move(*it)); }
Again, that’s not how for-range works. What you want is for (auto&& e : x) v.push_back(std::move(e)); But that doesn’t actually work, because a std::initializer_list never gives you non-const access to its elements. I notice that your test case doesn’t test this constructor. Sebastian