Why is there no vector(size_type n, const Allocator& alloc) in C++11?

Question

In C++11, std::vector has the constructor vector(size_type n) which will default construct n items in place, which can be used with default constructible, movable, non-copyable classes.

However, unlike every other vector constructor, there is no variant that takes an allocator, and I have resorted to the following:

// Foo is default constructible and moveable, but not copyable
const int n = 10; // Want 10 default constructed Foos
std::vector<Foo, CustomAllocator> foos(allocator);
foos.reserve(n);
for (int i = 0; i < n; ++i)
   foos.emplace_back();

Is there a better way to accomplish this? Is there a specific reason vector(size_type n, const Allocator& alloc) was omitted from the standard?

Answer 1

After thinking about it, it might not be a defect after all.

It is possible that allocator_type and value_type are perversely the same type. In that case, which function would vector(3, alloc) call? The constructor that takes a default value to copy-initialize into all of the elements, or the one that takes a size and an allocator? That's ambiguous, and thus a compile error.

Answer 2

First, instead of your reserve/loop thingy, you can simply use resize to achieve what your imagined constructor would do:

const int n = 10;
std::vector<Foo, Alloc> foos(allocator);
foo.resize(n);

Another option is to use the three argument version of the size_type n constructor:

const int n = 10;
std::vector<Foo, Alloc> foos(n, Foo(), allocator);

Though this actually copy constructs into the elements, which may or may not be acceptable.

On the rationale? No idea. Probably overlooked.