带有分配器参数的 std::vector 构造函数

Question

I'm trying out vector and its constructors . On cppreference , the second constructor is listed as:

constexpr explicit vector(const Allocator& alloc) noexcept;

and I couldn't figured out how to use it. I know I can use my own allocator, like:

std::vector<int, myAllocator<int>> iv;

but I wondered what is the use case for passing allocator as an argument for it. Maybe something like:

myallocator<int> ia;
std::vector<int, decltype(ia)> iv(ia);

I don't know what I'm missing, any tips are welcome.

Answer 1

but I wondered what is the use case for passing allocator as an argument for it. Maybe something like:

 myallocator<int> ia; std::vector<int, decltype(ia)> iv(ia);

Yes, that is how you pass an instance of the allocator into the mentioned constructor. That said, the non-allocator constructor value initialises the allocator, so there won't be a difference beyond the extra copying that your example does.