: dynarray vs std: : Vector

C + + 14出品 std::dynarray:

Dynarray 是一个 Vector，用一个在施工时固定并且在整个施工过程中不会变化的尺寸对象的生存期。

std::dynarray必须与 std::vector一样在运行时分配。

那么，std::dynarray的好处和用途是什么，而我们可以使用 std::vector，这是更动态(也可以重新调整大小) ？

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As you said yourself, std::dynarray is for a fixed-size dynamic array. It is not resizable. It's roughly speaking an improvement over new T[N] and over std::unique_ptr<T[]>(new T[N]).

Not needing to resize or manage capacity means you can implement the data structure with less complexity and in less space.

Moreover, std::dynarray is a weird animal that allows the implementation to implement it in different, non-specific ways, e.g. it's possible to put the array on the stack. Calling an allocation function is "optional". You can specify an allocator to construct the elements of the array, but that is not part of the type.

You might also wonder why we need std::dynarray and variable-length arrays. VLAs in C++14 are much more restrictive; they can only be local, automatic variables and offer no way to specify an allocation policy, and of course they don't have a standard container interface.

Some examples from 23.3.4.2 of a "current draft" (take that, Google cache):

explicit dynarray(size_type c);
Effects: Allocates storage for c elements. May or may not invoke the global operator new.

template <class Alloc>
dynarray(size_type c, const Alloc& alloc);
Effects: Equivalent to the preceding constructors except that each element is constructed with uses-allocator construction.

Whether or not you can use a given allocator to construct the array elements is a global trait:

template struct uses_allocator, Alloc> : true_type { };

Requires: Alloc shall be an Allocator (17.6.3.5). [Note: Specialization of this trait informs other library components that dynarray can be constructed with an allocator, even though it does not have a nested allocator_type.]

Edit: Jonathan Wakely's answer is bound to be far more authoritative and insightful.