MoveInsertable 和 CopyInsertable 之间的区别?

2022-01-17 00:00:00 compiler-construction c++ c++11

有人可以对这两个术语提供更清晰的解释吗?

Can someone provide a more lucid explanation of these two terms?

换句话说，请用例子做一些简单的解释.

In other words, some simple explanation with an example, please.

(来自:cppreference.com)

(from : cppreference.com)

MoveInsertable : 指定一个类型的右值可以复制到未初始化的存储中.

MoveInsertable : Specifies that a rvalue of the type can be copied in uninitialized storage.

CopyInsertable :指定一个实例type 可以在未初始化的存储中就地复制构造.

CopyInsertable : Specifies that an instance of the type can be copy-constructed in-place, in uninitialized storage.

推荐答案

这些需求是类型 T 和容器 X 之间的关系.容器有一个分配器类型，A，用于为其包含的对象分配内存.

These requirements are a relationship between a type T and a container X. A container has an allocator type, A, which it uses to allocate the memory for its contained objects.

如果 m 是这些分配器之一，p 是 T*，rv 是 rv 类型的右值code>T 和 v 类型为 T 的表达式:

If m is one of these allocators, p a T*, rv an rvalue of type T, and v an expression of type T:

CopyInsertable 由标准定义:

T is CopyInsertable into X 表示下面的表达式是良构的:

T is CopyInsertable into X means that the following expression is well-formed:

allocator_traits<A>::construct(m, p, v);

MoveInsertable 由标准定义:

T is MoveInsertable into X 表示下面的表达式是良构的:

T is MoveInsertable into X means that the following expression is well-formed:

allocator_traits<A>::construct(m, p, rv);

现在要理解这些定义，我们必须知道 allocator_traits<A>::construct 做了什么.很简单，在这种情况下它调用:

Now to understand these definitions, we must know what allocator_traits<A>::construct does. Quite simply, in this case it calls:

m.construct(p, v) // CopyInsertable case m.construct(p, rv) // MoveInsertable case

^{v 和 rv 在这里仍然有各自的值类别，因为 std::forward 应用于 allocator_traits<A>::construct.}

^{v and rv still have their respective value categories here because std::forward is applied to the argument of allocator_traits<A>::construct.}

那么分配器的construct 成员函数有什么作用呢?好吧，正如你所料，它在 p 位置构造了一个 T 类型的对象，方法是:

So what does an allocators construct member function do? Well, as you might expect, it constructs an object of type T at the location p by doing:

::new ((void*)p) T(v) // CopyInsertable case ::new ((void*)p) T(rv) // MoveInsertable case

^{同样，v 和 rv 是 std::forwarded.}

^{Again, v and rv are std::forwarded.}

当然，它们会分别调用复制或移动构造函数.

Of course, these will invoke the copy or move constructors respectively.

所以:

T is CopyInsertable into X:X的分配器可以放置-new 构造 T 的元素，传递 T
类型的表达式
T is MoveInsertable into X:X的分配器可以放置-new 构造 T 的元素，传递 T
类型的右值

T is CopyInsertable into X: the allocator for X can placement-new construct an element of T, passing an expression of type T

T is MoveInsertable into X: the allocator for X can placement-new construct an element of T, passing an rvalue of type T

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