对标准库分配器指针类型的要求 [英] Requirements on standard library allocator pointer types

问题描述

我想写一个四叉树稀疏矩阵类。简而言之， quadtree_matrix 是零矩阵或四元（ne，nw，se，sw） quadtree_matrix< T> 。

I am trying to write a quadtree sparse matrix class. In short, a quadtree_matrix<T> is either the zero matrix or a quadruple (ne, nw, se, sw) of quadtree_matrix<T>.

我想最终测试不同的分配方案，因为这可能会影响线性代数运算的性能。因此，我还将在标准分配器类型上使用模板 quadtree_matrix ，以便我可以重用现有的分配器。

I'd like eventually to test different allocation schemes since this will probably impact the performance of linear algebra operations. So I will also template quadtree_matrix on a standard allocator type, so that I can reuse existing allocators.

必须分配两种不同类型的数据： T 或节点，其中包含四个指针T或节点）。对于所有的算法，我会考虑，我知道确切的是什么样的数据，因为我知道什么是我在算法的任何点（我甚至不需要存储这些大小）的子矩阵的大小。

I will have to allocate two different kind of data: either a T, or a node, which contains four pointers (to either T or node). For all the algorithms I will consider, I know for sure what kind of data to expect because I know what are the sizes of the submatrices I am facing at any point of the algorithm (I don't even need to store these sizes).

我当然会使用两个不同的分配器：这是确定，因为分配器类型提供 rebind 模板，一个模板复制构造函数（并且意图用作值类型，因为 get_allocator 标准容器的成员建议通过返回一个副本）。

I will of course be using two different allocators: this is ok, since allocator types provide the rebind template and a template copy constructor (and are intended to be used as value types, as the get_allocator members of standard containers suggest by returning a copy).

问题是allocator的成员函数使用一个指针类型，这不是一个必须的指针。一些分配器（boost进程间分配器）广泛地使用此功能。

The problem is that allocator member functions use a certain pointer type, which is not required to be a vanilla pointer. Some allocators (boost interprocess allocators) use this feature extensively.

如果分配器指针类型是花园种类指针，我会没有问题：至少，我可以使用指针void和reinterpret_cast他们正确的类型 node * 或 T * ）。我也可以使用一个联合（可能更好）。

If the allocator pointer types were garden variety pointers, I would have no problems: at the very least, I could use pointers to void and reinterpret_cast them to the right type (either node* or T*). I could also use a union (probably better).

据我所知，没有要求PODness的 allocator ::指针类型。他们只需要是随机访问迭代器。

As far as I know, there is no requirement on the PODness of the allocator::pointer types. They are only required to be random access iterators.

现在，我的问题是：

分配器类模板 A （或其等效 A :: rebind< T>其他是否有任何保证：

Given an allocator class template A<T> (or its equivalent A::rebind<T>::other), is there any guarantee on:

1. 静态强制转换的能力 A< T& :: pointer to A< U> :: pointer 提供 U T ？


2. 静态强制转换的能力 A< T& ：指向到 A< U> ::指针提供 T U 并且castee的运行时类型（在此上下文中的任何内容）是 U ？


3. 类型 A< void> :: pointer （如果有意义）？ li>
   

1. The ability to static cast A<T>::pointer to A<U>::pointer provided U is an accessible base of T ?
2. The ability to static cast A<T>::pointer to A<U>::pointer provided T is an accessible base of U and the "runtime type" (whatever this means in this context) of the castee is U ?
3. The type A<void>::pointer (if this makes sense) ?

还是有我的问题的解决方案我没有想过？

Or is there a solution to my problem I didn't think about ?

推荐答案

从20.1.5 / 2中的表中可以清楚地看出， A< T> :: pointer 的类型必须是指向 T 。由于这些指针类型通常可以转换你的1和2是真的。然后， A< void> :: pointer 必须是 void * 。

From the tables in 20.1.5/2 it clearly indicates that the type of A<T>::pointer must be "pointer to T". Since those pointer types are normally convertible your 1 and 2 are true. It follows then that A<void>::pointer must be void*.

编辑：
在20.1.5 / 4中也有明确的措辞（它适用于什么标准容器可能假设的分配器）：

There's also explicit wording in 20.1.5/4 (it applies to what standard containers may assume about allocators):

typedef成员指针
const_pointer，size_type和
difference_type需要为
T *，T const *，size_t和ptrdiff_t，
。

The typedef members pointer, const_pointer, size_type, and difference_type are required to be T*,T const*, size_t, and ptrdiff_t, respectively.

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