为什么我需要std :: get_temporary_buffer？ [英] Why do I need std::get_temporary_buffer?

问题描述

为了什么目的，我应该使用 std :: get_temporary_buffer ？标准说明如下：

获取足以存储n个相邻T对象的指针。

我认为缓冲区将被分配在堆栈上，但这不是真的。根据C ++标准，这个缓冲区实际上不是临时的。这个函数对全局函数 :: operator new 有什么好处，它不会构造对象。我是对的，以下语句是等效的？

  int * x; 
 x = std :: get_temporary_buffer< int>（10）.first; 
 x = static_cast< int *>（:: operator new（10 * sizeof（int）））; 
  

此函数只对语法糖存在吗？为什么它的名称中有 temporary ？

---

在博士Dobb's Journal，1996年7月01日用于实现算法：

如果没有可分配的缓冲区，请求，算法仍然正常工作，它只是减慢。

解决方案

Stroustrup在C ++编程语言 §19.4.4，SE）：

想法是系统可能会保留多个固定 - 大小的缓冲区准备好快速分配，因此对于 n 对象的请求空间可以产生超过 n 的空间。它也可能产生较少，但是，使用 get_temporary_buffer（）的一种方法是乐观地要求很多，然后使用发生什么可用。

[...]因为 get_temporary_buffer（）是低级的，并且可能被优化用于管理临时缓冲区，它不应该用作新或 allocator :: allocate（）用于获取长期存储。

也开始引入两个函数：

算法通常需要临时空间来执行可接受的操作。

...但似乎没有在任何地方提供临时或长期的定义。 p>

anecdote .stackoverflow.com / amzn / click / 0321942043> 从数学到通用编程 提到，Stepanov在原STL设计中提供了一个伪造的占位符实现：

令人惊讶的是，他几年后发现所有提供STL实现的主要供应商仍在使用这个可怕的实现[...]

For what purpose I should use std::get_temporary_buffer? Standard says the following:

Obtains a pointer to storage sufficient to store up to n adjacent T objects.

I thought that the buffer will be allocated on the stack, but that is not true. According to the C++ Standard this buffer is actually not temporary. What advantages does this function have over the global function ::operator new, which doesn't construct the objects either. Am I right that the following statements are equivalent?

int* x;
x = std::get_temporary_buffer<int>( 10 ).first;
x = static_cast<int*>( ::operator new( 10*sizeof(int) ) );

Does this function only exist for syntax sugar? Why is there temporary in its name?

---

One use case was suggested in the Dr. Dobb's Journal, July 01, 1996 for implementing algorithms:

If no buffer can be allocated, or if it is smaller than requested, the algorithm still works correctly, It merely slows down.

解决方案

Stroustrup says in "The C++ Programming Language" (§19.4.4, SE):

The idea is that a system may keep a number of fixed-sized buffers ready for fast allocation so that requesting space for n objects may yield space for more than n. It may also yield less, however, so one way of using get_temporary_buffer() is to optimistically ask for a lot and then use what happens to be available.
[...] Because get_temporary_buffer() is low-level and likely to be optimized for managing temporary buffers, it should not be used as an alternative to new or allocator::allocate() for obtaining longer-term storage.

He also starts the introduction to the two functions with:

Algorithms often require temporary space to perform acceptably.

... but doesn't seem to provide a definition of temporary or longer-term anywhere.

An anecdote in "From Mathematics to Generic Programming" mentions that Stepanov provided a bogus placeholder implementation in the original STL design, however:

To his surprise, he discovered years later that all the major vendors that provide STL implementations are still using this terrible implementation [...]

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