什么是“跨度”？我什么时候应该使用一个？ [英] What is a "span" and when should I use one?

问题描述

最近我得到了在代码中使用 span< T> 的建议，或者在网站上看到了使用 span -应该是某种容器。但是-我在C ++ 17标准库中找不到类似的东西。

Recently I've gotten suggestions to use span<T>'s in my code, or have seen some answers here on the site which use span's - supposedly some kind of container. But - I can't find anything like that in the C++17 standard library.

那么这个神秘的 span< T> 是什么，以及为什么（或何时）

So what is this mysterious span<T>, and why (or when) is it a good idea to use it if it's non-standard?

推荐答案

是什么？

A span< T> 是：

• a的非常轻量级的抽象内存中某个位置的 T 类型的连续值序列。


• 基本上是结构{T * ptr; std :: size_t长度; } 和一堆便捷方法。


• 非所有者类型（即引用类型 ，而不是值类型）：它从不分配或取消分配任何东西，也不使智能指针保持活动状态。

• A very lightweight abstraction of a contiguous sequence of values of type T somewhere in memory.
• Basically a struct { T * ptr; std::size_t length; } with a bunch of convenience methods.
• A non-owning type (i.e. a "reference-type" rather than a "value type"): It never allocates nor deallocates anything and does not keep smart pointers alive.

它以前被称为 array_view 甚至更早于 array_ref 。

It was formerly known as an array_view and even earlier as array_ref.

首先，当不使用它时：

• Don'不要在可能需要任何启动和启动对的代码中使用它结束迭代器，例如 std :: sort ， std :: find_if ， std :: copy 以及所有这些超通用的模板化函数。


• 如果您具有标准库容器（或Boost容器等），则不要使用它知道最适合您的代码。

• Don't use it in code that could just take any pair of start & end iterators, like std::sort, std::find_if, std::copy and all of those super-generic templated functions.
• Don't use it if you have a standard library container (or a Boost container etc.) which you know is the right fit for your code. It's not intended to supplant any of them.

现在何时实际使用它：

使用 span< T> （分别是 span< const T> ），而不是具有长度值的独立的 T * （分别为 const T * ）。因此，替换如下函数：

Use span<T> (respectively, span<const T>) instead of a free-standing T* (respectively const T*) for which you have the length value. So, replace functions like:

  void read_into(int* buffer, size_t buffer_size);

具有：

  void read_into(span<int> buffer);

为什么要使用它？

哦，跨度很棒！使用 span ...

• 是可以使用的该指针+长度/开始+结束指针的组合，就像您使用的是带有花哨的标准库容器一样，例如：

• means that you can work with that pointer+length / start+end pointer combination like you would with a fancy, pimped-out standard library container, e.g.:

• for（auto& x：my_span）{/ *做东西* /}


• std :: find_if（my_span .begin（），my_span.end（），some_predicate）;
  
  
  
  

...但是绝不会产生任何开销，大多数容器类都会产生。

... but with absolutely none of the overhead most container classes incur.

让编译器有时为您做更多的工作。例如：

lets the compiler do more work for you sometimes. For example, this:

int buffer[BUFFER_SIZE];
read_into(buffer, BUFFER_SIZE);

变为：

int buffer[BUFFER_SIZE];
read_into(buffer);

...这将执行您想要的操作。另请参见准则P.5 。

... which will do what you would want it to do. See also Guideline P.5.

是在您期望将 const vector< T>& 传递给函数时的合理替代方案数据在内存中是连续的。

is the reasonable alternative to passing const vector<T>& to functions when you expect your data to be contiguous in memory. No more getting scolded by high-and-mighty C++ gurus!

使用 span ，您可以在 C ++核心指南中找到-

There's even more motivation for using spans, which you could find in the C++ core guidelines - but you catch the drift.

它在标准库中-但仅从C ++ 20开始。原因是它与 C ++结合使用时，它的当前形式仍然很新。核心指南项目，该项目从2015年才开始形成。（尽管评论者指出，它的历史更早。）

It is in the standard library - but only as of C++20. The reason is that it's still pretty new in its current form, conceived in conjunction with the C++ core guidelines project, which has only been taking shape since 2015. (Although as commenters point out, it has earlier history.)

它是核心准则的支持库（GSL）。实现：

It's part of the Core Guidelines's Support Library (GSL). Implementations:

• Microsoft / Neil Macintosh的 GSL 包含一个独立的实现： gsl / span


• GSL-Lite 是整个GSL的单头实现（它并不大，不用担心），包括 span< T> 。

• Microsoft / Neil Macintosh's GSL contains a standalone implementation: gsl/span
• GSL-Lite is a single-header implementation of the whole GSL (it's not that big, don't worry), including span<T>.

GSL实现通常假定平台实现C ++ 14支持[ 14 ]。这些替代的单头实现不依赖于GSL功能：

The GSL implementation does generally assume a platform that implements C++14 support [14]. These alternative single-header implementations do not depend on GSL facilities:

• martinmoene / span-lite 需要C ++ 98或更高版本


• tcbrindle / span 需要C ++ 11或更高版本

• martinmoene/span-lite requires C++98 or later
• tcbrindle/span requires C++11 or later

请注意，这些不同的span实现在它们所使用的方法/支持功能上有所不同。并且它们也可能与标准库C ++ 20中的版本有所不同。

Note that these different span implementations have some differences in what methods/support functions they come with; and they may also differ somewhat from the version going into the standard libraryin C++20.

更多阅读内容：：您可以在C ++ 17，P0122R7之前的最终正式建议中找到所有详细信息和设计注意事项： span：对象序列的边界安全视图，作者：尼尔·麦金托夫（Neal Macintosh）和斯蒂芬·拉瓦韦（Stephan J. Lavavej）虽然有点长。另外，在C ++ 20中，跨度比较语义发生了变化（这篇简短论文，作者：托尼·范·埃德（Tony van Eerd）。

Further reading: You can find all the details and design considerations in the final official proposal before C++17, P0122R7: span: bounds-safe views for sequences of objects by Neal Macintosh and Stephan J. Lavavej. It's a bit long though. Also, in C++20, the span comparison semantics changed (following this short paper by Tony van Eerd).

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