移动语义==自定义交换函数过时？

本文介绍了移动语义==自定义交换函数过时？的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

最近，许多 疑问 pop up 如何提供您自己的交换功能。使用C ++ 11， std :: swap 将使用 std :: move 并移动语义以交换给定值尽可能快。这当然只有当你提供一个移动构造函数和一个移动赋值操作符（或使用传递值）的时候才有效。

Recently, many questions pop up on how to provide your own swap function. With C++11, std::swap will use std::move and move semantics to swap the given values as fast as possible. This, of course, only works if you provide a move constructor and a move assignment operator (or one that uses pass-by-value).

现在， ，实际上是否需要在C ++ 11中编写自己的 swap 函数？我只能想到不可移动类型，但是再次，自定义 swap 通常通过某种指针交换（也称为移动）工作。也许有一些参考变量？ Hm ...

Now, with that given, is it actually necessary to write your own swap functions in C++11? I could only think of non-movable types, but then again, the custom swaps usually work through some kind of "pointer exchange" (aka moving). Maybe with certain reference variables? Hm...

推荐答案

这是一个判断的问题。我通常让std :: swap做原型代码的工作，但对于release代码编写一个自定义交换。我通常可以写一个自定义交换，大约是1移动建设+ 2移动任务+ 1无资源破坏的两倍快。

It is a matter of judgment. I will typically let std::swap do the job for prototyping code, but for release code write a custom swap. I can usually write a custom swap that is about twice as fast as 1 move construction + 2 move assignments + 1 resourceless destruction. However one may want to wait until std::swap actually proves to be a performance problem before going to the bother.

更新为Alf P. Steinbach：

Update for Alf P. Steinbach:

20.2.2 [utility.swap]指定 std :: swap（T&）有 noexcept 等效于：

20.2.2 [utility.swap] specifies that std::swap(T&, T&) has a noexcept equivalent to:

template <class T> void swap(T& a, T& b) noexcept ( is_nothrow_move_constructible<T>::value && is_nothrow_move_assignable<T>::value );

如果 T 上的移动操作为noexcept，则 std :: swap 在 T is notexcept。

I.e. if move operations on T are noexcept, then std::swap on T is noexcept.

请注意，此规范不需要移动成员。它只需要从右值的构造和赋值存在，如果它是noexcept，则交换将不会被忽略。例如：

Note that this spec doesn't require move members. It only requires that construction and assignment from rvalues exists, and if it is noexcept, then swap will be noexcept. E.g.:

class A { public: A(const A&) noexcept; A& operator=(const A&) noexcept; };

std :: swap< A> noexcept，即使没有移动成员。

std::swap<A> is noexcept, even without move members.