C ++ 17 Lambda捕获*此 [英] C++17 lambda capture *this

问题描述

C ++ 17将按值添加此对象的副本捕获，其中捕获规范为 [* this] 。

C++17 will add copy capture of this object by value, with a capture specification of [*this].

这有什么用？与捕获 this 有何不同？不能在C ++ 14中使用 [tmp = * this] 实现吗？

How is this useful? How is it different than capturing this? Can't this already be achieved in C++14 with [tmp = *this]?

奖金，用于解释为什么 P0018R3 使用 [=，tmp = * this] 代替 [tmp = * this] 在他们的例子中。如果他们使用 [tmp = * this] ，则将消除C ++ 14解决方案列出的所有缺点。

Bonus for explaining why P0018R3 uses [=, tmp = *this] instead of [tmp = *this] in their example. If they had used [tmp = *this], all the listed downsides of the C++14 solution would be eliminated.

推荐答案

它有什么用？当您需要 * this 的副本时非常有用-例如，当 * this 本身不再有效时，

How is it useful? It's useful when you need a copy of *this - for example, when *this itself is no longer valid by the time the lambda is evaluated.

与捕获 this 有何不同？它创建对象的副本，以便在评估lambda时，其 this 指针指向副本，而不是原始对象。

How is it different from capturing this? It makes a copy of the object, so that when the lambda is evaluated, its this pointer refers to the copy, rather than to the original object.

在C ++ 14中可以使用 [tmp = * this] 来实现吗？可以，但是 [* this] 更方便，因为可以在不使用 tmp前缀成员访问的情况下移动代码。 。否则，尤其是对于 [=，tmp = * this] ，当您要引用副本时，可能会意外地引用原始对象的成员（尤其是如果重新剪切/粘贴编程的习惯）。在这种情况下， [=，* this] 是一个更安全的选择，因为原始对象无法从lambda体内访问（至少不能通过 this 指针）。

Can it be achieved in C++14 with [tmp = *this]? It can, but [*this] is more convenient, as code can be moved without prefixing member access with tmp.. Otherwise, especially with [=, tmp = *this], it's possible to accidentally refer to members of the original object when you meant to refer to the copy (particularly if you're in the habit of cut+paste programming). [=,*this] is a safer alternative in this case, as the original object is not accessible from inside the lambda's body (at least, not via the this pointer).

这篇关于C ++ 17 Lambda捕获*此的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！