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C ++ 17 lambda capture * this

[英]C++17 lambda capture *this

原文 2017-01-13 14:59:25 5 2 c++/ lambda/ c++17/ c++-faq

C++17 will add copy capture of this object by value, with a capture specification of [*this] . C ++ 17将按值添加此对象的副本捕获 ，捕获规范为[*this] 。

How is this useful? 这有用吗？ How is it different than capturing this ? 它是如何捕捉比不同this ？ Can't this already be achieved in C++14 with [tmp = *this] ? 用[tmp = *this]在C ++ 14中已经不能实现[tmp = *this]吗？

Bonus for explaining why P0018R3 uses [=, tmp = *this] instead of [tmp = *this] in their example. 用于解释为什么P0018R3在其示例中使用[=, tmp = *this]而不是[tmp = *this]奖励。 If they had used [tmp = *this] , all the listed downsides of the C++14 solution would be eliminated. 如果他们使用了[tmp = *this] ，那么C ++ 14解决方案的所有列出的缺点都将被消除。

2 个解决方案

How is it useful? 它有用吗？ It's useful when you need a copy of *this - for example, when *this itself is no longer valid by the time the lambda is evaluated. 当你需要*this的副本时它很有用 - 例如，当lambda被评估时， *this本身不再有效。

How is it different from capturing this ? 它是如何从不同捕捉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. 它创建了对象的副本，因此在评估lambda时，它的this指针引用副本，而不是原始对象。

Can it be achieved in C++14 with [tmp = *this] ? 可以用[tmp = *this]在C ++ 14中实现吗？ It can, but [*this] is more convenient, as code can be moved without prefixing member access with tmp. 它可以，但[*this]更方便，因为可以移动代码而无需使用tmp.为成员访问添加前缀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). 否则，特别是[=, tmp = *this] ，当您打算引用副本时（特别是如果您习惯于剪切+粘贴编程），可能会意外地引用原始对象的成员。 [=,*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). [=,*this]在这种情况下是一个更安全的替代方案，因为原始对象不能从lambda体内部访问（至少不能通过this指针）。

Imagine that *this is a handle class, which maintains a shared_ptr to some shared state. 想象一下*this是一个句柄类，它将shared_ptr维护到某个共享状态。

The shared impl is (for example) a protocol handler state machine. 共享impl是（例如）协议处理程序状态机。

The handle class is passed through a series of asynchronous handlers, so itself must be copyable. handle类通过一系列异步处理程序传递，因此它本身必须是可复制的。 Each handler mutates the shared state. 每个处理程序都会改变共享状态。

A strong use case for this might be a protocol handler for use with a custom asio service (for example, an http_protocol_socket ). 一个强大的用例可能是与自定义asio服务一起使用的协议处理程序（例如， http_protocol_socket ）。

[=, tmp = *this] will promiscuously capture any variables by value, including, rather dangerously, the this pointer itself, as well as specifically capturing *this into tmp . [=, tmp = *this]将以值的形式捕获任何变量，包括相当危险的， this指针本身，以及特别是将*this捕获到tmp 。

In this use case it would be dangerous to inadvertently refer to this in the async handler, because it's likely to be a dangling pointer. 在这种情况下，使用它会是危险的不经意指this在异步处理程序，因为它很可能是一个悬摆指针。 This is a bug waiting to happen. 这是一个等待发生的错误。

[tmp=*this] will only capture *this . [tmp=*this]只捕获*this 。