C++核心準(zhǔn)則F.53:非局部使用變量時(shí)避免使用值捕捉

F.53: Avoid capturing by reference in lambdas that will be used nonlocally, including returned, stored on the heap, or passed to another thread

如果捕捉的變量可以能被非局部使用，則應(yīng)避免在lambda表達(dá)式中使用引用形式的捕捉。非局部使用包括返回，保存在堆中，或者傳遞給其他線(xiàn)程。

Reason（原因）

Pointers and references to locals shouldn't outlive their scope. Lambdas that capture by reference are just another place to store a reference to a local object, and shouldn't do so if they (or a copy) outlive the scope.

指向局部變量的指針和引用的生命周期不應(yīng)該長(zhǎng)于它們的作用域。通過(guò)引用捕捉變量的lambda表達(dá)式只是另外一個(gè)保存指向局部變量的引用的場(chǎng)所，如果這個(gè)lambda表達(dá)式（或拷貝）的生命周期超過(guò)作用域時(shí)不應(yīng)該使用引用形式的變量捕捉。

Example, bad（反面示例）

int local = 42;
// Want a reference to local.// Note, that after program exits this scope,// local no longer exists, therefore// process() call will have undefined behavior!thread_pool.queue_work([&]{ process(local); });
Example,?goodint?local?=?42;// Want a copy of local.// Since a copy of local is made, it will// always be available for the call.thread_pool.queue_work([=]{ process(local); });

• (Simple) Warn when capture-list contains a reference to a locally declared variable

  （簡(jiǎn)單）當(dāng)捕捉列表中包含本地聲明的變量時(shí)報(bào)警。
  

• (Complex) Flag when capture-list contains a reference to a locally declared variable and the lambda is passed to a non-const and non-local context

  （復(fù)雜）在捕捉列表中包含指向局部聲明的變量而且lambda表達(dá)式被傳遞給非常量或非局部上下文時(shí)做出提示。

???? 譯者注：第一種情況容易實(shí)現(xiàn)但并不完美。

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