C++核心准则F.53：非局部使用变量时避免使用值捕捉

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

如果捕捉的变量可以能被非局部使用，则应避免在lambda表达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，或者传递给其他线程。

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.

指向局部变量的指针和引用的生命周期不应该长于它们的作用域。通过引用捕捉变量的lambda表达式只是另外一个保存指向局部变量的引用的场所，如果这个lambda表达式（或拷贝）的生命周期超过作用域时不应该使用引用形式的变量捕捉。

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); });

Enforcement

• (Simple) Warn when capture-list contains a reference to a locally declared variable （简单）当捕捉列表中包含本地声明的变量时报警。
• (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 （复杂）在捕捉列表中包含指向局部声明的变量而且lambda表达式被传递给非常量或非局部上下文时做出提示。

译者注：第一种情况容易实现但并不完美。

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