返回值复制操作是在lock_guard析构函数之前还是之后执行的？ [英] Is a copy-on-return operation executed prior or after lock_guard destructor?

问题描述

get_a（）函数在竞争条件下是否安全？还是需要显式复制 str _ ？ get_b（）以便具有线程安全功能？

Is the get_a() function safe for race-conditions or do I need to explicitly copy str_ as in get_b() in order to have a thread-safe function?

class Class {
public:
  auto get_a() -> std::string {
    auto&& guard = std::lock_guard{mutex_};
    return str_;
  }
  auto get_b() -> std::string {
    auto&& guard = std::lock_guard{mutex_};
    auto str = str_;
    return str;
  }
private:
  std::mutex mutex_{};
  std::string str_{};
};

注意：我知道堆栈溢出有类似的问题，但是我

推荐答案

[stmt.return] p3 ：

调用结果的复制初始化在 return 语句的操作数建立的全表达式结束时，在临时变量销毁之前进行排序。依次在破坏 return 语句的块的局部变量之前进行排序。

The copy-initialization of the result of the call is sequenced before the destruction of temporaries at the end of the full-expression established by the operand of the return statement, which, in turn, is sequenced before the destruction of local variables of the block enclosing the return statement.

这意味着以下顺序发生：

This means that the following happen in order:

1. 返回对象已被复制初始化


2. return语句中的任何临时对象都将被破坏


3. 局部变量将被破坏

因此，我们可以推断 get_a 是完全安全的。

So, we can infer that get_a is completely safe.

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