std :: thread为什么将对象复制两次？

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问题描述

为什么在下面的示例代码中，对象被复制了两次？根据文档的构造方法，线程类会将所有参数复制到线程本地存储中，因此我们有理由进行第一次复制。那第二呢？

Why in the example code below, object is copied twice? According documentation constructor of thread class copies all arguments to thread-local storage so we have reason for the first copy. What about second?

class A {
public:
    A() {cout << "[C]" << endl;}
    ~A() {cout << "[~D]" << endl;}
    A(A const& src) {cout << "[COPY]" << endl;}
    A& operator=(A const& src) {cout << "[=}" << endl; return *this;}

    void operator() () {cout << "#" << endl;}
};

void foo()
{
    A a;
    thread t{a};
    t.join();
}

上面的输出：

[C]
[COPY]
[COPY]
[~D]
#
[~D]
[~D]

编辑：
是的，在添加move构造函数之后：

Well yes, after adding move constructor:

A(A && src) {cout << "[MOVE]" << endl;}

输出如下：

[C]
[COPY]
[MOVE]
[~D]
#
[~D]
[~D]

推荐答案

想移动或避免复制，更喜欢移动构造函数和 std :: move 。

For anything you want to move or avoid copies, prefer move constructors and std::move.

但是为什么

在C ++中移动是保守的。通常，只有在您明确编写 std :: move（）时，它才会移动。这样做是因为移动语义（如果扩展到非常明确的环境之外）可能会破坏旧代码。因此，自动移动通常仅限于非常谨慎的情况。

Move in C++ is conservative. It generally will only move if you explicitly write std::move(). This was done because move semantics, if extended beyond very explicit circumstances, might break older code. Automatic-moves are often restricted to very careful set of circumstances for this reason.

为了避免在这种情况下进行复制，您需要移动 a ，方法是使用 std :: move（a）（即使将其传递到 std :: thread ）。第一次创建副本的原因是因为std :: thread不能保证在完成std :: thread的构造之后（并且您没有明确地将其移入）该值将存在。因此，它会做安全的事并进行复制（不要引用或指向您传入的内容并进行存储：代码不知道您是否将其保存下来）。

In order to avoid copies in this situation, you need to shift a around by using std::move(a) (even when passing it into std::thread). The reason it makes a copy the first time around is because std::thread can't guarantee that the value will exist after you have finished constructing the std::thread (and you haven't explicitly moved it in). Thusly, it will do the safe thing and make a copy (not take a reference/pointer to what you passed in and store it: the code has no idea whether or not you'll keep it alive or not).

同时具有move构造函数和使用 std :: move 可使编译器最大程度地有效移动结构。如果您使用的是VC ++（不带CTP），则必须显式编写move构造函数，否则MSVC会（甚至有时会错误地）声明并使用Copy构造函数。

Having both a move constructor and using std::move will allow the compiler to maximally and efficiently move your structure. If you're using VC++ (with the CTP or not), you must explicitly write the move constructor, otherwise MSVC will (even sometimes erroneously) declare and use a Copy constructor.

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