c – std :: thread为什么对象被复制两次？

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,添加移动构造函数后：

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

输出是这样的：

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