使用默认构造函数生成的c++移动构造函数

Question

查看this question它提到了 C++11 及更高版本:

The move constructor is auto-generated if there is no user-declared copy constructor, copy assignment operator or destructor, and if the generated move constructor is valid (e.g. if it wouldn't need to assign constant members) (§12.8/10).

如果我有以下代码:

class Y
{
public:
    Y(const Y&) {}
};

struct hasY {
    hasY() = default;
    hasY(hasY&&) = default;
    Y mem; 
};
hasY hy, hy2 = std::move(hy); //this line fails as expected as Y has a user-defined copy constructor.

现在，如果我将默认构造函数添加到 Y:

Y() {}

错误消失了。
哪里说默认构造函数导致移动构造函数的创建？

(使用 VS 2015 更新 2)

Answer 1

class Y
{
public:
    Y(const Y&) {}
};

这个类没有默认构造函数，所以

struct hasY {
    hasY() = default;
    hasY(hasY&&) = default;
    Y mem;  // << requires default ctor
};

您遇到的错误与移动构造函数无关:

prog.cpp: In function 'int main()':
prog.cpp:13:7: error: use of deleted function 'hasY::hasY()'
  hasY hy;
   ^
prog.cpp:8:5: note: 'hasY::hasY()' is implicitly deleted because the default definition would be ill-formed:
     hasY() = default;

http://ideone.com/u46GWS