c++ move constructor generated with default constructor - c++

Question

Looking at this question it mentions C++11 and later only:
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).
So if I have the following code:
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.
Now if I add the default constructor to Y:
Y() {}
The error goes away.
Where does it say that the default constructor causes the creation of the move constructor?
(using VS 2015 update 2)

Answer 1

class Y
{
public:
Y(const Y&) {}
};
This class has no default constructor, so
struct hasY {
hasY() = default;
hasY(hasY&&) = default;
Y mem; // << requires default ctor
};
the error you were getting had nothing to do with move constructors:
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

Answer 2

It is nothing related to the move constructor It is about the default constructor. Try this:
class Y
{
public:
Y(const Y&) {}
};
struct hasY {
hasY() = default;
hasY(hasY&&) = default;
Y mem;
};
hasY hy; // This will cause an error because there is no default constructor
Now if you add the default constructor: Y(){}, the error will go away.
As #M.M commented, The copy constructor will be called in a such case.
You may try this code:
class Y{
public:
Y(){std::cout << "Default constructor\n";}
Y(const Y&) {std::cout << "Copy constructor\n";}
};
struct hasY {
hasY() = default;
hasY(hasY&&) = default;
Y mem;
};
int main(){
hasY hy;
hasY h=std::move(hy);
}
It will print:
Default constructor
Copy constructor
If you want to make the class not moveable you should delete the move constructor by yourself Y(Y&&)=delete;

Answer 3

Where does it say that the default constructor causes the creation of the move constructor?
There is no relationship between the creation of the move constructor and the existence, or lack thereof, of the default constructor.
From the C++11 Standard:
12.8 Copying and moving class objects
...
9 If the definition of a class X does not explicitly declare a move constructor, one will be implicitly declared
as defaulted if and only if
— X does not have a user-declared copy constructor,
— X does not have a user-declared copy assignment operator,
— X does not have a user-declared move assignment operator,
— X does not have a user-declared destructor, and
— the move constructor would not be implicitly defined as deleted.