Move constructor and non-const copy constructor - c++

Question

I am new in move constructor, I surveyed from some sites and tried using the Visual Studio 11 Express Beta..
Below is my testing code...
#include <iostream>
using namespace std;
class Foo
{
public:
Foo()
: Memory(nullptr)
{
cout<<"Foo Constructor"<<endl;
}
~Foo()
{
cout<<"~Foo Destructor"<<endl;
if(Memory != nullptr)
delete []Memory;
}
Foo(Foo& rhs)
: Memory(nullptr)
{
cout<<"Copy Constructor"<<endl;
//allocate
//this->Memory = new ....
//copy
//memcpy(this->Memory, rhs.Memory...);
}
Foo& operator=(Foo& rhs)
{
cout<<"="<<endl;
}
void* Memory;
Foo(int nBytes) { Memory = new char[nBytes]; }
Foo(Foo&& rhs)
{
cout<<"Foo Move Constructor"<<endl;
Memory = rhs.Memory;
rhs.Memory = nullptr;
}
};
Foo Get()
{
Foo f;
return f;
//return Foo();
}
void Set(Foo rhs)
{
Foo obj(rhs);
}
int main()
{
Set(Get());
return 0;
}
I don't know why it will not enter move constructor.
It's really a Rvalue from Get();
If I modified non-const copy constructor from const constructor,
it will enter move constructor. Behavior changed...
Could anyone kindly explain why it happened?

Answer 1

#include <iostream>
using namespace std;
class Foo
{
public:
Foo():
Memory(nullptr)
{
cout<< this << "Foo Constructor"<<endl;
}
~Foo()
{
cout<< this << "~Foo Destructor"<<endl;
if(Memory != nullptr)
delete []Memory;
}
Foo(Foo& rhs)
:Memory(nullptr)
{
cout<<this << "Copy Constructor"<<endl;
//allocate
//this->Memory = new ....
//copy
//memcpy(this->Memory, rhs.Memory...);
}
Foo& operator=(Foo& rhs)
{
cout<<"="<<endl;
}
void* Memory;
Foo(int nBytes) { Memory = new char[nBytes]; }
Foo(Foo&& rhs)
{
cout<<this << "Foo Move Constructor"<<endl;
Memory = rhs.Memory;
rhs.Memory = nullptr;
}
};
Foo Get()
{
Foo f;
cout << &f << "f" <<endl;
return f;
}
void Set(Foo rhs)
{
Foo obj(rhs);
cout << &obj << "obj"<<endl;
}
int main()
{
Set(Get());
return 0;
}
output...
0x7fffe38fa0a0 Foo Constructor
0x7fffe38fa0a0 f
0x7fffe38fa070 Copy Constructor
0x7fffe38fa070 obj
0x7fffe38fa070 ~Foo Destructor
0x7fffe38fa0a0 ~Foo Destructor
Answer: Due to the Named Return Value Optimization the parameter rhs is constructed inplace as an alias of local variable f. (That is to say rhs and f are the same instance).
As rhs is an lvalue, the copy constructor is used to copy construct obj from rhs.