Move member function generation

Question

Code:

#include 
#include 
#include 
#include 
#include 

struct value
{
    ~value(         


        

Answer 1

std::is_move_constructible<T> is true iff std::is_constructible<T, T&&> is true, but that doesn't imply that such a construction will call a move constructor, only that it is possible to construct the type from an rvalue of the same type. Such a construction might use a copy constructor.

When value::s is a unique_ptr the type's copy constructor and copy assignment operator are defined as deleted, because the s member is not copyable. It does not have a move constructor and move assignment operator because, as you pointed out, it has a user-declared destructor. That means it has no copy constructor and no move constructor (and no other user-defined constructors that could accept an argument of type value&&) so std::is_constructible<value, value&&> is false.

When value::s is a string the type's copy constructor and copy assignment operator are not defined as deleted, because the s member is copyable, and so value is also copyable, and a CopyConstructible type is also MoveConstructible, because it's valid in this context:

value v1;
value v2 = std::move(v1);  // calls copy constructor

That means std::is_constructible<value, value&&> is true, even though it invokes the copy constructor not a move constructor.