c++ - Can a move constructor be implicit?

Question

Consider the following class:

class A
{
public:
   std::string field_a;
   std::string field_b;
}

Now consider the following copy construction:

A a1(a2);

The copy construction will adequately copy A despite the lack of of an explicit copy constructor because the copy constructors for std::string will be called by the compiler generated implicit copy constructor.

What I wish to know is, is the same true for move construction?

EDIT: Testing here shows that:

A a2(std::move(a1));

Will actually result in a copy construction, unless the specific move constructor:

A( A && other ) : a(std::move(other.a)) {}

Is defined.

EDIT EDIT I pinged Stephan T Lavavej and asked him why VC 2012 doesn't seem to follow what draft 12.8 states regarding implicit move constructor generation. He was kind enough to explain:

It's more of a "feature not yet implemented" than a bug. VC currently implements what I refer to as rvalue references v2.0, where move ctors/assigns are never implicitly generated and never affect the implicit generation of copy ctors/assigns. C++11 specifies rvalue references v3.0, which are the rules you're looking at.

Answer 1

Yes, from the C++11 draft, 12.8:

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.

The last condition is specified with more detail later:

An implicitly-declared copy/move constructor is an inline public member of its class. A defaulted copy/move constructor for a class X is defined as deleted (8.4.3) if X has:

• a variant member with a non-trivial corresponding constructor and X is a union-like class,
• a non-static data member of class type M (or array thereof) that cannot be copied/moved because overload resolution (13.3), as applied to M’s corresponding constructor, results in an ambiguity or a function that is deleted or inaccessible from the defaulted constructor,
• a direct or virtual base class B that cannot be copied/moved because overload resolution (13.3), as applied to B’s corresponding constructor, results in an ambiguity or a function that is deleted or inaccessible from the defaulted constructor,
• any direct or virtual base class or non-static data member of a type with a destructor that is deleted or inaccessible from the defaulted constructor,
• for the copy constructor, a non-static data member of rvalue reference type, or
• for the move constructor, a non-static data member or direct or virtual base class with a type that does not have a move constructor and is not trivially copyable.

Plainly speaking, the move constructor will be implicitly declared if:

1. The class does not have user-declared any of the other special member functions.
2. The move constructor can be sensibly implemented by moving all its members and bases.

Your class obviously complies with these conditions.