string build_string()
{
 string result;

 //... build code

 return result;
}

//VS2010
#define __STL_MOVE_VALUE(x)      std::move(x)
#define __STL_FORWARD_VALUE(T,x) std::forward<T>(x)

//old compilers
#define __STL_MOVE_VALUE(x) (x)
#define __STL_FORWARD_VALUE(T,x) (x)

//class B is movable
class A
{
public:
  //я чего-то не понимаю или тут реально не нужно && ? ведь это ненужное ограничение на клиента
  void SetB(B&& b)
  {
    m_b = std::move(b);
  }

  B m_b;
};

//variant 1
class A
{
public:
  A& operator=(A&& other)
  {
    CloseResource();
    std::swap(m_Resource, other.m_Resource);
    return *this;
  }

  ~A()
  {
    CloseResource();
  }

private:
  ResourceType m_Resource;
};

//variant 2
class A
{
public:
  A& operator=(A&& other)
  {
    std::swap(m_Resource, other.m_Resource);
  }

  ~A()
  {
    //free resource
  }
private:
  ResourceType m_Resource;
};

//variant 3
class A
{
public:
  A& operator=(A&& other)
  {
    A tmp(std::move(other));
    std::swap(tmp.m_Resource, m_Resource);
    return *this;
  }

private:
  ResourceType m_Resource;
};

//class B is movable
class A
{
 public:
  //я чего-то не понимаю или тут реально не нужно && ? ведь это ненужное ограничение на клиента
  void SetB(B&& b)
  {
    m_b = std::move(b);
  }

  B m_b;
};

КД>string build_string()
КД>{
КД> string result;

КД> //... build code

КД> return result;
КД>}
КД>

When the criteria for elision of a copy operation are met or would be met save for the fact that the source object is a function parameter, and the object to be copied is designated by an lvalue, overload resolution to select the constructor for the copy is first performed as if the object were designated by an rvalue. If overload resolution fails, or if the type of the first parameter of the selected constructor is not an rvalue reference to the object’s type (possibly cv-qualified), overload resolution is performed again, considering the object as an lvalue. [ Note: This two-stage overload resolution must be performed regardless of whether copy elision will occur. It determines the constructor to be called if elision is not performed, and the selected constructor must be accessible even if the call is elided. —end note ]

[ Example:

class Thing {
public:
  Thing();
  ~Thing();
  Thing(Thing&&);
private:
  Thing(const Thing&);
};

Thing f(bool b) {
  Thing t;
  if (b)
    throw t;         // OK: Thing(Thing&&) used (or elided) to throw t
  return t;          // OK: Thing(Thing&&) used (or elided) to return t
}

Thing t2 = f(false); // OK: Thing(Thing&&) used (or elided) to construct t2

—end example ]

КД>>Лично я определяю SetB(B&& b) только в довесок к нормальному "void SetB(const B& b)"

void SetB(B b)
{
 m_std_vector_of_B.push_back(b);
}

void SetB(const B& b)
{
 m_std_vector_of_B.push_back(b);
}