If you assign to an uninitialized
the effect is to bind (for the first time) to the object. Clearly, there is
no other choice.
int x = 1 ; int& rx = x ; optional<int&> ora ; optional<int&> orb(x) ; ora = orb ; // now 'ora' is bound to 'x' through 'rx' *ora = 2 ; // Changes value of 'x' through 'ora' assert(x==2);
If you assign to a bare C++ reference, the assignment is forwarded to the referenced object; its value changes but the reference is never rebound.
int a = 1 ; int& ra = a ; int b = 2 ; int& rb = b ; ra = rb ; // Changes the value of 'a' to 'b' assert(a==b); b = 3 ; assert(ra!=b); // 'ra' is not rebound to 'b'
Now, if you assign to an initialized
the effect is to rebind to the new object
instead of assigning the referee. This is unlike bare C++ references.
int a = 1 ; int b = 2 ; int& ra = a ; int& rb = b ; optional<int&> ora(ra) ; optional<int&> orb(rb) ; ora = orb ; // 'ora' is rebound to 'b' *ora = 3 ; // Changes value of 'b' (not 'a') assert(a==1); assert(b==3);
Rebinding semantics for the assignment of initialized
optional references has been
chosen to provide consistency among initialization states
even at the expense of lack of consistency with the semantics of bare C++ references.
It is true that
to behave as much as possible as
does whenever it is initialized; but in the case when
doing so would result in inconsistent behavior w.r.t to the lvalue initialization
forwarding assignment to the referenced object (thus changing the referenced
object value but not rebinding), and consider the following code:
optional<int&> a = get(); int x = 1 ; int& rx = x ; optional<int&> b(rx); a = b ;
What does the assignment do?
a is uninitialized,
the answer is clear: it binds to
(we now have another reference to
But what if
a is already initialized?
it would change the value of the referenced object (whatever that is); which
is inconsistent with the other possible case.
would assign just like
does, you would never be able to use Optional's assignment without explicitly
handling the previous initialization state unless your code is capable of functioning
whether after the assignment,
aliases the same object as
That is, you would have to discriminate in order to be consistent.
If in your code rebinding to another object is not an option, then it is very
likely that binding for the first time isn't either. In such case, assignment
to an uninitialized
shall be prohibited. It is quite possible that in such a scenario it is a precondition
that the lvalue must be already initialized. If it isn't, then binding for
the first time is OK while rebinding is not which is IMO very unlikely. In
such a scenario, you can assign the value itself directly, as in: