From: Fernando Cacciola (fernando_cacciola_at_[hidden])
Date: 2005-02-15 12:54:19
"Joe Gottman" <jgottman_at_[hidden]> escribió en el mensaje
news:curjg1$2vq$1_at_sea.gmane.org...
> I just discovered that optional<T> is not safe under self-assignment.
Ouch!! This is a major oversight from my part...
Every C++ 101 book warns about it
> Consider the following code fragment:
>
> boost::optional<long> x(1); //At this point x is initialized and *x ==
> 1
> x = x; //x should be unchanged, but instead x is now uninitialized
>
> Optional's assignment operator looks like
>
> optional& operator= ( optional const& rhs )
> {
> this->assign( rhs ) ;
> return *this ;
> }
>
> and assign is defined by
>
> void assign ( argument_type val )
> {
> destroy();
> construct(val);
> }
>
Well, this is not the correct overload, but you are right all the same.
> Thus, if (this == &rhs) in operator=(), any value held by rhs will be
> destroyed.
Indeed (what was I thinking!)
>
> I can see two possible fixes for this. the first is to put an "if (this
> != &rhs) " around the call to assign in operator=(). This is simplest,
> but it slows the code down in order to protect against a very rare
> situation.
> The other is to rewrite assign as follows:
>
> void assign(argument_type val)
> {
> if (m_initialized) {
> get() = val; // call assignment operator
> } else {
> construct(val); //call copy constructor
> }
> return *this;
> }
>
> This makes optional self-assignment safe.
Right.
> It has the added advantage that if T's assignment operator has the strong
> exception safety guarantee, so does optional<T>'s assignment operator.
Well, this is a good point.
It is typical for classes with a throwing copy ctor to have a strong
assignment.
> It has the disadvantage of making optional depend on both assignment
> operator and copy constructor to copy, but very few classes have a copy
> constructor but don't have an assignment operator.
>
Right, I don't think this extra requirement will have any problems in
practice.
Thanks for the report!
Fernando Cacciola