The new ?? and ??? operators

[Arlen Albert Keshabyan]

Stewart Gordon Wrote:

> "Arlen Albert Keshabyan" <arlen.albert at gmail.com> wrote in message
> news:fd611h$4co$1 at digitalmars.com...
> > It would be the sugar syntactic to add '??' operator to D.
> > Consider the example code:
> >
> > string error_message = getErrorMessage() ??  "no errors";
> > A a = x.getPreparedAObject() ??  y.getPreparedAObject() ??  new A();
> >
> > This operator is supposed to do the same thing as C# does: lvalue
> > evaluates to the first non-null value or to null if all rvalues
> > contains null.
> 
> So effectively, it works like || in JavaScript and the like.  I guess the 
> return type of a ??  expression would be determined by the same rules that 
> govern that of a ConditionalExpression.  The expression would evaluate to 
> the first subexpression whose value when implicitly converted to a boolean 
> is true.  Otherwise ...  to the .init of the return type?
> 
> > This operator might be extended to '???' to evaluate to a value
> > that conforms to some conditions.  The lvalue gets the first rvalue
> > that evaluates to true (evaluation goes from left to right).  If no
> > conditions evaluates to true then the lvalue stays unchanged.  If
> > no conditions are given explicitly then those conditions evaluates
> > to true (so, the best place for them is at the end of a sequence).
> <snip>
> 
> I don't really like this:
> - It would cause the semantics of the = operator to depend on the form of 
> the RHS.
> - What if the ??? expression isn't the RHS of an = operator?
> - The null case of this operator doesn't match semantically as they're 
> normally expected to.  (I'm not sure if there's any better way to word 
> this.)  To see what I mean, compare the meanings of
> 
> a = b ??? c ??? d;
> a = b ??? c;
> a = b;
> 
> Stewart.
> 
> -- 
> My e-mail address is valid but not my primary mailbox.  Please keep replies 
> on the 'group where everybody may benefit. 
> 

If you'd read my first post carefully you'd never ask the question like this.

> a = b ??? c ??? d;
> a = b ??? c;
> a = b;

the conditions must be explicit (!). Inexplicit conditions always evaluates to TRUE despite types involved in conditions. So, you will got the following:

> a = b ??? c ??? d;
a = b; //any type, even boolean


> a = b ??? c;
a = b;

> a = b;
a = b; //so no ??? ternary operator. Treated just like no (?:) operator :)



bool a = true
bool b = true
bool c = false
bool d = true;

a = b != true ??? c != false ??? d == false ??? false;

now a == false because no conditions evaluates to true except for the last one 'false'. Yes, 'false' evaluates to true :) so it's assigned to the 'a' variable.