Re: [boost] [optional] from_optional

From: Václav Veselý (vaclav.vesely_at_[hidden])
Date: 2006-03-13 01:41:38

Ulrich Eckhardt wrote:
> On Sunday 12 March 2006 11:38, Václav Veselý wrote:
>> There is a common need of getting a value from an optional with
possibility to
>> define a default value in case of the optional is uninitialized. I
suggest two
>> alternatives:
>>
>> optional<int> o;
>> int i = o.get(123); // a meber function get with an additional parameter
>> int j = from_optional(o, 123); // a free function 
>
> I don't like the first one, it's just not intuitive enough.

Maybe better selected name could be more intuitive.

> Also, if there is a
> sensible default, there is no need for optional<> anyways.

I don't think so. Library functions can return optionals and it depends on
the user of library if there is a default value and what is is.

> Also, they don't provide much advantage to the ternary operator
>   int i = o ? *o : 123;
> just that you only have one 'o' in there instead of two.

There are two points:
1) As Sebastian Redl in another post said, o can be returned from function.
2) I think, that my proposal is more readable and elegant.

> However, I just had a
> crazy idea, how about overloading an operator for them:
>   int i = o | 123;
>   int j = o / 456;
>   int k = *(o|123);

I don't like this much. I prefer descriptive function names to weirdly
overloaded operators.

> or maybe putting this choice into the name like
>   int l = o.get_value_or(012);

I would be satisfied with any other reasonable name.

Except of replacing missing value with default there there are more
strategies. For example Property Tree library (which is scheduled for
review) has four (!) methods for getting value with for ways of handle the
case of missing value. That strategies are throwing an exception, return
default value, return optional and get the value in an out parameter and
returning only the flag. I think, that all these strategies should be solved
in optional library. The method names could be:

    val = opt.get_or_exception()
    val = opt.get_or_default(123)
    tie(flag, val) = opt.get_tuple()
    flag = opt.get_xxx(val) // reference to a variable
       

I'm not sure with the neme of the last version.

Regards,
Vaclav