Re: [boost] [optional] Safe optional

Le 17/11/14 22:12, Vladimir Batov a écrit :

On 11/17/2014 09:12 PM, Mostafa wrote:

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On Sun, 16 Nov 2014 23:04:43 -0800, Andrzej Krzemienski wrote:

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Hi Everyone, I would like to run an idea through everyone in this list. There is a recurring complaint about Boost.Optional that it allows you to do "unsafe" things: 1. Inadvertent mixed comparisons between T and optional<T> 2. Unintended conversion from T to optional<T>

The problem with optional is that it tries to be a drop-in replacement proxy for its underlying type, and, unfortunately, it can't fully do that. So it ends up with an identity crisis. IMO, optional should be treated as a first class object, not a thin wrapper around T, that means no implicit conversions to/from T, no implicit comparisons with T, etc... Last time I looked at this, that will solve the reference rebinding gotcha. That is, you'll have one behavior for optional<T> and optional....

1. With all due respect I do not feel I can agree with the above... and I do not believe "optional" has a "problem"... especially of the described magnitude. IMO the result of applying the conceptual/design changes described above to the existing "optional" won't be "optional" as we know it... not even close. And after using "optional" quite a bit I can say I personally won't be very happy. IMO "optional" has been born as a practical solution to a real problem and IMO it solves it quite well. Yes, it has certain behavior that the user needs to be aware of... but what class does not impose restrictions of that kind? Any potential functional/behavioral change has to be looked at individually.

For example, I do agree that that there should not be implicit optional<T> to T conversion. I was not even aware there was such. This conversion must be explicit if any.

However, implicit T to optional<T> conversion has a very practical purpose. For example,

int value(optional<int> =optional<int>());

allows me to shrink the API as value getter and setter are merged into one function. Namely,

int k = value(); // Get the value value(22); // Set the value. Implicit conversion of T to optional<T>

Please don't do that. Use just overload.

Instead, asking the user to call explicitly

value(optional<22>)

is a professional suicide.

2. As for the separate/additional "safe" optional, I am personally not that thrilled by the idea as of now. IMO that'll result in user-base fragmentation, incompatibilities and inevitably more confusion in the end. Anything useful for safe_optional is useful for optional. So I see safe_optional as a view of a restricted interface. This doesn't mean that conversions from one to the other are not possible.

Vicente