Re: [boost] compact_optional -- prompting interest

On Tue, Sep 29, 2015 at 2:16 AM, Andrzej Krzemienski wrote:

2015-09-29 9:11 GMT+02:00 Gonzalo BG :

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@Andrzej: Right now your implementation allows constructing a compact_optional from the sentinel value, which results in an empty compact_optional. What is the motivation for this?

In my fork I've disabled it for the following reasons: - I'd rather use compact_optional's default constructor to explicitly

state

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that i'm constructing an empty optional, this made it easier to reason about my code, - the other constructors from value_type can then hint the compiler that the compact_optional is not empty (but I did not profile so I don't know if this has any impact at all).

The motivation for this was to enable an integration of compact_optional with older/other parts of the program that still use magical values:

``` using Index = compact_optional> string s = /*...*/ Index i {s.find("substr")}; ```

In the above example, I will not be changing std, but want to change to compact_optional as soon as possible.

But in fact, I could provide a dedicated conversion function for this use case. This aspect is still under consideration, so I may go with your suggestion.

Regards, &rzej

In fact, I think constructing from the magic value is the primary use case for such a class. This makes it easy to integrate with, for instance, a networking protocol that has a magic value meaning "no value here". Now that I've thought about it more, compact_optional is a way of saying that you care exactly how that "not present" state is constructed.