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Subject: Re: [boost] compact_optional -- prompting interest
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2015-09-25 11:37:10

On 25.09.2015 17:35, Andrzej Krzemienski wrote:
> Hi Everyone,
> I would like to inquire if there would be an interest in Boost in another
> library for storing optional objects, but working under different design
> goals.
> Compact optional T has (or can have) the same sizeof as T. It uses one
> indicated value of T to represent the "empty" (or "singular") value. You
> can declare it like this:
> compact_optional<evp_int<int, -1>> oi;
> This reads: we have an optional int, with type int inside, where -1
> represents the empty value. It can never have a genuine (non-empty value
> -1). This can be used, for instance, to wrap the std::string::npos into:
> compact_optional<evp_int<string::size_type, string::npos>>
> With the same memory layout as std::string::size_type, but with the special
> syntax for managing the singular value.
> It is not meant to be an alternative to Boost.Optional: it targets a
> different application space.

The idea looks interesting - I sometimes have to deal with magic values.
Currently I prefer to wrap the object into optional<> or at least make
the special value not so magic (i.e. so that the code always does the
right thing without checking for the magic value).

However, I'm not sure I agree with your rationale on the reduced
interface and possibly the compact_optional naming.

1. You chose not to provide relational operators for compact_optional
because you don't know how to order 'empty' values. I think you don't
have to make that decision and simply forward the call to the underlying
type. I mean, you always have the stored object constructed in some
state and as long as it implements operators you can always use them.

2. compact_optional does not provide direct assignment of the values of
the stored type, requiring to manually construct a
compact_optional-wrapped value. To me, this is too cumbersome to use
while I don't see any wins from this restriction. Besides more typing,
this essentially requires to use a typedef to declare and use the
compact_optional variable.

3. Nitpick: the typical name for the getter operation is get(), not
value(). I would also have used empty() to test for the magic value but
maybe that makes you feel it like a container.

4. Regarding compact_optional naming. While the class can be used for
similar purpose as optional, its interface and behavior are
significantly different. Perhaps a different name would be better to
avoid confusion (e.g. nullable<>).

5. A suggestion: add evp_zero and evp_empty policies. The first uses
literal zero as the special value and can be used with numeric (integer
and fp) and pointer types. The second uses a default constructed value
as the magic value and a member empty() function to test for magic
value. This could be useful with containers, strings and ranges.

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