Re: [boost] compact_optional -- prompting interest

On 25.09.2015 18:37, Andrey Semashev wrote:

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> 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>

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.

6. Also, I think unsafe_raw_value() is a too scary name. There's nothing unsafe about obtaining the special value from the wrapper. I would name it get_raw() (which is in addition to get() which is now value()).