Re: [boost] compact_optional -- prompting interest

On Fri, 25 Sep 2015, 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.

In my opinion, this is not an alternative to Boost.Optional because it should be part of Boost.Optional. For the interface, you could either add a second template parameter to boost::optional specifying the policy, or specialize boost::optional.

The (single-header) implementation is at: https://github.com/akrzemi1/compact_optional

The documentation is at: https://github.com/akrzemi1/compact_optional/blob/master/documentation.md

For the motivation and design rationale see this article: https://akrzemi1.wordpress.com/2015/07/15/efficient-optional-values/

Any feedback is welcome.

-- Marc Glisse