An educational C++20 implementation of Boost.Optional that also is-a C++20 std::optional.

The primary motivation for this implementation is the exploration of these C++20 features:

• concepts
• restricted templates instead of SFINAE and std::enable_if to control overload sets
• modules and implementation techniques regarding their various flavours
• three-way comparison

This implementation of Boost.Optional offers these major features:

• single header
• conformant to the specification of Boost.Optional
• conformant to the C++20 specification of std::optional
• the namespace is configurable, by default it is boost
• the underlying optional base class is configurable, by default it is std::optional
• can be used as a legacy header #include <optional/optional.hpp>
• can be compiled into a header module and used as import <optional/optional.hpp>;
• can be compiled into a named module boost.optional and used as import boost.optional; This is what people should aim for!
• the module name is configurable

So far, MSVC 16.8-pre3 is capable of compiling all module flavours and the assorted examples. Clang trunk and gcc 10 accept the code at least as #include, I couldn't yet figure out how to compile it as modules on Compiler Explorer.

All you actually need is in directory optional:

• optional.hpp is the full implementation of

  • optional<T>
  • optional<T &>
  • the related free functions and deduction guides

  as specified by C++20 and Boost

• optional.cpp is the minimal module interface unit source, a stub translation unit required by the syntactic rules of C++20.

  It is both

  • universal, because it may turn every single-header-only library into a module given sufficient preparation.
  • minimal, because you cannot get away with fewer logical lines and preprocessing tokens to do so.

Directory VisualStudio contains a solution and projects to build

• a named module (with invisible details)
• a header module (with invisible details)
• an example using optional as a legacy include (with unavoidably visible details)
• an example using optional as an imported header module
• an example using optional as an imported named module