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

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boost20.optional

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.

How to use it

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

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

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