zeus::expected

Backporting std::expected to C++17.

Features

Implemented C++ Standard Proposals:

• [x] P0323R12 <expected>
• [x] P2505R5 Monadic Functions For expected
• [x] P2549R1 std::unexpected<E> should have error() as member accessor

Implemented LWG Issues:

• [x] LWG-3836 std::expected<bool, E1> conversion constructor expected(const expected<U, G>&) should take precedence over expected(U&&) with operator bool
• [x] LWG-3843 std::expected<T,E>::value() & assumes E is copy constructible
• [x] LWG-3940 std::expected<void, E>::value() also needs E to be copy constructible
• [x] LWG-4026 Assignment operators of std::expected should propagate triviality
• [x] LWG-3877 incorrect constraints on const-qualified monadic overloads for std::expected

Enhancements:

• Enhanced noexcept (covered by tests from MSVC's STL)

Compiler supports

Any compiler that supports C++17 and later should work. Usage against higher language standard is also supported, in which you'll gain some benefits (e.g. better constexpr).

While heavily tested under MSVC and C++20, it's only tested slightly under C++17, since newly-made. May assume that it could just work if it compiled successfully. Feedbacks are welcome.

Table of known compiler status.

Building and testing

Catch2 is required to build the tests. You may choose any preferred package manager to introduce the requirements. Here is the conan one.

# install the requirements
conan install . -s build_type=Debug -b missing
# glance the cmake preset names
cmake --list-presets
cmake --build --list-presets
ctest --list-presets
# configure, build and test per preset names

Acknowledgements

• tl-expected, the original code base this library came from.
• MSVC's STL's massive, strong and robust tests.