This repository contains a never-valueless C++14 implementation of std::variant in variant.hpp and an implementation of
expected<T, E...> in expected.hpp that is an extended version of
expected<T, E> as proposed in P0323R1 and the subsequent D0323R2.
The code requires mp11 and Boost.Config. The repository is intended to be placed into the
libs/variant2 directory of a Boost clone or release, with mp11 in
libs/mp11, but the headers will also work standalone if mp11.hpp or mp11_single.hpp is included beforehand.
- g++ 5 or later with -std=c++14 or -std=c++1z
- clang++ 3.5 or later with -std=c++14 or -std=c++1z
- Visual Studio 2017
boost::variant2::variant<T...> is an almost conforming implementation of
std::variant with the following differences:
- A converting constructor from, e.g.
variant<float, double, int>is provided as an extension;
- The reverse operation, going from
variant<float, double, int>to
variant<int, float>is provided as the member function
subset<U...>. (This operation can throw if the current state of the variant cannot be represented.)
To avoid going into a valueless-by-exception state, this implementation falls back to using double storage unless
- all the contained types are nothrow move constructible, or
- the first alternative is the type
If the second bullet doesn't hold, but the first does, the variant uses single storage, but
emplace constructs a temporary and moves it into place if the construction of the object can throw. In case this is undesirable, one can force
emplace into always constructing in-place by adding
valueless as a first alternative.
boost::variant2::expected<T, E...> represents the return type of an operation that may potentially fail. It contains either the expected result of type
T, or a reason for the failure, of one of the error types in
E.... Internally, this is stored as
See its documentation for more information.