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# variant2
This repository contains a never-valueless C++14 implementation of [std::variant](http://en.cppreference.com/w/cpp/utility/variant) in [variant.hpp](include/boost/variant2/variant.hpp) and an implementation of `expected<T, E...>` in [expected.hpp](include/boost/variant2/expected.hpp) that is an extended version of `extended<T, E>` as proposed in [P0323R1](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0323r1.pdf) and the subsequent [D0323R2](https://github.com/viboes/std-make/blob/master/doc/proposal/expected/d0323r2.md).
This repository contains a never-valueless C++14 implementation of [std::variant](http://en.cppreference.com/w/cpp/utility/variant) in [variant.hpp](include/boost/variant2/variant.hpp) and an implementation of `expected<T, E...>` in [expected.hpp](include/boost/variant2/expected.hpp) that is an extended version of `expected<T, E>` as proposed in [P0323R1](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0323r1.pdf) and the subsequent [D0323R2](https://github.com/viboes/std-make/blob/master/doc/proposal/expected/d0323r2.md).
The code requires [mp11](https://github.com/pdimov/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](https://github.com/pdimov/mp11/blob/master/include/boost/mp11.hpp) or [mp11_single.hpp](https://github.com/pdimov/mp11/blob/master/include/boost/mp11_single.hpp) is included beforehand.
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* clang++ 3.5 or later with -std=c++14 or -std=c++1z
* Visual Studio 2017
Tested on [Travis](https://travis-ci.org/pdimov/variant2/) and [Appveyor](https://ci.appveyor.com/project/pdimov/variant2/).
## variant.hpp
The class `boost::variant2::variant<T...>` is an almost conforming implementation of `std::variant` with the following differences:
* The function `valueless_by_exception()` is not present, as the variant is never valueless;
* A converting constructor from, e.g. `variant<int, float>` to `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 the valueless state, this implementation falls back to using double storage unless
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
* at least one contained type has a nothrow default constructor.
* the list of alternatives contains the type `valueless`.
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, the recommended practice is to add an alternative that has a nonthrowing default constructor.
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` to the list of alternatives.
## expected.hpp