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35 lines
2.6 KiB
Markdown
35 lines
2.6 KiB
Markdown
# variant2
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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).
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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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Supported compilers:
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* g++ 5 or later with -std=c++14 or -std=c++1z
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* clang++ 3.5 or later with -std=c++14 or -std=c++1z
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* Visual Studio 2017
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Tested on [Travis](https://travis-ci.org/pdimov/variant2/) and [Appveyor](https://ci.appveyor.com/project/pdimov/variant2/).
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## variant.hpp
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The class `boost::variant2::variant<T...>` is an almost conforming implementation of `std::variant` with the following differences:
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* A converting constructor from, e.g. `variant<int, float>` to `variant<float, double, int>` is provided as an extension;
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* 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.)
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To avoid going into a valueless-by-exception state, this implementation falls back to using double storage unless
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* all the contained types are nothrow move constructible, or
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* the first alternative is the type `valueless`.
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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.
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## expected.hpp
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The class `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 `variant<T, E...>`.
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See [its documentation](doc/expected.md) for more information.
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