A never-valueless implementation of std::variant
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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.

Supported compilers:

  • 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

Tested on Travis and Appveyor.


The class boost::variant2::variant<T...> is an almost conforming implementation of std::variant with the following differences:

  • 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 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 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, one can force emplace into always constructing in-place by adding valueless as a first alternative.


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...>.

See its documentation for more information.