# value_or_error Concept

Something not really mentioned until now is how Outcome interoperates with the proposed
P0323 `std::expected<T, E>`

, whose design lands in between `unchecked<T, E = varies>`

and `checked<T, E = varies>`

(both of which are type aliases hard coding no-value
policies as previously covered in this tutorial).

Expected and Outcome are isomorphic to one another in design intent, but interoperation
for code using Expected and Outcome ought to be seamless thanks to the proposed `ValueOrError`

concept framework, a subset of which Outcome implements.

The `explicit basic_result(concepts::value_or_error<T, E> &&)`

and `explicit basic_outcome(concepts::value_or_error<T, E> &&)`

constructors will explicitly construct from any type matching the `concepts::value_or_error<T, E>`

concept, which includes `std::expected<A, B>`

, if `A`

is constructible to `X`

, and `B`

is
constructible to `Y`

. The `value_or_error`

concept in turn is true if and only if the input type has:

- A
`.has_value()`

observer returning a`bool`

. `.value()`

and`.error()`

observers.

## Implementation

Outcome’s machinery for implementing `concepts::value_or_error`

conversion is user extensible by injection
of specialisations of the `value_or_error<T, U>`

type into the `BOOST_OUTCOME_V2_NAMESPACE::convert`

namespace.

Outcome’s default `convert::value_or_error<T, U>`

implementation explicitly
excludes Outcome `result`

and `outcome`

types from the default mechanism as
there is a major gotcha: the `value_or_error`

matched type’s `.value()`

is often
not callable in constexpr as it can throw, which makes this conversion mechanism
pretty much useless for constexpr code. Besides, `outcome`

has a converting
constructor overload for `result`

inputs which is constexpr capable.

Note that if you do enable `outcome`

inputs, a `result`

will match an input
`outcome`

, but silently dropping any exception state. This is probably undesirable.

Examples of how to implement your own `convert::value_or_error<T, U>`

converter
is demonstrated in the worked example, next.