Expected< T, E > Class Template Reference

A discriminated union holding either a success value or an error. More...

#include <expected.h>

Public Types
using	value_type = T
using	error_type = E

Public Member Functions
	Expected (T &&v) noexcept(is_nothrow_move_constructible_v< T >)
	Construct in the success state by moving a value in.
	Expected (Unexpected< E > u) noexcept
	Construct in the error state from an Unexpected tag.
	Expected (Expected &&other) noexcept(is_nothrow_move_constructible_v< T >)
	Move constructor.
Expected &	operator= (Expected &&other) noexcept(is_nothrow_move_constructible_v< T >)
	Move assignment.
	Expected (const Expected &)=delete
	Deleted — Expected is move-only to prevent accidental copies.
Expected &	operator= (const Expected &)=delete
	Deleted — Expected is move-only to prevent accidental copies.
	~Expected () noexcept
	Destructor. Destroys the active union member.
	operator bool () const noexcept
	Returns true if the Expected holds a success value.
bool	has_value () const noexcept
	Returns true if the Expected holds a success value.
T &	value () noexcept
	Access the success value by reference.
const T &	value () const noexcept
	Access the success value by const reference.
T &	operator* () noexcept
	Dereference operator — access the success value.
const T &	operator* () const noexcept
	Dereference operator (const) — access the success value.
T *	operator-> () noexcept
	Arrow operator — access members of the success value.
const T *	operator-> () const noexcept
	Arrow operator (const) — access members of the success value.
E	error () const noexcept
	Access the error code.

Detailed Description

template<typename T, typename E>
class Expected< T, E >

A discriminated union holding either a success value or an error.

Designed for failable operations in the fused library (decompositions, solvers, etc.). Uses a union internally so that only the active member is ever constructed — on the error path, T is never touched.

Move-only by design to prevent accidental copies of large results (e.g. matrix types) on embedded targets. With C++17 guaranteed copy elision and NRVO, returning by value from algorithms is zero-cost.

Marked [[nodiscard]] so the compiler warns if a caller discards the result without checking the error state.

Template Parameters

T	Success value type.
E	Error type (e.g. MatrixStatus, FilterStatus). No default — callers must always specify the error type explicitly.

Example — returning success:

Expected<Matrix3f, MatrixStatus> cholesky(const Matrix3f& A) {
    Matrix3f L;
    // ... compute L ...
    return move(L);  // or just `return L;` with NRVO
}

Example — returning an error:

if (!A.isSymmetric())
    return Unexpected{MatrixStatus::NotSymmetric};

Example — caller side:

auto result = cholesky(A);
if (!result) {
    handle(result.error());
    return;
}
auto& L = *result;

Member Typedef Documentation

error_type

template<typename T , typename E >

using Expected< T, E >::error_type = E

value_type

template<typename T , typename E >

using Expected< T, E >::value_type = T

Constructor & Destructor Documentation

Expected() [1/4]

template<typename T , typename E >

Expected< T, E >::Expected ( T &&  v )

inlinenoexcept

Construct in the success state by moving a value in.

Parameters

v	The success value (moved from).

Expected() [2/4]

template<typename T , typename E >

Expected< T, E >::Expected ( Unexpected< E >  u )

inlinenoexcept

Construct in the error state from an Unexpected tag.

Parameters

u	An Unexpected wrapper carrying the error code.

Expected() [3/4]

template<typename T , typename E >

Expected< T, E >::Expected ( Expected< T, E > &&  other )

inlinenoexcept

Move constructor.

Placement-new constructs the active member from the source. The source is left in a valid but unspecified state.

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Expected() [4/4]

template<typename T , typename E >

Expected< T, E >::Expected ( const Expected< T, E > &  )

delete

Deleted — Expected is move-only to prevent accidental copies.

~Expected()

template<typename T , typename E >

Expected< T, E >::~Expected ( )

inlinenoexcept

Destructor. Destroys the active union member.

Member Function Documentation

error()

template<typename T , typename E >

E Expected< T, E >::error ( ) const

inlinenoexcept

Access the error code.

Warning

Undefined behavior if in the success state.

has_value()

template<typename T , typename E >

bool Expected< T, E >::has_value ( ) const

inlinenoexcept

Returns true if the Expected holds a success value.

operator bool()

template<typename T , typename E >

Expected< T, E >::operator bool ( ) const

inlineexplicitnoexcept

Returns true if the Expected holds a success value.

operator*() [1/2]

template<typename T , typename E >

const T & Expected< T, E >::operator* ( ) const

inlinenoexcept

Dereference operator (const) — access the success value.

Warning

Undefined behavior if in the error state.

operator*() [2/2]

template<typename T , typename E >

T & Expected< T, E >::operator* ( )

inlinenoexcept

Dereference operator — access the success value.

Warning

Undefined behavior if in the error state.

operator->() [1/2]

template<typename T , typename E >

const T * Expected< T, E >::operator-> ( ) const

inlinenoexcept

Arrow operator (const) — access members of the success value.

Warning

Undefined behavior if in the error state.

operator->() [2/2]

template<typename T , typename E >

T * Expected< T, E >::operator-> ( )

inlinenoexcept

Arrow operator — access members of the success value.

Warning

Undefined behavior if in the error state.

operator=() [1/2]

template<typename T , typename E >

Expected & Expected< T, E >::operator= ( const Expected< T, E > &  )

delete

Deleted — Expected is move-only to prevent accidental copies.

operator=() [2/2]

template<typename T , typename E >

Expected & Expected< T, E >::operator= ( Expected< T, E > &&  other )

inlinenoexcept

Move assignment.

Destroys the current active member, then placement-new constructs from the source.

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value() [1/2]

template<typename T , typename E >

const T & Expected< T, E >::value ( ) const

inlinenoexcept

Access the success value by const reference.

Warning

Undefined behavior if in the error state.

value() [2/2]

template<typename T , typename E >

T & Expected< T, E >::value ( )

inlinenoexcept

Access the success value by reference.

Warning

Undefined behavior if in the error state.

Member Data Documentation

err_

template<typename T , typename E >

E Expected< T, E >::err_

val_

template<typename T , typename E >

T Expected< T, E >::val_

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The documentation for this class was generated from the following file:

• core/include/utilities/expected.h