Aleph::expected< T, E > Class Template Reference

Minimal std::expected-style result type for C++20. More...

#include <ah-cpp-compat.H>

Public Types
using	value_type = T
using	error_type = E
using	unexpected_type = unexpected< E >

Public Member Functions
constexpr	expected ()
	Default-construct with a value.
template<class U = T, class = std::enable_if_t< std::is_constructible_v<T, U &&> and not std::is_same_v<cpp_compat_detail::remove_cvref_t<U>, expected> and not std::is_same_v<cpp_compat_detail::remove_cvref_t<U>, unexpect_t>>>
constexpr	expected (U &&v)
	Construct from a value.
template<class G >
constexpr	expected (const unexpected< G > &u)
	Construct an error from unexpected (lvalue).
template<class G >
constexpr	expected (unexpected< G > &&u)
	Construct an error from unexpected (rvalue).
template<class... Args>
constexpr	expected (unexpect_t, Args &&...args)
	Construct an error in place.
	expected (const expected &o)
	Copy constructor.
	expected (expected &&o) noexcept(std::is_nothrow_move_constructible_v< T > and std::is_nothrow_move_constructible_v< E >)
	Move constructor.
expected &	operator= (const expected &o)
	Copy assignment.
expected &	operator= (expected &&o) noexcept(std::is_nothrow_move_constructible_v< T > and std::is_nothrow_move_constructible_v< E >)
	Move assignment.
	~expected ()
	Destructor.
constexpr bool	has_value () const noexcept
	Check if a value is held.
constexpr	operator bool () const noexcept
	Contextual conversion to bool.
constexpr const T &	operator* () const &noexcept
	Access the value (const lvalue).
constexpr T &	operator* () &noexcept
	Access the value (lvalue).
constexpr const T *	operator-> () const noexcept
	Access the value via pointer (const).
constexpr T *	operator-> () noexcept
	Access the value via pointer (mutable).
constexpr const T &	value () const &
	Access the value with exception on error (const lvalue).
constexpr T &	value () &
	Access the value with exception on error (lvalue).
constexpr const E &	error () const &noexcept
	Access the error (const lvalue).
constexpr E &	error () &noexcept
	Access the error (lvalue).
template<class U >
constexpr T	value_or (U &&default_value) const &
	Get the value or a default.
template<class F >
auto	and_then (F &&f) const &
	Monadic bind: chain another fallible operation.
template<class F >
auto	transform (F &&f) const &
	Monadic map: transform the value.
template<class F >
auto	or_else (F &&f) const &
	Monadic error recovery: handle the error.

Private Member Functions
void	destroy () noexcept

Private Attributes
union {
T   val_
E   err_
};
bool	has_

Detailed Description

template<class T, class E>
class Aleph::expected< T, E >

Minimal std::expected-style result type for C++20.

Holds either a value of type T or an error of type E. Aliased to std::expected when the toolchain provides <expected>. See the file header for the supported surface.

Template Parameters

T	value type.
E	error type.

Definition at line 285 of file ah-cpp-compat.H.

Member Typedef Documentation

error_type

template<class T , class E >

using Aleph::expected< T, E >::error_type = E

Definition at line 307 of file ah-cpp-compat.H.

unexpected_type

template<class T , class E >

using Aleph::expected< T, E >::unexpected_type = unexpected<E>

Definition at line 308 of file ah-cpp-compat.H.

value_type

template<class T , class E >

using Aleph::expected< T, E >::value_type = T

Definition at line 306 of file ah-cpp-compat.H.

Constructor & Destructor Documentation

expected() [1/7]

template<class T , class E >

constexpr Aleph::expected< T, E >::expected ( )

inlineconstexpr

Default-construct with a value.

Constructs the value member via default-initialization.

Precondition

T must be default-constructible.

Postcondition

has_value() == true.

Exceptions

Any	exception thrown by T's default constructor.

Complexity O(1) plus T construction.

Exception Safety Basic guarantee: if construction throws, no object.

Definition at line 320 of file ah-cpp-compat.H.

expected() [2/7]

template<class T , class E >

template<class U = T, class = std::enable_if_t< std::is_constructible_v<T, U &&> and not std::is_same_v<cpp_compat_detail::remove_cvref_t<U>, expected> and not std::is_same_v<cpp_compat_detail::remove_cvref_t<U>, unexpect_t>>>

constexpr Aleph::expected< T, E >::expected ( U &&  v )

inlineconstexpr

Construct from a value.

Implicit conversion from U to expected<T,E> when T is constructible from U. Enables patterns like return 42; from a function returning expected<int, Error>.

Template Parameters

U	Value type (forwarded to T).

Parameters

v	The value to store.

Postcondition

has_value() == true and *this == v.

Exceptions

Any	exception thrown by T(std::forward<U>(v)).

Complexity O(1) plus T construction.

Exception Safety Basic guarantee.

Definition at line 340 of file ah-cpp-compat.H.

expected() [3/7]

template<class T , class E >

template<class G >

constexpr Aleph::expected< T, E >::expected ( const unexpected< G > &  u )

inlineconstexpr

Construct an error from unexpected (lvalue).

Template Parameters

G	Error type of the unexpected (must convert to E).

Parameters

u	The unexpected error wrapper.

Postcondition

has_value() == false and error() == u.error().

Exceptions

Any	exception thrown by E(u.error()).

Complexity O(1) plus E construction.

Definition at line 353 of file ah-cpp-compat.H.

expected() [4/7]

template<class T , class E >

template<class G >

constexpr Aleph::expected< T, E >::expected ( unexpected< G > &&  u )

inlineconstexpr

Construct an error from unexpected (rvalue).

Template Parameters

G	Error type of the unexpected (must convert to E).

Parameters

u	The unexpected error wrapper.

Postcondition

has_value() == false and error() == std::move(u).error().

Exceptions

Any	exception thrown by E(std::move(u).error()).

Complexity O(1) plus E construction.

Definition at line 366 of file ah-cpp-compat.H.

expected() [5/7]

template<class T , class E >

template<class... Args>

constexpr Aleph::expected< T, E >::expected ( unexpect_t  , Args &&...  args  )

inlineexplicitconstexpr

Construct an error in place.

Pass Aleph::unexpect as the first argument to construct the error from arbitrary arguments forwarded to E's constructor:

expected<int, std::string> e(unexpect, 5, 'x'); // E constructed as string(5,'x')

Template Parameters

Args	Argument types forwarded to E.

Parameters

args	Arguments forwarded to E's constructor.

Postcondition

has_value() == false.

Exceptions

Any	exception thrown by E(std::forward<Args>(args)...).

Complexity O(1) plus E construction.

Definition at line 386 of file ah-cpp-compat.H.

expected() [6/7]

template<class T , class E >

Aleph::expected< T, E >::expected ( const expected< T, E > &  o )

inline

Copy constructor.

Copies the active member (value or error) from o.

Parameters

o	Source expected to copy from.

Postcondition

has_value() == o.has_value() and active member equals o's.

Exceptions

Any	exception thrown by T or E copy constructor.

Complexity O(1) plus active member copy.

Exception Safety Basic guarantee.

Definition at line 401 of file ah-cpp-compat.H.

References Aleph::blossom_maximum_cardinality_matching(), Aleph::expected< T, E >::err_, Aleph::expected< T, E >::has_, and Aleph::expected< T, E >::val_.

expected() [7/7]

template<class T , class E >

Aleph::expected< T, E >::expected ( expected< T, E > &&  o )

inlinenoexcept

Move constructor.

Moves the active member (value or error) from o.

Parameters

o	Source expected to move from.

Postcondition

has_value() == o.has_value() and active member moved from o.

Complexity O(1) plus active member move.

Exception Safety Conditional noexcept: noexcept if both T and E

have noexcept move constructors, otherwise basic guarantee.

Definition at line 419 of file ah-cpp-compat.H.

References Aleph::blossom_maximum_cardinality_matching(), Aleph::expected< T, E >::err_, Aleph::expected< T, E >::has_, and Aleph::expected< T, E >::val_.

~expected()

template<class T , class E >

Aleph::expected< T, E >::~expected ( )

inline

Destructor.

Destroys the active member (value or error).

Complexity O(1) plus active member destruction.

Exception Safety No-throw guarantee.

Definition at line 493 of file ah-cpp-compat.H.

References Aleph::expected< T, E >::destroy().

Member Function Documentation

and_then()

template<class T , class E >

template<class F >

auto Aleph::expected< T, E >::and_then ( F &&  f ) const &

inline

Monadic bind: chain another fallible operation.

If a value is held, invokes f(value) and returns its result (which must be an expected). If an error is held, propagates the error without calling f. Enables chaining fallible operations:

auto res = parse(input)
  .and_then([](int v) -> expected<int, Error> { return validate(v); })
  .and_then([](int v) -> expected<int, Error> { return process(v); });\n  

Template Parameters

F	Callable type: F(const T&) -> expected<U, E>.

Parameters

f	Function to apply to the value.

Returns

Result of f(value) if value is held, otherwise expected<U,E>(error).

Complexity O(1) plus f invocation if value is held.

Definition at line 621 of file ah-cpp-compat.H.

References Aleph::expected< T, E >::err_, Aleph::expected< T, E >::has_, and Aleph::expected< T, E >::val_.

Referenced by TEST(), and TEST().

destroy()

template<class T , class E >

void Aleph::expected< T, E >::destroy ( )

inlineprivatenoexcept

Definition at line 297 of file ah-cpp-compat.H.

References Aleph::expected< T, E >::err_, Aleph::expected< T, E >::has_, and Aleph::expected< T, E >::val_.

Referenced by Aleph::expected< T, E >::~expected(), Aleph::expected< void, E >::~expected(), Aleph::expected< T, E >::operator=(), Aleph::expected< void, E >::operator=(), Aleph::expected< void, E >::operator=(), and Aleph::expected< T, E >::operator=().

error() [1/2]

template<class T , class E >

constexpr E & Aleph::expected< T, E >::error ( ) &

inlineconstexprnoexcept

Access the error (lvalue).

Returns

Mutable reference to the stored error.

Precondition

has_value() == false. Behavior is undefined if a value is held.

Complexity O(1).

Definition at line 582 of file ah-cpp-compat.H.

References Aleph::expected< T, E >::err_.

error() [2/2]

template<class T , class E >

constexpr const E & Aleph::expected< T, E >::error ( ) const &

inlineconstexprnoexcept

Access the error (const lvalue).

Returns

Const reference to the stored error.

Precondition

has_value() == false. Behavior is undefined if a value is held.

Complexity O(1).

Definition at line 575 of file ah-cpp-compat.H.

References Aleph::expected< T, E >::err_.

Referenced by TEST(), and TEST().

has_value()

template<class T , class E >

constexpr bool Aleph::expected< T, E >::has_value ( ) const

inlineconstexprnoexcept

Check if a value is held.

Returns

true if the object holds a value, false if it holds an error.

Complexity O(1).

Definition at line 501 of file ah-cpp-compat.H.

References Aleph::expected< T, E >::has_.

Referenced by TEST(), TEST(), TEST(), and TEST().

operator bool()

template<class T , class E >

constexpr Aleph::expected< T, E >::operator bool ( ) const

inlineexplicitconstexprnoexcept

Contextual conversion to bool.

Returns

true if the object holds a value, false if it holds an error.

Complexity O(1).

Usage Enables if (e) ... where e is an expected.

Definition at line 508 of file ah-cpp-compat.H.

References Aleph::expected< T, E >::has_.

operator*() [1/2]

template<class T , class E >

constexpr T & Aleph::expected< T, E >::operator* ( ) &

inlineconstexprnoexcept

Access the value (lvalue).

Returns

Mutable reference to the stored value.

Precondition

has_value() == true. Behavior is undefined if an error is held.

Complexity O(1).

Definition at line 522 of file ah-cpp-compat.H.

References Aleph::expected< T, E >::val_.

operator*() [2/2]

template<class T , class E >

constexpr const T & Aleph::expected< T, E >::operator* ( ) const &

inlineconstexprnoexcept

Access the value (const lvalue).

Returns

Const reference to the stored value.

Precondition

has_value() == true. Behavior is undefined if an error is held.

Complexity O(1).

Definition at line 515 of file ah-cpp-compat.H.

References Aleph::expected< T, E >::val_.

operator->() [1/2]

template<class T , class E >

constexpr const T * Aleph::expected< T, E >::operator-> ( ) const

inlineconstexprnoexcept

Access the value via pointer (const).

Returns

Const pointer to the stored value.

Precondition

has_value() == true. Behavior is undefined if an error is held.

Complexity O(1).

Definition at line 529 of file ah-cpp-compat.H.

References Aleph::expected< T, E >::val_.

operator->() [2/2]

template<class T , class E >

constexpr T * Aleph::expected< T, E >::operator-> ( )

inlineconstexprnoexcept

Access the value via pointer (mutable).

Returns

Mutable pointer to the stored value.

Precondition

has_value() == true. Behavior is undefined if an error is held.

Complexity O(1).

Definition at line 539 of file ah-cpp-compat.H.

References Aleph::expected< T, E >::val_.

operator=() [1/2]

template<class T , class E >

expected & Aleph::expected< T, E >::operator= ( const expected< T, E > &  o )

inline

Copy assignment.

Replaces the active member with a copy of o's active member. Uses a copy-and-move idiom to provide strong exception safety: if construction of the new member throws, *this is left in its original state.

Parameters

o	Source expected to assign from.

Returns

*this.

Postcondition

has_value() == o.has_value() and active member equals o's.

Exceptions

Any	exception thrown by T or E copy/move constructor.

Complexity O(1) plus destruction + construction.

Exception Safety Strong guarantee: if copy fails, *this unchanged.

Definition at line 442 of file ah-cpp-compat.H.

References Aleph::blossom_maximum_cardinality_matching(), Aleph::expected< T, E >::destroy(), Aleph::expected< T, E >::err_, Aleph::expected< T, E >::has_, and Aleph::expected< T, E >::val_.

operator=() [2/2]

template<class T , class E >

expected & Aleph::expected< T, E >::operator= ( expected< T, E > &&  o )

inlinenoexcept

Move assignment.

Replaces the active member by moving from o. Uses move-and-swap idiom to provide strong exception safety.

Parameters

o	Source expected to move from.

Returns

*this.

Postcondition

has_value() == o.has_value() and active member moved from o.

Complexity O(1) plus destruction + move construction.

Exception Safety Strong guarantee if move is noexcept; otherwise basic.

Conditional noexcept if both T and E have noexcept move ctors.

Definition at line 469 of file ah-cpp-compat.H.

References Aleph::blossom_maximum_cardinality_matching(), Aleph::expected< T, E >::destroy(), Aleph::expected< T, E >::err_, Aleph::expected< T, E >::has_, and Aleph::expected< T, E >::val_.

or_else()

template<class T , class E >

template<class F >

auto Aleph::expected< T, E >::or_else ( F &&  f ) const &

inline

Monadic error recovery: handle the error.

If an error is held, invokes f(error) and returns its result (which must be an expected). If a value is held, propagates the value without calling f. Use this to recover from errors or provide fallbacks:

auto recovered = parse(input)
  .or_else([](const Error &e) -> expected<int, Error> {
    log(e);
    return 0;  // default value on error
  });

Template Parameters

F	Callable type: F(const E&) -> expected<T, F>.

Parameters

f	Function to apply to the error.

Returns

Result of f(error) if error is held, otherwise expected<T,F>(value).

Complexity O(1) plus f invocation if error is held.

Definition at line 672 of file ah-cpp-compat.H.

References Aleph::expected< T, E >::err_, Aleph::expected< T, E >::has_, and Aleph::expected< T, E >::val_.

Referenced by TEST(), and TEST().

transform()

template<class T , class E >

template<class F >

auto Aleph::expected< T, E >::transform ( F &&  f ) const &

inline

Monadic map: transform the value.

If a value is held, invokes f(value) and wraps the result in an expected<U, E>. If an error is held, propagates the error. Use this to apply pure transformations without changing the error channel:

auto doubled = parse(input).transform([](int v) { return v * 2; });

Template Parameters

F	Callable type: F(const T&) -> U.

Parameters

f	Function to apply to the value.

Returns

expected<U,E>(f(value)) if value is held, otherwise expected<U,E>(error).

Complexity O(1) plus f invocation if value is held.

Definition at line 644 of file ah-cpp-compat.H.

References Aleph::blossom_maximum_cardinality_matching(), Aleph::expected< T, E >::err_, Aleph::expected< T, E >::has_, and Aleph::expected< T, E >::val_.

Referenced by TEST().

value() [1/2]

template<class T , class E >

constexpr T & Aleph::expected< T, E >::value ( ) &

inlineconstexpr

Access the value with exception on error (lvalue).

Returns

Mutable reference to the stored value.

Exceptions

bad_expected_access<E>

if an error is held.

Complexity O(1).

Exception Safety Strong guarantee: throws if error is held.

Definition at line 563 of file ah-cpp-compat.H.

References Aleph::blossom_maximum_cardinality_matching(), Aleph::expected< T, E >::err_, Aleph::expected< T, E >::has_, and Aleph::expected< T, E >::val_.

value() [2/2]

template<class T , class E >

constexpr const T & Aleph::expected< T, E >::value ( ) const &

inlineconstexpr

Access the value with exception on error (const lvalue).

Returns

Const reference to the stored value.

Exceptions

bad_expected_access<E>

if an error is held.

Complexity O(1).

Exception Safety Strong guarantee: throws if error is held.

Definition at line 550 of file ah-cpp-compat.H.

References Aleph::blossom_maximum_cardinality_matching(), Aleph::expected< T, E >::err_, Aleph::expected< T, E >::has_, and Aleph::expected< T, E >::val_.

Referenced by TEST(), TEST(), and TEST().

value_or()

template<class T , class E >

template<class U >

constexpr T Aleph::expected< T, E >::value_or ( U &&  default_value ) const &

inlineconstexpr

Get the value or a default.

Returns the stored value if present, otherwise returns default_value converted to T. Useful for providing fallbacks without branching:

int x = parse(input).value_or(0);  // 0 if parse failed

Template Parameters

U	Type of the default value (must convert to T).

Parameters

default_value

Fallback value to return if an error is held.

Returns

The stored value if present, otherwise static_cast<T>(default_value).

Complexity O(1) plus T copy if value is held, or T construction from U if not.

Definition at line 598 of file ah-cpp-compat.H.

References Aleph::expected< T, E >::has_, and Aleph::expected< T, E >::val_.

Referenced by TEST(), and TEST().

Member Data Documentation

[union]

union { ... } Aleph::expected< T, E >

err_

template<class T , class E >

E Aleph::expected< T, E >::err_

Definition at line 293 of file ah-cpp-compat.H.

Referenced by Aleph::expected< T, E >::expected(), Aleph::expected< void, E >::expected(), Aleph::expected< void, E >::expected(), Aleph::expected< T, E >::expected(), Aleph::expected< void, E >::and_then(), Aleph::expected< T, E >::and_then(), Aleph::expected< T, E >::destroy(), Aleph::expected< void, E >::destroy(), Aleph::expected< T, E >::error(), Aleph::expected< void, E >::error(), Aleph::expected< T, E >::error(), Aleph::expected< void, E >::error(), Aleph::expected< T, E >::operator=(), Aleph::expected< void, E >::operator=(), Aleph::expected< void, E >::operator=(), Aleph::expected< T, E >::operator=(), Aleph::expected< void, E >::or_else(), Aleph::expected< T, E >::or_else(), Aleph::expected< T, E >::transform(), Aleph::expected< T, E >::value(), Aleph::expected< void, E >::value(), and Aleph::expected< T, E >::value().

has_

template<class T , class E >

bool Aleph::expected< T, E >::has_

private

Definition at line 295 of file ah-cpp-compat.H.

Referenced by Aleph::expected< T, E >::expected(), Aleph::expected< void, E >::expected(), Aleph::expected< void, E >::expected(), Aleph::expected< T, E >::expected(), Aleph::expected< void, E >::and_then(), Aleph::expected< T, E >::and_then(), Aleph::expected< T, E >::destroy(), Aleph::expected< void, E >::destroy(), Aleph::expected< T, E >::has_value(), Aleph::expected< void, E >::has_value(), Aleph::expected< T, E >::operator bool(), Aleph::expected< void, E >::operator bool(), Aleph::expected< T, E >::operator=(), Aleph::expected< void, E >::operator=(), Aleph::expected< void, E >::operator=(), Aleph::expected< T, E >::operator=(), Aleph::expected< void, E >::or_else(), Aleph::expected< T, E >::or_else(), Aleph::expected< T, E >::transform(), Aleph::expected< T, E >::value(), Aleph::expected< void, E >::value(), Aleph::expected< T, E >::value(), and Aleph::expected< T, E >::value_or().

val_

template<class T , class E >

T Aleph::expected< T, E >::val_

Definition at line 292 of file ah-cpp-compat.H.

Referenced by Aleph::expected< T, E >::expected(), Aleph::expected< T, E >::expected(), Aleph::expected< T, E >::and_then(), Aleph::expected< T, E >::destroy(), Aleph::expected< T, E >::operator*(), Aleph::expected< T, E >::operator*(), Aleph::expected< T, E >::operator->(), Aleph::expected< T, E >::operator->(), Aleph::expected< T, E >::operator=(), Aleph::expected< T, E >::operator=(), Aleph::expected< T, E >::or_else(), Aleph::expected< T, E >::transform(), Aleph::expected< T, E >::value(), Aleph::expected< T, E >::value(), and Aleph::expected< T, E >::value_or().

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

• ah-cpp-compat.H