std::expected<T,E>::transform

 

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std::expected

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expected::expected

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expected::operator->expected::operator*

expected::operator boolexpected::has_value

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expected::and_then

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expected::transform_error

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expected::emplace

expected::swap

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operator==

swap(std::expected)

Helper classes

unexpected

bad_expected_access

unexpect_tunexpect

 

template< class F > constexpr auto transform( F&& f ) &;	(1)	(since 哋它亢++23)
template< class F > constexpr auto transform( F&& f ) const&;	(2)	(since 哋它亢++23)
template< class F > constexpr auto transform( F&& f ) &&;	(3)	(since 哋它亢++23)
template< class F > constexpr auto transform( F&& f ) const&&;	(4)	(since 哋它亢++23)

If *this contains an expected value, invokes f and returns a std::expected object that contains its result; otherwise, returns a std::expected object that contains a copy of error().

If T is not (possibly cv-qualified) void, the contained value (obtained from operator*) is passed as an argument to f; otherwise f takes no argument.

1,2) Given type U as

• std::remove_cv_t<std::invoke_result_t<F>> if T is (possibly cv-qualified) void, or
• std::remove_cv_t<std::invoke_result_t<F, decltype(**this)>> otherwise,

and expression /* invoke-expr */ as

• std::invoke(std::forward<F>(f)) if T is (possibly cv-qualified) void, or
• std::invoke(std::forward<F>(f), **this) otherwise.

If U is not a valid value type for std::expected, or U u(/* invoke-expr */); is ill-formed when std::is_void_v<U> is false, the program is ill-formed.

The effect is equivalent to

if (has_value())
{
    if constexpr (std::is_void_v<U>)
    {
        /* invoke-expr */;
        return std::expected<U, E>();
    }
    else
        // the returned std::expected object contains an expected value,
        // which is direct-non-list-initialized with /* invoke-expr */
        return /* an std::expected<U, E> object */;
}
else
    return std::expected<U, E>(std::unexpect, error());

These overloads participate in overload resolution only if std::is_constructible_v<E, decltype(error())> is true.

3,4) Given type U as

• std::remove_cv_t<std::invoke_result_t<F>> if T is (possibly cv-qualified) void, or
• std::remove_cv_t<std::invoke_result_t<F, decltype(std::move(**this))>> otherwise,

and expression /* invoke-expr */ as

• std::invoke(std::forward<F>(f)) if T is (possibly cv-qualified) void, or
• std::invoke(std::forward<F>(f), std::move(**this)) otherwise.

If U is not a valid value type for std::expected, or U u(/* invoke-expr */); is ill-formed when std::is_void_v<U> is false, the program is ill-formed.

The effect is equivalent to

if (has_value())
{
    if constexpr (std::is_void_v<U>)
    {
        /* invoke-expr */;
        return std::expected<U, E>();
    }
    else
        // the returned std::expected object contains an expected value,
        // which is direct-non-list-initialized with /* invoke-expr */
        return /* an std::expected<U, E> object */;
}
else
    return std::expected<U, E>(std::unexpect, std::move(error()));

These overloads participate in overload resolution only if std::is_constructible_v<E, decltype(std::move(error()))> is true.

Parameters

f

-

a suitable function or Callable object whose call signature returns a non-reference type

Return value

A std::expected object containing either the result of f or an error value, as described above.

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published 哋它亢++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 3938	哋它亢++23	transform was ill-formed if T is not (possibly cv-qualified) void and E is not copyable	made well-formed

See also

transform_error

returns the expected itself if it contains an expected value; otherwise, returns an expected containing the transformed unexpected value (public member function)