std::expected

 

C++

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std::expected

Member functions

expected::expected (C++23)

expected::~expected (C++23)

expected::operator= (C++23)

Observers

expected::operator->expected::operator* (C++23)(C++23)

expected::operator boolexpected::has_value (C++23)(C++23)

expected::value (C++23)

expected::error (C++23)

expected::value_or (C++23)

Monadic operations

expected::and_then (C++23)

expected::or_else (C++23)

expected::transform (C++23)

expected::transform_error (C++23)

Modifiers

expected::emplace (C++23)

expected::swap (C++23)

Non-member functions

operator== (C++23)

swap (C++23)

Helper classes

unexpected (C++23)

bad_expected_access (C++23)

unexpect_tunexpect (C++23)(C++23)

 

Defined in header <expected>
template< class T, class E > class expected;		(since C++23)

The class template std::expected provides a way to store either of two values. An object of std::expected at any given time either holds an expected value of type T, or an unexpected value of type E. std::expected is never valueless.

The stored value is allocated directly within the storage occupied by the expected object. No dynamic memory allocation takes place.

A program is ill-formed if it instantiates an expected with a reference type, a function type, or a specialization of std::unexpected. In addition, T must not be std::in_place_t or std::unexpect_t.

Template parameters

T	-	the type of the expected value. The type must either be (possibly cv-qualified) void, or meet the Destructible requirements (in particular, array and reference types are not allowed).
E	-	the type of the unexpected value. The type must meet the Destructible requirements, and must be a valid template argument for std::unexpected (in particular, arrays, non-object types, and cv-qualified types are not allowed).

Member types

Member type	Definition
value_type (C++23)	T
error_type (C++23)	E
unexpected_type (C++23)	std::unexpected<E>
rebind (C++23)	template< class U > using rebind = expected<U, error_type>;

Member functions

(constructor) (C++23)	constructs the expected object (public member function)
(destructor) (C++23)	destroys the expected object, along with its contained value (public member function)
operator= (C++23)	assigns contents (public member function)
Observers
operator->operator* (C++23)	accesses the expected value (public member function)
operator boolhas_value (C++23)	checks whether the object contains an expected value (public member function)
value (C++23)	returns the expected value (public member function)
error (C++23)	returns the unexpected value (public member function)
value_or (C++23)	returns the expected value if present, another value otherwise (public member function)
Monadic operations
and_then (C++23)	returns the result of the given function on the expected value if it exists; otherwise, returns the expected itself (public member function)
transform (C++23)	returns an expected containing the transformed expected value if it exists; otherwise, returns the expected itself (public member function)
or_else (C++23)	returns the expected itself if it contains an expected value; otherwise, returns the result of the given function on the unexpected value (public member function)
transform_error (C++23)	returns the expected itself if it contains an expected value; otherwise, returns an expected containing the transformed unexpected value (public member function)
Modifiers
emplace (C++23)	constructs the expected value in-place (public member function)
swap (C++23)	exchanges the contents (public member function)

Non-member functions

operator== (C++23)	compares expected objects (function template)
swap(std::expected) (C++23)	specializes the std::swap algorithm (function)

Helper classes

unexpected (C++23)	represented as an unexpected value (class template)
bad_expected_access (C++23)	exception indicating checked access to an expected that contains an unexpected value (class template)
unexpect_tunexpect (C++23)	in-place construction tag for unexpected value in expected (class) (constant)

Notes

Types with the same functionality are called Result in Rust and Either in Haskell.

Feature-test macro	Value	Std	Comment
__cpp_lib_expected	202202L	(C++23)	class template std::expected and associated helper classes
	202211L	(C++23)	Monadic functions for std::expected

Example

Run this code

#include <cmath>
#include <expected>
#include <iomanip>
#include <iostream>
#include <string_view>
 
enum class parse_error
{
    invalid_input,
    overflow
};
 
auto parse_number(std::string_view& str) -> std::expected<double, parse_error>
{
    const char* begin = str.data();
    char* end;
    double retval = std::strtod(begin, &end);
 
    if (begin == end)
        return std::unexpected(parse_error::invalid_input);
    else if (std::isinf(retval))
        return std::unexpected(parse_error::overflow);
 
    str.remove_prefix(end - begin);
    return retval;
}
 
int main()
{
    auto process = [](std::string_view str)
    {
        std::cout << "str: " << std::quoted(str) << ", ";
        if (const auto num = parse_number(str); num.has_value())
        {
            std::cout << "value: " << *num << '\n';
            // If num did not have a value, dereferencing num
            // would cause an undefined behavior, and
            // num.value() would throw std::bad_expected_access.
            // num.value_or(123) uses specified default value 123.
        }
        else if (num.error() == parse_error::invalid_input)
        {
            std::cout << "error: invalid input\n";
        }
        else if (num.error() == parse_error::overflow)
        {
            std::cout << "error: overflow\n";
        }
        else
        {
            std::cout << "unexpected!\n"; // or invoke std::unreachable();
        }
    };
 
    for (auto src: { "42", "42abc", "meow", "inf" })
        process(src);
}

Output:

str: "42", value: 42
str: "42abc", value: 42
str: "meow", error: invalid input
str: "inf", error: overflow

References

• C++23 standard (ISO/IEC 14882:2023):

See also

variant (C++17)	a type-safe discriminated union (class template)
optional (C++17)	a wrapper that may or may not hold an object (class template)