std::expected

 

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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)

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 <expected>
#include <iostream>
#include <string_view>
 
enum class parse_error {
    invalid_char,
    overflow
};
 
std::expected<double, parse_error>
parse_number(std::string_view& str) {
    const char* begin = str.data();
    char* end;
    double retval = std::strtod(begin, &end);
 
    if (begin == end) {
        return std::unexpected(parse_error::invalid_char);
    }
    // else if (retval == std::numeric_limits<double>::infinity) {
    //     return std::unexpected(parse_error::overflow);
    // }
 
    str.remove_prefix(end - begin);
    return retval;
}
 
auto main(void) -> int {
 
    std::string_view src = "12";
    // change the src to "meow" or something invalid
    // that is can not be converted to a number and observe the output
    auto num = parse_number(src);
 
    if (num.has_value()) {
        // if num does not have a value
        // dereferencing num will cause an undefined behaviour
        std::cout << *num << "\n";
        // num.value():     throws std::bad_expected_access
        // num.value_or(0): uses specified default value
    }
    else if (num.error() == parse_error::invalid_char) {
        // handle ...
    }
    else if (num.error() == parse_error::overflow) {
        // handle ...
    }
    else {
        // do another thing
    }
 
    return EXIT_SUCCESS;
}

Output:

12

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)