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std::expected<T,E>:: operator bool, std::expected<T,E>:: has_value

From cppreference.net
C++
Utilities library
std::expected
constexpr explicit operator bool ( ) const noexcept ;
(1) (since C++23)
constexpr bool has_value ( ) const noexcept ;
(2) (since C++23)

Checks whether * this represents an expected value.

Contents

Return value

has_val

Notes

A std::expected object is never valueless. If has_value() returns true , operator*() can be used to access the expected value; otherwise, error() can be used to access the unexpected value.

Example

Run this code 
#include <charconv>
#include <concepts>
#include <cstdint>
#include <expected>
#include <print>
#include <string>
#include <string_view>
#include <system_error>
template<std::integral Int = int>
constexpr std::expected<Int, std::string> to_int(std::string_view str)
{
    Int value{};
    const auto [_, ec] = std::from_chars(str.data(), str.data() + str.size(), value);
    if (ec == std::errc())
        return value;
    return std::unexpected{std::move(std::make_error_code(ec).message())};
}
int main()
{
    if (auto result = to_int("42"); result.has_value())
        std::println("{}", *result); // after the check it is safe to use operator*
    else
        std::println("{}", result.error());
    if (const auto result = to_int("not a number"); result)
        std::println("{}", *result);
    else
        std::println("{}", result.error());
    if (const auto result{to_int<std::int16_t>("32768")}) // implicitly calls (1)
        std::println("{}", *result);
    else
        std::println("{}", result.error());
}

Possible output: 

42
Invalid argument
Numerical result out of range

See also

accesses the expected value
(public member function)
returns the unexpected value
(public member function)
checks whether the object contains a value
(public member function of std::optional<T> )