std::expected<T,E>::operator bool, std::expected<T,E>::has_value
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std::expected
expected::operator boolexpected::has_value
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
[edit] Return value
[edit] 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.
[edit] 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