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/*
* This source code is released into the public domain.
*/
module;
/*
* error: a type representing an error.
*
* An error consists of an immediate cause, which may be a string or
* std:error_code, and an optional proximate cause, which is another error
* object. Any number of error objects may be stacked.
*
* For example, a failure to open a file might be a stack of two errors:
*
* - string, "failed to open /etc/somefile",
* - std::error_code, "No such file or directory".
*
* Calling .str() will format the entire stack starting at that error,
* for example: "failed to open /etc/somefile: No such file or directory".
*
* Errors may be moved and (relatively) cheaply copied, since the cause
* chain is refcounted.
*
* error derives from std::exception, so it may be thrown and caught and
* provides a useful what(). When throwing errors, creating a derived
* error will make it easier to distinguish errors when catching them.
*/
#include <iosfwd>
#include <format>
#include <memory>
#include <optional>
#include <string>
#include <system_error>
#include <utility>
#include <variant>
export module nihil.error;
namespace nihil {
// Things which can be errors.
using error_t = std::variant<
std::monostate,
std::string,
std::error_code,
std::error_condition
>;
export struct error : std::exception {
// Create an empty error, representing success.
error();
// Destroy an error.
~error() override;
// Create an error from a freeform string.
error(std::string_view what, error cause);
explicit error(std::string_view what);
template<typename Cause>
requires(std::is_error_code_enum<Cause>::value ||
std::is_error_condition_enum<Cause>::value)
error(std::string_view what, Cause &&cause)
: error(what, error(std::forward<Cause>(cause)))
{}
// Create an error from an std::error_code.
error(std::error_condition what, error cause);
explicit error(std::error_condition what);
// Create an error from an std::error_condition.
error(std::error_code what, error cause);
explicit error(std::error_code what);
// Create an error from an std::error_code enum.
error(auto errc, error cause)
requires(std::is_error_code_enum<decltype(errc)>::value)
: error(make_error_code(errc), std::move(cause))
{}
explicit error(auto errc)
requires(std::is_error_code_enum<decltype(errc)>::value)
: error(make_error_code(errc))
{}
// Create an error from an std::error_condition enum.
error(auto errc, error cause)
requires(std::is_error_condition_enum<decltype(errc)>::value)
: error(make_error_condition(errc), std::move(cause))
{}
explicit error(auto errc)
requires(std::is_error_condition_enum<decltype(errc)>::value)
: error(make_error_condition(errc))
{}
error(error const &);
error(error &&) noexcept;
auto operator=(this error &, error const &) -> error &;
auto operator=(this error &, error &&) noexcept -> error &;
// Return the cause of this error.
[[nodiscard]] auto cause(this error const &) -> std::shared_ptr<error>;
// Return the root cause of this error, which may be this object.
// For errors caused by an OS error, this will typically be the
// error_code error.
[[nodiscard]] auto root_cause(this error const &) -> error const &;
// Format this error as a string.
[[nodiscard]] auto str(this error const &) -> std::string;
// Return this error's error_code, if any.
[[nodiscard]] auto code(this error const &)
-> std::optional<std::error_code>;
// Return this error's error_condition, if any.
[[nodiscard]] auto condition(this error const &)
-> std::optional<std::error_condition>;
[[nodiscard]] auto what() const noexcept -> char const * final;
private:
friend auto operator==(error const &, error const &) -> bool;
friend auto operator<=>(error const &, error const &)
-> std::strong_ordering;
// This error.
error_t m_error = make_error_code(std::errc());
// The cause of this error, if any.
std::shared_ptr<error> m_cause;
// For std::exception::what(), we need to keep the string valid
// until we're destroyed.
mutable std::optional<std::string> m_what;
};
/*
* Format an error and its cause(s) as a string.
*/
export [[nodiscard]] auto to_string(error const &) -> std::string;
// Compare an error to another error. This only compares the error itself,
// not any nested causes.
export [[nodiscard]] auto operator==(error const &, error const &)
-> bool;
export [[nodiscard]] auto operator<=>(error const &, error const &)
-> std::strong_ordering;
// Compare an error to an std::error_code.
export [[nodiscard]] auto operator==(error const &, std::error_code const &)
-> bool;
// Compare an error to an std::error_condition.
export [[nodiscard]] auto operator==(error const &,
std::error_condition const &)
-> bool;
// Compare an error to an std::error_code enum.
export [[nodiscard]] auto operator==(error const &lhs, auto rhs) -> bool
requires(std::is_error_code_enum<decltype(rhs)>::value)
{
return lhs.code() == rhs;
}
// Compare an error to an std::error_condition enum.
export [[nodiscard]] auto operator==(error const &lhs, auto rhs) -> bool
requires(std::is_error_condition_enum<decltype(rhs)>::value)
{
return lhs.condition() == rhs;
}
// Print an error to an ostream.
export [[nodiscard]] auto operator<<(std::ostream &, error const &)
-> std::ostream &;
} // namespace nihil
// Make error formattable.
export template<>
struct std::formatter<nihil::error, char>
{
template<typename ParseContext>
constexpr auto parse(ParseContext &ctx) -> ParseContext::iterator
{
return ctx.begin();
}
template<typename FormatContext>
auto format(nihil::error const &e, FormatContext &ctx) const
-> FormatContext::iterator
{
return std::ranges::copy(to_string(e), ctx.out()).out;
}
};
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