[systemd-devel] For C++ coders: dbus::Error

[Carlo Wood]

Hi,

I finished an implementation of dbus::Error.

dbus::Error is derived from the POD struct ErrorConst
and has no variable members of its own.

ErrorConst is just a wrapper around sd_bus_error.

    struct ErrorConst {
      sd_bus_error m_error;
    };

    class Error : protected ErrorConst {
    {
      ... only functions here...
    };

This allows to create constexpr errors with
static initialization; for example,

    static constexpr ErrorConst foo_error =
        { SD_BUS_ERROR_MAKE_CONST("org.example.name", "Error description") };

The member functions of dbus::Error support
    SD_BUS_ERROR_NULL (default constructor)
    sd_bus_error_copy (constructing from a ErrorConst (shared), or
                       from an Error that is not shared (deep copy),
                       assignment operator)
    sd_bus_error_move (move constructor, move assignment operator)
    sd_bus_error_free (destructor)
    sd_bus_error_set (constructing from a name with optional message)
    sd_bus_error_is_set (accessor `is_set`)

Finally it can be implicitly be converted to std::error_code!

    operator std::error_code() const;

The latter is the more interesting thing that took 99% of the time
to design this ;).

You can see this in action here:
https://github.com/CarloWood/dbus-task/blob/master/tests/error_test.cxx

The idea is that when, for example, you receive a callback with
a sd_bus_error* (in the message), that it is easy converted to
a std::error_code.

For example,

    int on_reply_concatenate(sd_bus_message* m, void* userdata, sd_bus_error*)
    {
      int is_error = sd_bus_message_is_method_error(m, nullptr);
      if (is_error < 0)
        THROW_ALERTC(-is_error, "sd_bus_message_is_method_error");
      if (is_error)
      {
        sd_bus_error const* error = sd_bus_message_get_error(m);
        dbus::Error dbus_error = error;
        std::error_code error_code = dbus_error;
        std::cout << "error_code = " << error_code << " [" << error_code.message() << "]\n";

See
https://github.com/CarloWood/dbus-task/blob/master/tests/client_side_concatenate_test.cxx
for all the context.

Regards,
Carlo Wood