Hi, as described in "Genode Foundations", all types which are used as argument or return types in RPC functions must have a default constructor. I do not think that such a limitation is really necessary. So I tried to write a proof of concept [1]. These modifications should allow RPC functions to pass most of the non-default-constructible objects which satisfy the rest of the requirements for RPC argument and return types. I extended the "hello_tutorial" with a little test code to show that it works [2].
Software patterns like RAII [3] are a possible reason for removing that limitation. Quite often classes cannot provide a useful default-constructor which does more than merely setting the object state to "invalid". Because often there are no useful default values. So if a class provides a default-constructor which just sets the object to "invalid", the code complexity increases since care has to be taken that the object is fed with data up to a specific point of time. Unfortunately, my implementation does not remove default-constructor calls completely: 1. Capabilities still need default-constructors. Msgbuf stores them directly in an array and I did not find the time to check changes of this storage behavior for possible side effects. 2. Output-only _arguments_ have to be default-constructible too. Please note, that non-default-constructible _return_ types are allowed. So this affects only RPC arguments which are explicitly declared as Output-only, for example by adding custom specializations of Rpc_direction. The reason for this limitation is that my Pod_tuple version still stores the RPC arguments directly. While input arguments are copy-constructed, it is hard to copy from an object that does not yet exist. For the second point, I see no simple solutions: we could create a simple byte buffer instead of a default-constructed output-only argument. Since "normal" RPC arguments have to be trivially copyable [4], a RPC function could easily write to this buffer, which would be accessible by the correctly type-casted reference or pointer. A minimalist version of boost::optional [5] or the like as member of Server_args could make sense. However, it would require more template specializations and case differentiations because the special RPC types Capability and Native_capability are not trivially copyable. I successfully tested the code with the repositories "hello_tutorial" and "demo" on base-linux. But if you decide to use my code contribution, please note that it is rather a proof of concept. There is room for improvements: * I tried to minimize the code modifications. So some refactoring may be useful: I expanded Meta::Empty by constructors and a typedef, I inserted possibly confusing forward declarations, and Meta::Pod_tuples and Meta::Pod_args could be renamed because they do not only accept POD types [6]. * I completely ignored the trace-related code. I do not know if there is a need of modifications. Thanks to Johannes Schlatow who gave me the idea of grappling with this topic. [1] https://github.com/3dik/genode/tree/nodef [2] https://github.com/3dik/genode/commit/c35a8d4a3166da74c46423f5e0b4a35933174f3d [3] https://en.wikipedia.org/wiki/Resource_Acquisition_Is_Initialization [4] http://en.cppreference.com/w/cpp/concept/TriviallyCopyable [5] http://www.boost.org/doc/libs/1_62_0/libs/optional/doc/html/boost_optional/tutorial/when_to_use_optional.html [6] http://en.cppreference.com/w/cpp/concept/PODType ------------------------------------------------------------------------------ The Command Line: Reinvented for Modern Developers Did the resurgence of CLI tooling catch you by surprise? Reconnect with the command line and become more productive. Learn the new .NET and ASP.NET CLI. Get your free copy! http://sdm.link/telerik _______________________________________________ genode-main mailing list genode-main@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/genode-main
