API: C++ and MSVC extensions

[PMeira]

Regarding the API side (not implementation of the library), it could be useful to add a header that maps the C-API to the equivalent COM presentation of the API using MSVC properties (non-standard C++): https://docs.microsoft.com/en-us/cpp/cpp/property-cpp?view=msvc-170

This shouldn't be too hard to create or maintain but would take a few days to implement and test. This would complement our current offerings to provide almost drop-in replacements for the COM module (at the moment, we have DSS Python, DSS Sharp, and DSS MATLAB).

The extension has also been implemented in Clang for a while (several years, needs -fms-extensions or similar compiler flags), so this could be a path to migrate Windows-specific C++ code to Linux/macOS without too much hassle. Apparently this is also available in GCC but I couldn't find a reference in the docs, but there are comments like https://blog.aaronballman.com/2011/11/an-almost-useful-language-extension/#comment-208718 that suggest it really is available. Even in the Wikipedia section it lists GCC as supporting it: https://en.wikipedia.org/wiki/Property_%28programming%29#C++ -- I couldn't find anything in GCC's testsuite, so maybe this was removed. There are always other "less clean" ways to implement a similar API in C++ if it comes to that.

Unless more users request this, it's is not a high priority at the moment, it's just "nice to have" by v1.0.

[PMeira]

Unfortunately -fms-extensions pollutes the class namespace since the getter and setter must be public, and using templates together to that doesn't work. Since actual COM usage in C++ would be worse (handling of VARIANT arrays), I left only the function-style access (like OpenDSSDirect.jl/.py), for now.

The new Obj_* and Batch_* APIs are implemented too, so we can bypass the parser:

#include "dss.hpp"

//...

    dss::APIUtil ctx_util(ctx);
    dss::IDSS dss(&ctx_util);

    dss.Text.Command("redirect '../electricdss-tst/Version8/Distrib/IEEETestCases/NEVTestCase/NEVMASTER.DSS'");
    ctx_util.create<dss::obj::Load>("a_new_load").bus1("loadbus.1.2.3").kW(1234).pf(0.85).Vminpu(0.8).end_edit();
    dss.ActiveCircuit.Solution.Solve();
    auto voltages = dss.ActiveCircuit.AllBusVolts<std::vector<double>>();

So, it shouldn't look too alien for most users. OpenDSS errors are already mapped to exceptions.

The initial set of headers will be uploaded here soon.

This would complement our current offerings to provide almost drop-in replacements for the COM module (at the moment, we have DSS Python, DSS Sharp, and DSS MATLAB).

Like mentioned in dss-extensions/OpenDSSDirect.py#80 (comment), we will probably provide a COM implementation in the future.

Alternatively, I'm thinking about wrapping the official COM API in dss_hpp, exposing the same C++ to the user, without the need to handle VARIANT arrays and things like that. This, of course, would only be useful on Windows, or maybe when running via Wine.