[WIP] Versioning, back-porting, version-detection & forward/backward compatibility

[follower]

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TL;DW:

It would be useful/nice to have:

• A godot-cpp-specific version #define or similar that could be used to conditionally include different code when building with different branches, especially in situations like Ref<> where workarounds that were required in earlier versions cause crashes in later versions. (See: #417)

• Significant godot-cpp fixes (like the Ref<> one above) back-ported from 3.2 to 3.1 so that in many cases one code-base can support both Godot 3.1 & 3.2. (Or, prioritize 3.1 and forward-port to 3.2.)

Update: It seems like the Ref<> issue is compatible with earlier versions also.

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• Versioning, back-porting, version-detection & forward/backward compatibility

• Back-porting to branches of godot-cpp specific fixes/improvements.

• Compile-time version detection

• Run-time version detection

Maybe in:

• https://github.com/godotengine/godot-cpp/blob/master/include/core/Godot.hpp
• https://github.com/godotengine/godot-cpp/blob/master/include/core/GodotGlobal.hpp
• https://github.com/godotengine/godot-cpp/blob/master/include/core/Defs.hpp

Related:

• https://github.com/GodotNativeTools/godot_headers/issues/53
• https://github.com/GodotNativeTools/godot_headers/issues/70
• https://github.com/godotengine/godot/issues/35464
• https://github.com/godot-rust/godot-rust/issues/396
• https://github.com/godotengine/godot/pull/39183
• https://github.com/godotengine/godot/pull/39064
• https://github.com/GodotNativeTools/godot_headers/pull/74
• https://github.com/GodotNativeTools/godot_headers/commit/f2122198d51f230d903f9585527248f6cf411494
• https://github.com/GodotNativeTools/godot_headers/issues/72

[WIP]

[Calinou]

See also https://github.com/godotengine/godot-docs/pull/5744. We are discussing what to do for the 3.x branch and whether it's worth trying to support multiple minor versions with a single compiled add-on, but it doesn't seem to be feasible or worth it.