The main differences ROOT 6 from ROOT 5

ROOT
1

Dear experts!
It often happens that the program written in ROOT 5 does not work in ROOT 6 and vice versa. Could you describe the main differences between ROOT 5 and ROOT 6 or give a link where it is described.

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ROOT 6 is strictly C++ (Cling). ROOT 5 is not (cint)… It allows some constructs which are not valid C++ … In ROOT 5, to be strictly C++, you can use Aclic

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See also https://root.cern.ch/root/html600/notes/release-notes.html