/usr/share/man/man1/x86_64-linux-gnu-g++.1.gz is in g++ 4:7.3.0-3ubuntu2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/man/man1/g++-7.1.gz
/usr/share/man/man1/g++-7.1.gz