Install libstdc++-5-doc by entering the following commands in the terminal:
sudo apt update sudo apt install libstdc++-5-doc
Description:
GNU Standard C++ Library v3 (documentation files)
This package contains documentation files for the GNU stdc++ library. . One set is the distribution documentation, the other set is the source documentation including a namespace list, class hierarchy, alphabetical list, compound list, file list, namespace members, compound members and file members.
Homepage: http://gcc.gnu.org/
Version: 5.5.0-12ubuntu1
Section: universe/doc