Is it bad practice to use C features in C++?

Question

For example printf instead of cout, scanf instead of cin, using #define macros, etc?

Answer 1

You should definitely use printf in place of cout. The latter does let you make most or all of the formatting controls printf allows, but it does so in a stateful way. I.e. the current formatting mode is stored as part of the (global) object. This means bad code can leave cout in a state where subsequent output gets misformatted unless you reset all the formatting every time you use it. It also wreaks havoc with threaded usage.