How can I let TextMate show line numbers when I'm coding C++?

Solution 1:

Press Cmd-Opt-L or select View » Gutter » Line Numbers.

I don't think it's possible to automatically enable display of line numbers dependent on your current language. But with the default keyboard shortcut this command is easy enough to reach.

Solution 2:

View->Gutter->Line Numbers

Enjoy! Its a fun editor.