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.