I used to be able to use g++ from the terminal, but now it's missing. I still have XCode installed
I had the same problem at one point with gcc.  gcc and g++ usually link to a version-specific binary.  Look in /Developer/usr/bin for something like g++-x.y, where x.y is the installed version of g++.  You'll probably see gcov, gcc, etc. there as well.  Just create a symbolic link from the versioned g++ to g++.

For example, on my machine, I would type ln -sf g++-4.2 g++.

Hope this helps.  If the versioned compiler tools aren't there, you may need to reinstall the Developer Tools.