You can configure apt to install select packages from testing while leaving the rest of the system on stable, and g++ should be something that you can handle that way without too much difficulty since g++-4.7 is a separate package that is simultaneously installable with older versions.
In brief, add:
Package: * Pin: release a=testing Pin-Priority: 100
/etc/apt/preferences or a file in
/etc/apt/preferences.d/, which ensures that the packages aren't installed by default, and then add the wheezy repository to your
deb http://ftp.us.debian.org/debian testing main deb-src http://ftp.us.debian.org/debian testing main
and after an
apt-get update you should be able to install g++-4.7. Hopefully it won't pull in too many other packages. You will then be able to use g++-4.7 as a compiler (without changing the default g++).