How to install g++ 4.7.2 & c++11 on CentOS 5.x?

From this answer to "Install gcc 4.7 on CentOS [6.x]", the easiest way to get g++ 4.7, and the required tools and libraries, for CentOS 5.x is via the devtools package:

cd /etc/yum.repos.d
wget http://people.centos.org/tru/devtools/devtools.repo 
yum --enablerepo=testing-devtools-6 install devtoolset-1.0

Since you're running g++ manually (as opposed to through make), you'll need to update your $PATH variable so your shell will use the new gcc, g++, etc. binaries:

export PATH=/opt/centos/devtoolset-1.0/root/usr/bin/:$PATH

At this point, your g++ should be version 4.7.0

$ g++ --version
g++ (GCC) 4.7.0 20120507 (Red Hat 4.7.0-5)
Copyright (C) 2012 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

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Be aware that installing gcc and associated tools and libraries in this manner relies on the repository maintainer to keep their packages up to date.

If you're interested in keeping your gcc up to date, you may want to consider building gcc from source.

Also, compiling programs with a different version of g++ and libstdc++ than are installed on your system can cause all kinds of Fun, coping with which is beyond the scope of this answer. It may be worth moving to a Linux distribution that has support for what you're doing.