How to obtain (almost) unique system identifier in a cross platform way?

I'm looking for a way to get a number which will almost surely change when running the code on different machines and almost surely stay the same between two runs on the same machine.

If I were doing this as a shell script in Linux, I would use something like this:

{ uname -n ; cat /proc/meminfo | head -n1 ; cat /proc/cpuinfo ; } | md5sum

But I need this in C++ (with boost) and at least on Windows, Linux and Mac.