SI++ is a header file for C++ that defines a few custom suffixes for floating point values. It allows you to specify real-world physical quantities in a sensible manner:
qspeed v = 180_km/h; qtime week = 7_d; qcurrent I = 10_kA; qmass m = 1_t;
It also converts Celsius and Fahrenheit to Kelvin.
Because SI++ depends on custom literal suffixes, which is a C++11 feature, you'll need a compiler that supports it (and you'll need to compile in C++11 mode).
This program is free software. It comes without any warranty, to the extent permitted by applicable law. You can redistribute it and/or modify it under the terms of the Do What The Fuck You Want To Public License, Version 2, as published by Sam Hocevar. See http://sam.zoy.org/wtfpl/COPYING for more details.