Here is my c++ code if you want to play with it. You'll need to find constrained_value for boost (or I can provide it as a patch if you request). As for semantics, different people actually want/expect different behavior. I chose the 'pythonic' approach. If you do fpa @ fpb, where fpa,b are fixed point numbers of same/different sizes and @ is a binary op, my library will _not_ promote the output. Explicit is better. If you want a(32 bit) x b(32 bit) -> c(64 bit), you promote the operands explicitly first.