Naturally, this library is demanding of the optimizer. I believe someone else has verified that it is possible to achieve zero overhead under Visual C++ 8.0 as well...
unit_example_14.cpp passes double by value and quantity by reference. When I looked at the assembler output from msvc 8.0 it looked like this had a noticable effect (the quantity code was quite a bit more complex).
Interesting. I checked both permutations : double by const reference and quantity by value and get const reference : f(x,y,z) took 4.28 seconds to run 1e+09 iterations with double = 9.34579e+08 flops f(x,y,z) took 4.29 seconds to run 1e+09 iterations with quantity<double> = 9.32401e+08 flops g(x,y,z) took 4.27 seconds to run 1e+09 iterations with double = 9.36768e+08 flops g(x,y,z) took 4.3 seconds to run 1e+09 iterations with quantity<double> = 9.30233e+08 flops value : f(x,y,z) took 4.29 seconds to run 1e+09 iterations with double = 9.32401e+08 flops f(x,y,z) took 4.28 seconds to run 1e+09 iterations with quantity<double> = 9.34579e+08 flops g(x,y,z) took 4.28 seconds to run 1e+09 iterations with double = 9.34579e+08 flops g(x,y,z) took 4.28 seconds to run 1e+09 iterations with quantity<double> = 9.34579e+08 flops These differences look negligible to me...obviously, this depends on the compiler/optimizer since there is no reason why there should be any runtime cost... Matthias