Hello- I don't think I will have time to write a full review of units (and I probably don't know enough to seriously critique it anyway), but FWIW, I think it should be accepted, and it looks quite useful and well designed to me. I did try some quick performance tests out of curiosity on a few different systems. Here is the output of the unit_example_14 test program for: 1) gcc 3.4.4 on WinXP / MinGW (Intel P4 2.8 GHz) ------------------------------------ f(x,y,z) took 6.593 seconds to run 1e+009 iterations with double = 6.06704e+008 flops f(x,y,z) took 10.782 seconds to run 1e+009 iterations with quantity<double> = 3.70989e+008 flops g(x,y,z) took 6.625 seconds to run 1e+009 iterations with double = 6.03774e+008 flops g(x,y,z) took 9.906 seconds to run 1e+009 iterations with quantity<double> = 4.03796e+008 flops ------------------------------------ 2) gcc 3.4.4 on WinXP / cygwin (Intel P4 2.8 GHz) ------------------------------------ f(x,y,z) took 6.5 seconds to run 1e+09 iterations with double = 6.15385e+08 flops f(x,y,z) took 10.375 seconds to run 1e+09 iterations with quantity<double> = 3.85542e+08 flops g(x,y,z) took 6.719 seconds to run 1e+09 iterations with double = 5.95327e+08 flops g(x,y,z) took 10.531 seconds to run 1e+09 iterations with quantity<double> = 3.79831e+08 flops ------------------------------------ 3) gcc 3.4.6 on Linux (Intel Xeon 3.2 GHz) ------------------------------------ f(x,y,z) took 13.76 seconds to run 1e+09 iterations with double = 2.90698e+08 flops f(x,y,z) took 17.34 seconds to run 1e+09 iterations with quantity<double> = 2.30681e+08 flops g(x,y,z) took 13.82 seconds to run 1e+09 iterations with double = 2.89436e+08 flops g(x,y,z) took 14.76 seconds to run 1e+09 iterations with quantity<double> = 2.71003e+08 flops ------------------------------------ 4) gcc 3.4.6 on Linux (AMD Athlon 1.6GHz) ------------------------------------ f(x,y,z) took 27.83 seconds to run 1e+09 iterations with double = 1.4373e+08 flops f(x,y,z) took 33.22 seconds to run 1e+09 iterations with quantity<double> = 1.20409e+08 flops g(x,y,z) took 27.54 seconds to run 1e+09 iterations with double = 1.45243e+08 flops g(x,y,z) took 30.68 seconds to run 1e+09 iterations with quantity<double> = 1.30378e+08 flops ------------------------------------ So gcc, at least, seems to be able to tell the difference when the units are being used, at the level of a few tens of percent. This was compiled with g++ -s -O3 -DNDEBUG I also tried compiling with gcc 3.3.3 but got a large number of errors... I attached the gzipped output in case that is interesting, but maybe this compiler is not supposed to be supported anyway? -Lewis