On 4/29/2013 12:18 PM, Vicente Botet wrote:
No, this is not solved by implicit/explicit conversion, but using closed arithmetic. That is
fixed<8>* fixed<8>-> fixed<8>
The C++1y proposal uses open arithmetic and
fixed<8>* fixed<8>-> fixed<16>
Using closed arithmetic the user must explicitly convert to the desired result
fixed<8> a; fixed<16> b = fixed<16>(a) * fixed<16>(a);
Whenever possible fixed-point multiply is done in a larger temporary then scaled back down. If you can't use a larger internal type it becomes much more complicated and takes many more instructions to do the operation. I don't see how this explicit conversion of the operands helps. You've gone from needing a 32bit internal type to a 64bit internal type. If you promote on multiply things get out of hand pretty quickly when you chain operations writing natural code. fixed<16,16> a; auto b = a*a*a*a; b is a 128bit fixed<64,64> There is a middle ground solution whereby: a*a -> promoted_fixed<32,32> so a*a*a*a would be: fixed<16,16> * fixed<16,16> -> promoted_fixed<32,32> promoted_fixed<32,32> * fixed<16,16> -> promoted_fixed<32,32> promoted_fixed<32,32> * fixed<16,16> -> promoted_fixed<32,32> Then do the scaling down only at the end. fixed<16,16> b = a*a*a*a; Unfortunately auto no longer does the intuitive thing here, but there is precedence for that in the language. This is what I've chosen in the past.