Le 20/04/13 06:14, Michael Marcin a écrit :
On 4/19/2013 7:15 PM, Vicente J. Botet Escriba wrote:
Le 18/04/13 15:39, Dmitriy Gorbel a écrit :
I want to provide my proposal to the Boost community.
Please, lets discuss it! I will be grateful for your reviews and advices. How can I improve it? I appreciate any feedback you may have.
proposal_Dmitriy_Gorbel.pdf <http://boost.2283326.n4.nabble.com/file/n4645577/proposal_Dmitriy_Gorbel.pdf>
<snip very good feedback>
There is a typo: The range must be *grater* then the resolution
I don't understand your types.
cardinal<16> 0 <= n <= 65536
This seems to be a 16 bit unsigned type but requires 17 bits to store this range. It should probably be 0 <= n <= 65535.
integral<4> -16 <= n <= 16
Similar here this seem to be a 5 it signed integer but requires 6 bits to store this range. It should probably be -16 <= n <= 15.
The intention of Laurence was to be able to return the same type while applying the unary minus operator. With your range the result of -integral<4> is integral<5> or the operation needs to check for overflow, which is not good neither. Using an additional bit allows to overcome this deficiency but of course lost a possible value out of the 2^n. Of course this would needs consensus on this ML.
nonnegative<8,-4> -256 < n < 256 in increments of 2^-4 = 1/16
I don't understand how a type nonnegative can store values in (-256,0).
Yes this should be 0 < n < 256.
negatable<16,-8> -65536 < n < 65536 in increments of 2^-8 = 1/ 256
This seems close to a fixed point type as I'm used to seeing it. Although again the ranges seem wrong.
This depend on the definition. With the definition in http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3352.html this is correct.
I'm much more accustom to seeing fixed point number specified as <Magnitude bits, Fractional Bits> i.e. <16,8> instead of <16,-8>.
The problem I see with this notation is that to represent the numbers -65536<n<65536 in increments of 2^8=256 you need to use a fractional bits that is negative, which is in some way counterintuitive with the name of the template parameter, or maybe we need to find a name for which a negative number is intuitive.
Still this representation makes sense because it specifies both parameters in terms 2^x. It also supports something like <17,1> to give a 16 bit type that has the range [-131072, 131071] in increments of 2.
Still it might be surprising to those familiar with the more common fixed-point notation.
As I said there are several notations and there is no real one that would make happy everyone. So I think that the library should take in account this point and provide some aliases (c++11)/type traits(c++98) for the most common notations. Choosing the default notation is critical and having a consensus on it would be difficult. Do you think that it is worth proposing several default notations and request the boost community to choose the default one? Best, Vicente