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Subject: Re: [std-proposals] Extending std::bitset
From: Jake Arkinstall (jake.arkinstall_at_[hidden])
Date: 2020-03-12 15:00:42


I think it's more about the code, when a large binary representation
unnecessarily gets in the way compared with something a quarter of the size.

strtoull and co have base arguments, so having the same thing for a bitset
makes some sense IMO.

On Thu, 12 Mar 2020, 19:53 Jorg Brown via Std-Proposals, <
std-proposals_at_[hidden]> wrote:

> Ramy -
>
> You're saying that using strings to initialize bitsets is infeasible
> because of their size. To me, this sounds like you're concerned about, in
> this case, 80 bytes of RAM?
>
> You go on to suggest that the bitset string constructor should accept
> another argument, indicating the base of the string, and that the to_stirng
> member function should accept a base.
>
> But this also infeasible, albeit for a different reason: you're now
> suggesting that the standard library should be able to perform an
> arbitrary-precision conversion of a string from one base to another.
>
> That would cost a lot more than 80 bytes of code... and std::bitset is an
> odd place to put that conversion operation.
>
> I believe that, for your needs, it should be easy to write a pair of
> functions:
>
> template<size_t N>
> std::bitset<N> bitset_from_hex(std::string_view);
>
> template<size_t N>
> std::string hex_from_bitset(const std::bitset<N>&);
>
> Does that work for you?
>
> -- Jorg
>
>
> On Thu, Mar 12, 2020 at 12:25 PM ramy koudsi via Std-Proposals <
> std-proposals_at_[hidden]> wrote:
>
>> Apologies, I must have been referring to an old spec by accident.
>> C++11 and newer does indeed accept a string in one of the constructors.
>>
>> However, the accepted string must be zeroes and ones. This doesn't seem
>> very feasible, especially as we get larger bitsets.
>> Using current C++11 or newer, to initialize a bitset with a value of 80
>> bits, we would have to pass an 80 character string.
>>
>> For this reason I see a need for the bitset string constructor to accept
>> another argument, indicating the base of the string.
>> This way, initializing a value can be done with a much smaller string.
>>
>> A base can also be passed to the to_string member function, telling it to
>> output a string in that base, instead of just zeroes and ones.
>>
>> Thank you,
>>
>> Ramy
>>
>>
>> On March 12, 2020 at 6:07 AM, Magnus Fromreide <magfr_at_[hidden]>
>> wrote:
>>
>> On Thu, Mar 12, 2020 at 01:49:42AM -0400, ramy koudsi via Std-Proposals
>> wrote:
>>
>> Hi,
>>
>>
>> std::bitset does not have a size limit, however it has an initialization
>> size limit. Bitsets cannot currently be initialized with anything bigger
>> than a long long int.
>>
>> To remove this limitation I propose adding another constructor that
>> accepts a std::string object.
>>
>>
>> std::bitset already (since c++11) have a constructor accepting a
>> std::basic_string.
>>
>> /MF
>>
>> I see its use like:
>>
>> std::bitset<80> bigbs(“FFFF95D3FFFFAC27FFFF”);
>>
>>
>> The example above uses hex strings, but can easily be changed to accept
>> strings of any base by adding another constructor accepting a string and an
>> argument showing the base.
>>
>> std::bitset<12> littlebs(“AFD”, 16);
>>
>>
>> In addition, a member function to_string(int base) would be nice.
>>
>> std::string littlestr = littlebs.to_string(16);
>>
>> // littlestr is “AFD”
>>
>>
>> I wanted to hear your thoughts about this. Has anyone thought of this and
>> come up with a better solution?
>>
>>
>> Thank you
>>
>>
>> Ramy
>>
>> --
>>
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>>
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>>
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>>
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