Re: [boost] [config/multiprecision/units/general] Do we have a policy for user-defined-literals?

Subject: Re: [boost] [config/multiprecision/units/general] Do we have a policy for user-defined-literals?
From: John Maddock (john_at_[hidden])
Date: 2013-04-28 08:46:24

>> Note however, that constructors may be less efficient in general - 
>> cpp_int users would have to fall back on a construct-from-string rather 
>> than constexpr initialisation (the issue is you can't write a number with 
>> enough digits unless it has a user-defined-suffix).
>
> Can't you construct from string constexpr? Both gcc and clang are happy 
> with code like this (just an experiment to see what constexpr accepts):
>
> struct uint128 {
>   unsigned long h, l;
>   constexpr uint128(unsigned long h_, unsigned long l_):h(h_),l(l_){}
>   constexpr uint128 lshift(int i)const{
>     return uint128{h<<4|l>>60,l<<4|i};
>   }
>   static constexpr int chartoint(char c){
>     return (c>='0'&&c<='9')?c-'0':(c-'a'+10);
>   }
>   static constexpr uint128 from_string(uint128 tmp, const char* s){
>     return (*s==0)?tmp:from_string(tmp.lshift(chartoint(*s)),s+1);
>   }
> };
>
> int main(){
>   constexpr uint128 
> a=uint128::from_string(uint128{0,0},"1234567890abcdef123");
>   static_assert(a.l==0x4567890abcdef123,"");
>   static_assert(a.h==0x123,"");
> }

I had no idea you could do that!  It's still a lot easier to code a 
user-defined-literal though ;-)

Thanks, John.