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Re: Template argument deduction mystery

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On 05/09/2019 01:55, Juha Nieminen wrote:
> In C++17 you can do this:
> 
> //-----------------------------------------------------
> #include <iostream>
> #include <algorithm>
> #include <functional>
> 
> int main(int argc, char** argv)
> {
>      double values[5] = { 1.1, 2.2, 3.3, 4.4, 5.5 };
> 
>      // Notice this:
>      std::sort(values, values+5, std::greater());
> 
>      for(double v: values) std::cout << v << " ";
>      std::cout << "\n";
> }
> //-----------------------------------------------------
> 
> You don't actually need to specify the template parameter of std::greater.
> It will be automatically deduced as 'double'. That's really nice.
> 
> But wait... How?!?
> 
> There's nothing telling std::greater that the template parameter should be
> of type double. It's not taking any constructor parameters or anything.
> And the declaration of std::sort() is ostensibly like this:
> 
>    template<class RandomIt, class Compare>
>    void sort(RandomIt first, RandomIt last, Compare comp);
> 
> so there's nothing there telling it that it should be double either.
> 
> So how?

Magic?  I assume it's related to the changes in C++17 which extend 
template argument deduction to template classes as well as functions.

All of the information is there, given the signature of Compare is bool 
comp(const Type1 &a, const Type2 &b) and Type1 and Type2 can be deduced 
from RandomIt.

-- 
Ian.

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Template argument deduction mystery Juha Nieminen <nospam@thanks.invalid> - 2019-09-04 13:55 +0000
  Re: Template argument deduction mystery Ian Collins <ian-news@hotmail.com> - 2019-09-05 07:16 +1200
  Re: Template argument deduction mystery Bonita Montero <Bonita.Montero@gmail.com> - 2019-09-05 03:03 +0200
    Re: Template argument deduction mystery Juha Nieminen <nospam@thanks.invalid> - 2019-09-05 21:50 +0000
      Re: Template argument deduction mystery Pavel <pauldontspamtolk@removeyourself.dontspam.yahoo> - 2019-09-05 20:04 -0400
        Re: Template argument deduction mystery Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2019-09-06 14:55 +0100
          Re: Template argument deduction mystery Pavel <pauldontspamtolk@removeyourself.dontspam.yahoo> - 2019-09-06 23:56 -0400
            Re: Template argument deduction mystery Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2019-09-07 15:25 +0100
              Re: Template argument deduction mystery Juha Nieminen <nospam@thanks.invalid> - 2019-09-07 18:55 +0000
                Re: Template argument deduction mystery Bo Persson <bo@bo-persson.se> - 2019-09-07 21:28 +0200
                Re: Template argument deduction mystery Ian Collins <ian-news@hotmail.com> - 2019-09-08 12:29 +1200
                Re: Template argument deduction mystery Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2019-09-07 23:06 +0100
              Re: Template argument deduction mystery Pavel <pauldontspamtolk@removeyourself.dontspam.yahoo> - 2019-09-07 18:35 -0400

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