illuminate create_window code in D0424R0.pdf

d0424r0_test.cpp

//Purpose:
//  Illustrate the workings of the create_window code found in attachment to:
/*
https://groups.google.com/a/isocpp.org/forum/?utm_medium=email&utm_source=footer#!msg/std-proposals/b1X597zMx9s/4fnQB6gBCwAJ
 */
//Modifications:
//  Renamed create_window to named_args and renamed many other
//  types and variables to make them more generic or readable.
//  Added several comments explaining purpose of various parts.
//  Added extensive print statements to illustrate how code
//  worked.
//=========================
#include <iostream>
template <char ...s>
struct argument 
{
  using this_arg_name_t=argument<s...>;
  template<typename ArgType>
  using this_arg_name_val_t=std::pair<this_arg_name_t,ArgType>;
  
    static std::string
  this_name()
    {
      static char const str[sizeof...(s)+1]={s...,'\0'};
      return str;
    }
    static std::string
  this_type_name()
    {
      std::string prefix("argument<");
      std::string suffix(">");
      return prefix+this_name()+suffix;
    }
  argument()
    {
      std::cout
        <<this_type_name()<<"::default CTOR"
        <<"\n";
    }
    
  /**@brief
   *  Used to generates *actual* arguments to a function.
   */
    template <typename ArgType>
    this_arg_name_val_t<ArgType> 
  operator=(ArgType arg_value) 
    {
      std::cout
        <<this_type_name()<<"::operator=(ArgType arg_value)"
        <<":arg_value="
        <<arg_value
        <<"\n";
      return {{}, arg_value};
    }

  /**@brief
   *  Used in body of function to extract the desired value
   *  of an argument with name, this_arg_name_t::this_name().
   *  This is used in combination with next auto operator()
   *  to linearly search the arguments in the body of a function
   *  to extract the desired argument values.
   */
    template 
    < typename ThisArgType
    , typename ...TailArgNameVals
    >
  auto operator()
    ( this_arg_name_val_t<ThisArgType> a
    , TailArgNameVals ...
    ) const 
    {
      std::cout
        <<this_type_name()
        <<"::operator()(this_arg_name_t<ThisArgType> a, TailArgNameVals...)const"
        <<":a.second="<<a.second
        <<"\n";
      return a.second;
    }

  /**@brief
   *  Used in body of function to extract the desired value
   *  of an argument with name, this_arg_name_t::this_name().
   *  In this call, HeadArgNameVal does *not* match
   *  this_arg_name_val_t<ThisArgType> for any ThisArgType; hence, 
   *  the TailArgNameVals are searched for a match in the
   *  recursive call.
   *  This is used in combination with previous auto operator()
   *  to linearly search the arguments in the body of a function
   *  to extract the desired argument values.
   */
    template 
    < typename HeadArgName 
      //instance of argument<t...> but where t... != s...
    , typename HeadArgType
    , typename ...TailArgNameVals
    >
  auto operator()
    ( std::pair<HeadArgName,HeadArgType>
    , TailArgNameVals ...args
    ) const 
    {
      unsigned const tail_size=sizeof...(TailArgNameVals);
      std::cout
        <<this_type_name()
        <<"::operator()(HeadArgNameVal, TailArgNameVals...)const"
        <<":head_name="<<HeadArgName::this_name()
        <<":tail_size="<<tail_size
        <<"\n";
      static_assert(tail_size != 0, "missing argument");
      return (*this)(args...);
    }
};

template <typename CharT, CharT ...s>
argument<s...> operator"" _arg() 
{
  std::cout<<argument<s...>::this_type_name()<<" operator \"\" _arg()\n"; 
  return {}; 
}

template <typename ...NameVals>
void named_args(NameVals ...name_vals) 
{
  std::cout<<"named_args(NameVals... name_vals)\n";
  int var0 = "var0"_arg(name_vals...);
  std::cout<<"var0="<<var0<<"\n";
  int var1 = "var1"_arg(name_vals...);
  std::cout<<"var1="<<var1<<"\n";
  int var2 = "var2"_arg(name_vals...);
  std::cout<<"var2="<<var2<<"\n";
}

int main() {
  named_args
  ( "var0"_arg = 0
  , "var2"_arg = 2
  , "var1"_arg = 1
  );
  return 0;
}
//invocation&output:
/*
/tmp/build/clangxx3_8_pkg/clang/p0424ro/p0424r0_test.exe 
argument<var0> operator "" _arg()
argument<var0>::default CTOR
argument<var0>::operator=(ArgType arg_value):arg_value=0
argument<var0>::default CTOR
argument<var2> operator "" _arg()
argument<var2>::default CTOR
argument<var2>::operator=(ArgType arg_value):arg_value=2
argument<var2>::default CTOR
argument<var1> operator "" _arg()
argument<var1>::default CTOR
argument<var1>::operator=(ArgType arg_value):arg_value=1
argument<var1>::default CTOR
named_args(NameVals... name_vals)
argument<var0> operator "" _arg()
argument<var0>::default CTOR
argument<var0>::operator()(this_arg_name_t<ThisArgType> a, TailArgNameVals...)const:a.second=0
var0=0
argument<var1> operator "" _arg()
argument<var1>::default CTOR
argument<var1>::operator()(HeadArgNameVal, TailArgNameVals...)const:head_name=var0:tail_size=2
argument<var1>::operator()(HeadArgNameVal, TailArgNameVals...)const:head_name=var2:tail_size=1
argument<var1>::operator()(this_arg_name_t<ThisArgType> a, TailArgNameVals...)const:a.second=1
var1=1
argument<var2> operator "" _arg()
argument<var2>::default CTOR
argument<var2>::operator()(HeadArgNameVal, TailArgNameVals...)const:head_name=var0:tail_size=2
argument<var2>::operator()(this_arg_name_t<ThisArgType> a, TailArgNameVals...)const:a.second=2
var2=2

Compilation finished at Tue Aug 16 06:56:03
 */