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  1. #include <iostream>
  2.  
  3. namespace  variadic_toolbox
  4. {
  5.   template<unsigned  count,
  6.     template<unsigned...> class  meta_functor, unsigned...  indices>
  7.   struct  apply_range
  8.   {
  9.     typedef  typename apply_range<count - 1, meta_functor, count - 1, indices...>::result  result;
  10.   };
  11.  
  12.   template<template<unsigned...> class  meta_functor, unsigned...  indices>
  13.   struct  apply_range<0, meta_functor, indices...>
  14.   {
  15.     typedef  typename meta_functor<indices...>::result  result;
  16.   };
  17. }
  18.  
  19. namespace  compile_time
  20. {
  21.   template<char...  str>
  22.   struct  string
  23.   {
  24.     static  constexpr  const char  chars[] = { str..., '\0' };
  25.   };
  26.  
  27.   template<char...  str>
  28.   constexpr  const char string<str...>::chars[];
  29.  
  30.   template<typename  lambda_str_type>
  31.   struct  string_builder
  32.   {
  33.     template<unsigned... indices>
  34.     struct  produce
  35.     {
  36.       typedef  string<lambda_str_type {}.chars[indices]...>  result;
  37.     };
  38.   };
  39.  
  40.   template<char...  str0, char...  str1>
  41.   string<str0..., str1...>  operator+(string<str0...>, string<str1...>)
  42.   {
  43.     return {};
  44.   }
  45. }
  46.  
  47. #define  CSTRING(string_literal)                                                        \
  48.     []{                                                                                 \
  49.         struct  constexpr_string_type { const char * chars = string_literal; };         \
  50.         return  variadic_toolbox::apply_range<sizeof(string_literal)-1,                 \
  51.             compile_time::string_builder<constexpr_string_type>::produce>::result{};    \
  52.     }()
  53.  
  54. int main()
  55. {
  56.   auto  str_hello = CSTRING("hello");
  57.   auto  str_world = CSTRING(" world");
  58.  
  59.   const char* concat = (str_hello + str_world).chars;
  60.  
  61.   std::cout << "runtime concat: " << str_hello.chars << str_world.chars << "\n <=> \n";
  62.   std::cout << "compile concat: " << concat << std::endl;
  63. }
  64.  
Success #stdin #stdout 0s 3460KB
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stdin
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Standard input is empty
stdout
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runtime concat: hello world
 <=> 
compile concat: hello world
https://ideone.com/8Ft2xu
language:
C++14 (gcc 8.3)
created:
7 years ago
visibility:
public

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