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  1. #include <array>
  2. #include <cstddef>
  3. #include <cstring>
  4.  
  5. namespace my
  6. {
  7.  
  8.   namespace detail
  9.   {
  10.  
  11.     template <typename CharT>
  12.     constexpr auto
  13.     strlen_c(const CharT *const string) noexcept
  14.     {
  15.       auto count = static_cast<std::size_t>(0);
  16.       for (auto s = string; *s; ++s)
  17.         ++count;
  18.       return count;
  19.     }
  20.  
  21.     template <std::size_t N, std::size_t M, typename CharT>
  22.     struct truncation_helper
  23.     {
  24.       template <typename... CharTs>
  25.       static constexpr auto
  26.       help(const CharT *const string, const std::size_t length, const CharTs... chars) noexcept
  27.       {
  28.         static_assert(sizeof...(chars) == M, "wrong instantiation");
  29.         const auto c = (length > M) ? string[M] : static_cast<CharT>(0);
  30.         return truncation_helper<N, M + 1, CharT>::help(string, length, chars..., c);
  31.       }
  32.     };
  33.  
  34.     template <std::size_t N, typename CharT>
  35.     struct truncation_helper<N, N, CharT>
  36.     {
  37.       template <typename... CharTs>
  38.       static constexpr auto
  39.       help(const CharT *, const std::size_t, const CharTs... chars) noexcept
  40.       {
  41.         static_assert(sizeof...(chars) == N, "wrong instantiation");
  42.  
  43.         std::array<CharT, N + 1> result = { chars..., static_cast<CharT>(0) };
  44.         return result;
  45.       }
  46.     };
  47.  
  48.   }  // namespace detail
  49.  
  50.   template <std::size_t N, typename CharT>
  51.   constexpr auto
  52.   truncate(const CharT *const string) noexcept
  53.   {
  54.     const auto length = detail::strlen_c(string);
  55.     return detail::truncation_helper<N, 0, CharT>::help(string, length);
  56.   }
  57.  
  58. }  // namespace my
  59.  
  60. #ifndef SOMETEXT
  61. #  define SOMETEXT "example"
  62. #endif
  63.  
  64. namespace /* anonymous */
  65. {
  66.   constexpr auto limit = static_cast<std::size_t>(8);
  67.   constexpr auto text = my::truncate<limit>(SOMETEXT);
  68. }
  69.  
  70. int
  71. main()
  72. {
  73.   std::printf("text = \"%s\"\n", text.data());
  74.   std::printf("len(text) = %lu <= %lu\n", std::strlen(text.data()), limit);
  75.   std::printf("text = \"%s\"\n", my::truncate<4>(SOMETEXT).data());
  76. }
  77.  
Success #stdin #stdout 0s 3456KB
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Standard input is empty
stdout
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text = ""
len(text) = 0 <= 8
text = "exam"
https://ideone.com/um0IWd
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
secret

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