std::basic_string

namespace std {
  template <class charT,
            class traits = char_traits<charT>,
            class Allocator = allocator<charT> >
  class basic_string;

  using string    = basic_string<char>;
  using u8string  = basic_string<char8_t>;   // C++20から
  using u16string = basic_string<char16_t>;  // C++11から
  using u32string = basic_string<char32_t>;  // C++11から
  using wstring   = basic_string<wchar_t>;


  namespace pmr {  // C++17から
    template <class charT, class traits = char_traits<charT>>
      using basic_string =
        std::basic_string<charT, traits, polymorphic_allocator<charT>>;

    using string    = basic_string<char>;
    using u8string  = basic_string<char8_t>; // C++20から
    using u16string = basic_string<char16_t>;
    using u32string = basic_string<char32_t>;
    using wstring   = basic_string<wchar_t>;
  }
}

#include <cassert>
#include <string>

constexpr bool f()
{
  std::string s = "Hello";
  s += " World";

  auto* p = s.data();
  assert(p);

  return true;
}

int main()
{
  static_assert(f());
}