text: //===----------------------------------------------------------------------===//
  //
  // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
  // See https://llvm.org/LICENSE.txt for license information.
  // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
  //
  //===----------------------------------------------------------------------===//
  
  // <ostream>
  
  // template <class charT, class traits = char_traits<charT> >
  //   class basic_ostream;
  
  // basic_ostream<charT,traits>& operator<<(basic_streambuf<charT,traits>* sb);
  
  #include <ostream>
  #include <cassert>
  
  template <class CharT>
  class testbuf
      : public std::basic_streambuf<CharT>
  {
      typedef std::basic_streambuf<CharT> base;
      std::basic_string<CharT> str_;
  public:
      testbuf()
      {
      }
      testbuf(const std::basic_string<CharT>& str)
          : str_(str)
      {
          base::setg(const_cast<CharT*>(str_.data()),
                     const_cast<CharT*>(str_.data()),
                     const_cast<CharT*>(str_.data() + str_.size()));
      }
  
      std::basic_string<CharT> str() const
          {return std::basic_string<CharT>(base::pbase(), base::pptr());}
  
  protected:
  
      virtual typename base::int_type
          overflow(typename base::int_type ch = base::traits_type::eof())
          {
              if (ch != base::traits_type::eof())
              {
                  int n = static_cast<int>(str_.size());
                  str_.push_back(static_cast<CharT>(ch));
                  str_.resize(str_.capacity());
                  base::setp(const_cast<CharT*>(str_.data()),
                             const_cast<CharT*>(str_.data() + str_.size()));
                  base::pbump(n+1);
              }
              return ch;
          }
  };
  
  int main(int, char**)
  {
      {
          testbuf<char> sb;
          std::ostream os(&sb);
          testbuf<char> sb2("testing...");
          assert(sb.str() == "");
          os << &sb2;
          assert(sb.str() == "testing...");
      }
  
    return 0;
  }
  