text: // Copyright 2017 The Abseil Authors.
  //
  // Licensed under the Apache License, Version 2.0 (the "License");
  // you may not use this file except in compliance with the License.
  // You may obtain a copy of the License at
  //
  //      http://www.apache.org/licenses/LICENSE-2.0
  //
  // Unless required by applicable law or agreed to in writing, software
  // distributed under the License is distributed on an "AS IS" BASIS,
  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  // See the License for the specific language governing permissions and
  // limitations under the License.
  
  #ifndef ABSL_STRINGS_INTERNAL_OSTRINGSTREAM_H_
  #define ABSL_STRINGS_INTERNAL_OSTRINGSTREAM_H_
  
  #include <cassert>
  #include <ostream>
  #include <streambuf>
  #include <string>
  
  #include "absl/base/port.h"
  
  namespace absl {
  inline namespace lts_2018_06_20 {
  namespace strings_internal {
  
  // The same as std::ostringstream but appends to a user-specified std::string,
  // and is faster. It is ~70% faster to create, ~50% faster to write to, and
  // completely free to extract the result std::string.
  //
  //   std::string s;
  //   OStringStream strm(&s);
  //   strm << 42 << ' ' << 3.14;  // appends to `s`
  //
  // The stream object doesn't have to be named. Starting from C++11 operator<<
  // works with rvalues of std::ostream.
  //
  //   std::string s;
  //   OStringStream(&s) << 42 << ' ' << 3.14;  // appends to `s`
  //
  // OStringStream is faster to create than std::ostringstream but it's still
  // relatively slow. Avoid creating multiple streams where a single stream will
  // do.
  //
  // Creates unnecessary instances of OStringStream: slow.
  //
  //   std::string s;
  //   OStringStream(&s) << 42;
  //   OStringStream(&s) << ' ';
  //   OStringStream(&s) << 3.14;
  //
  // Creates a single instance of OStringStream and reuses it: fast.
  //
  //   std::string s;
  //   OStringStream strm(&s);
  //   strm << 42;
  //   strm << ' ';
  //   strm << 3.14;
  //
  // Note: flush() has no effect. No reason to call it.
  class OStringStream : private std::basic_streambuf<char>, public std::ostream {
   public:
    // The argument can be null, in which case you'll need to call str(p) with a
    // non-null argument before you can write to the stream.
    //
    // The destructor of OStringStream doesn't use the std::string. It's OK to destroy
    // the std::string before the stream.
    explicit OStringStream(std::string* s) : std::ostream(this), s_(s) {}
  
    std::string* str() { return s_; }
    const std::string* str() const { return s_; }
    void str(std::string* s) { s_ = s; }
  
   private:
    using Buf = std::basic_streambuf<char>;
  
    Buf::int_type overflow(int c) override;
    std::streamsize xsputn(const char* s, std::streamsize n) override;
  
    std::string* s_;
  };
  
  }  // namespace strings_internal
  }  // inline namespace lts_2018_06_20
  }  // namespace absl
  
  #endif  // ABSL_STRINGS_INTERNAL_OSTRINGSTREAM_H_
  