THUAI6/CAPI/cpp/grpc/include/absl/strings/internal/ostringstream.h

// 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
//
//      https://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
{
    ABSL_NAMESPACE_BEGIN
    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
    ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_STRINGS_INTERNAL_OSTRINGSTREAM_H_