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+// Copyright 2022 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.
+//
+// -----------------------------------------------------------------------------
+// File: log/log_streamer.h
+// -----------------------------------------------------------------------------
+//
+// This header declares the class `LogStreamer` and convenience functions to
+// construct LogStreamer objects with different associated log severity levels.
+
+#ifndef ABSL_LOG_LOG_STREAMER_H_
+#define ABSL_LOG_LOG_STREAMER_H_
+
+#include <ios>
+#include <memory>
+#include <ostream>
+#include <string>
+#include <utility>
+
+#include "absl/base/config.h"
+#include "absl/base/log_severity.h"
+#include "absl/log/absl_log.h"
+#include "absl/strings/internal/ostringstream.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/optional.h"
+#include "absl/utility/utility.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// LogStreamer
+//
+// Although you can stream into `LOG(INFO)`, you can't pass it into a function
+// that takes a `std::ostream` parameter. `LogStreamer::stream()` provides a
+// `std::ostream` that buffers everything that's streamed in.  The buffer's
+// contents are logged as if by `LOG` when the `LogStreamer` is destroyed.
+// If nothing is streamed in, an empty message is logged.  If the specified
+// severity is `absl::LogSeverity::kFatal`, the program will be terminated when
+// the `LogStreamer` is destroyed regardless of whether any data were streamed
+// in.
+//
+// Factory functions corresponding to the `absl::LogSeverity` enumerators
+// are provided for convenience; if the desired severity is variable, invoke the
+// constructor directly.
+//
+// LogStreamer is movable, but not copyable.
+//
+// Examples:
+//
+//   ShaveYakAndWriteToStream(
+//       yak, absl::LogInfoStreamer(__FILE__, __LINE__).stream());
+//
+//   {
+//     // This logs a single line containing data streamed by all three function
+//     // calls.
+//     absl::LogStreamer streamer(absl::LogSeverity::kInfo, __FILE__, __LINE__);
+//     ShaveYakAndWriteToStream(yak1, streamer.stream());
+//     streamer.stream() << " ";
+//     ShaveYakAndWriteToStream(yak2, streamer.stream());
+//     streamer.stream() << " ";
+//     ShaveYakAndWriteToStreamPointer(yak3, &streamer.stream());
+//   }
+class LogStreamer final {
+ public:
+  // LogStreamer::LogStreamer()
+  //
+  // Creates a LogStreamer with a given `severity` that will log a message
+  // attributed to the given `file` and `line`.
+  explicit LogStreamer(absl::LogSeverity severity, absl::string_view file,
+                       int line)
+      : severity_(severity),
+        line_(line),
+        file_(file),
+        stream_(absl::in_place, &buf_) {
+    // To match `LOG`'s defaults:
+    stream_->setf(std::ios_base::showbase | std::ios_base::boolalpha);
+  }
+
+  // A moved-from `absl::LogStreamer` does not `LOG` when destroyed,
+  // and a program that streams into one has undefined behavior.
+  LogStreamer(LogStreamer&& that) noexcept
+      : severity_(that.severity_),
+        line_(that.line_),
+        file_(std::move(that.file_)),
+        buf_(std::move(that.buf_)),
+        stream_(std::move(that.stream_)) {
+    if (stream_.has_value()) stream_->str(&buf_);
+    that.stream_.reset();
+  }
+  LogStreamer& operator=(LogStreamer&& that) {
+    ABSL_LOG_IF(LEVEL(severity_), stream_).AtLocation(file_, line_) << buf_;
+    severity_ = that.severity_;
+    file_ = std::move(that.file_);
+    line_ = that.line_;
+    buf_ = std::move(that.buf_);
+    stream_ = std::move(that.stream_);
+    if (stream_.has_value()) stream_->str(&buf_);
+    that.stream_.reset();
+    return *this;
+  }
+
+  // LogStreamer::~LogStreamer()
+  //
+  // Logs this LogStreamer's buffered content as if by LOG.
+  ~LogStreamer() {
+    ABSL_LOG_IF(LEVEL(severity_), stream_.has_value()).AtLocation(file_, line_)
+        << buf_;
+  }
+
+  // LogStreamer::stream()
+  //
+  // Returns the `std::ostream` to use to write into this LogStreamer' internal
+  // buffer.
+  std::ostream& stream() { return *stream_; }
+
+ private:
+  absl::LogSeverity severity_;
+  int line_;
+  std::string file_;
+  std::string buf_;
+  // A disengaged `stream_` indicates a moved-from `LogStreamer` that should not
+  // `LOG` upon destruction.
+  absl::optional<absl::strings_internal::OStringStream> stream_;
+};
+
+// LogInfoStreamer()
+//
+// Returns a LogStreamer that writes at level LogSeverity::kInfo.
+inline LogStreamer LogInfoStreamer(absl::string_view file, int line) {
+  return absl::LogStreamer(absl::LogSeverity::kInfo, file, line);
+}
+
+// LogWarningStreamer()
+//
+// Returns a LogStreamer that writes at level LogSeverity::kWarning.
+inline LogStreamer LogWarningStreamer(absl::string_view file, int line) {
+  return absl::LogStreamer(absl::LogSeverity::kWarning, file, line);
+}
+
+// LogErrorStreamer()
+//
+// Returns a LogStreamer that writes at level LogSeverity::kError.
+inline LogStreamer LogErrorStreamer(absl::string_view file, int line) {
+  return absl::LogStreamer(absl::LogSeverity::kError, file, line);
+}
+
+// LogFatalStreamer()
+//
+// Returns a LogStreamer that writes at level LogSeverity::kFatal.
+//
+// The program will be terminated when this `LogStreamer` is destroyed,
+// regardless of whether any data were streamed in.
+inline LogStreamer LogFatalStreamer(absl::string_view file, int line) {
+  return absl::LogStreamer(absl::LogSeverity::kFatal, file, line);
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_LOG_STREAMER_H_