Avoid holding entire file in memory

Previously we need a file buffer that is as large as the file size and
this could be a problem when user has less free memory available or
having very large data. Now with the help of `FileOutputIterator`,
we can have a much smaller, fixed size immediate file buffer and also
the code looks nice with `lt::bencode()`.

src/base/utils/io.h

@@ -0,0 +1,63 @@
+/*
+ * Bittorrent Client using Qt and libtorrent.
+ * Copyright (C) 2020  Mike Tzou (Chocobo1)
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+ *
+ * In addition, as a special exception, the copyright holders give permission to
+ * link this program with the OpenSSL project's "OpenSSL" library (or with
+ * modified versions of it that use the same license as the "OpenSSL" library),
+ * and distribute the linked executables. You must obey the GNU General Public
+ * License in all respects for all of the code used other than "OpenSSL".  If you
+ * modify file(s), you may extend this exception to your version of the file(s),
+ * but you are not obligated to do so. If you do not wish to do so, delete this
+ * exception statement from your version.
+ */
+
+#pragma once
+
+#include <iterator>
+#include <memory>
+
+class QByteArray;
+class QFileDevice;
+
+namespace Utils
+{
+    namespace IO
+    {
+        // A wrapper class that satisfy LegacyOutputIterator requirement
+        class FileDeviceOutputIterator
+            : public std::iterator<std::output_iterator_tag, void, void, void, void>
+        {
+        public:
+            explicit FileDeviceOutputIterator(QFileDevice &device, const int bufferSize = (4 * 1024));
+            FileDeviceOutputIterator(const FileDeviceOutputIterator &other) = default;
+            ~FileDeviceOutputIterator();
+
+            // mimic std::ostream_iterator behavior
+            FileDeviceOutputIterator &operator=(char c);
+            // TODO: make these `constexpr` in C++17
+            FileDeviceOutputIterator &operator*();
+            FileDeviceOutputIterator &operator++();
+            FileDeviceOutputIterator &operator++(int);
+
+        private:
+            QFileDevice *m_device;
+            std::shared_ptr<QByteArray> m_buffer;
+            int m_bufferSize;
+        };
+    }
+}