[C++] Split flatbuffers.h into separate files (#6868)

* split flatbuffers.h into separate files

* wrong variable in cmakelists for android

* readded two accidentally deleted includes

* created buffer.h and moved buffer related things over

include/flatbuffers/detached_buffer.h

@@ -0,0 +1,132 @@
+/*
+ * Copyright 2021 Google Inc. All rights reserved.
+ *
+ * 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 FLATBUFFERS_DETACHED_BUFFER_H_
+#define FLATBUFFERS_DETACHED_BUFFER_H_
+
+#include "flatbuffers/allocator.h"
+#include "flatbuffers/base.h"
+#include "flatbuffers/default_allocator.h"
+
+namespace flatbuffers {
+
+// DetachedBuffer is a finished flatbuffer memory region, detached from its
+// builder. The original memory region and allocator are also stored so that
+// the DetachedBuffer can manage the memory lifetime.
+class DetachedBuffer {
+ public:
+  DetachedBuffer()
+      : allocator_(nullptr),
+        own_allocator_(false),
+        buf_(nullptr),
+        reserved_(0),
+        cur_(nullptr),
+        size_(0) {}
+
+  DetachedBuffer(Allocator *allocator, bool own_allocator, uint8_t *buf,
+                 size_t reserved, uint8_t *cur, size_t sz)
+      : allocator_(allocator),
+        own_allocator_(own_allocator),
+        buf_(buf),
+        reserved_(reserved),
+        cur_(cur),
+        size_(sz) {}
+
+  // clang-format off
+  #if !defined(FLATBUFFERS_CPP98_STL)
+  // clang-format on
+  DetachedBuffer(DetachedBuffer &&other)
+      : allocator_(other.allocator_),
+        own_allocator_(other.own_allocator_),
+        buf_(other.buf_),
+        reserved_(other.reserved_),
+        cur_(other.cur_),
+        size_(other.size_) {
+    other.reset();
+  }
+  // clang-format off
+  #endif  // !defined(FLATBUFFERS_CPP98_STL)
+  // clang-format on
+
+  // clang-format off
+  #if !defined(FLATBUFFERS_CPP98_STL)
+  // clang-format on
+  DetachedBuffer &operator=(DetachedBuffer &&other) {
+    if (this == &other) return *this;
+
+    destroy();
+
+    allocator_ = other.allocator_;
+    own_allocator_ = other.own_allocator_;
+    buf_ = other.buf_;
+    reserved_ = other.reserved_;
+    cur_ = other.cur_;
+    size_ = other.size_;
+
+    other.reset();
+
+    return *this;
+  }
+  // clang-format off
+  #endif  // !defined(FLATBUFFERS_CPP98_STL)
+  // clang-format on
+
+  ~DetachedBuffer() { destroy(); }
+
+  const uint8_t *data() const { return cur_; }
+
+  uint8_t *data() { return cur_; }
+
+  size_t size() const { return size_; }
+
+  // clang-format off
+  #if !defined(FLATBUFFERS_CPP98_STL)
+  // clang-format on
+  // These may change access mode, leave these at end of public section
+  FLATBUFFERS_DELETE_FUNC(DetachedBuffer(const DetachedBuffer &other));
+  FLATBUFFERS_DELETE_FUNC(
+      DetachedBuffer &operator=(const DetachedBuffer &other));
+  // clang-format off
+  #endif  // !defined(FLATBUFFERS_CPP98_STL)
+  // clang-format on
+
+ protected:
+  Allocator *allocator_;
+  bool own_allocator_;
+  uint8_t *buf_;
+  size_t reserved_;
+  uint8_t *cur_;
+  size_t size_;
+
+  inline void destroy() {
+    if (buf_) Deallocate(allocator_, buf_, reserved_);
+    if (own_allocator_ && allocator_) { delete allocator_; }
+    reset();
+  }
+
+  inline void reset() {
+    allocator_ = nullptr;
+    own_allocator_ = false;
+    buf_ = nullptr;
+    reserved_ = 0;
+    cur_ = nullptr;
+    size_ = 0;
+  }
+};
+
+}  // namespace flatbuffers
+
+#endif  // FLATBUFFERS_DETACHED_BUFFER_H_