Make mojo::internal::Buffer a non-virtual class.

mojo/public/cpp/bindings/lib/buffer.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MOJO_PUBLIC_CPP_BINDINGS_LIB_BUFFER_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_LIB_BUFFER_H_
 
 #include <stddef.h>
 
+#include "base/logging.h"
+#include "base/macros.h"
+#include "mojo/public/cpp/bindings/lib/bindings_internal.h"
+
 namespace mojo {
 namespace internal {
 
-// Buffer provides a way to allocate memory. Allocations are 8-byte aligned and
-// zero-initialized. Allocations remain valid for the lifetime of the Buffer.
+// Buffer provides an interface to allocate memory blocks which are 8-byte
+// aligned and zero-initialized. It doesn't own the underlying memory. Users
+// must ensure that the memory stays valid while using the allocated blocks from
+// Buffer.
 class Buffer {
  public:
-  virtual ~Buffer() {}
-  virtual void* Allocate(size_t num_bytes) = 0;
+  Buffer() {}
+
+  // The memory must have been zero-initialized. |data| must be 8-byte
+  // aligned.
+  void Initialize(void* data, size_t size) {
+    DCHECK(IsAligned(data));
+
+    data_ = data;
+    size_ = size;
+    cursor_ = reinterpret_cast<uintptr_t>(data);
+    data_end_ = cursor_ + size;
+  }
+
+  size_t size() const { return size_; }
+
+  void* data() const { return data_; }
+
+  // Allocates |num_bytes| from the buffer and returns a pointer to the start of
+  // the allocated block.
+  // The resulting address is 8-byte aligned, and the content of the memory is
+  // zero-filled.
+  void* Allocate(size_t num_bytes) {
+    num_bytes = Align(num_bytes);
+    uintptr_t result = cursor_;
+    cursor_ += num_bytes;
+    if (cursor_ > data_end_ || cursor_ < result) {
+      NOTREACHED();
+      cursor_ -= num_bytes;
+      return nullptr;
+    }
+
+    return reinterpret_cast<void*>(result);
+  }
+
+ private:
+  void* data_ = nullptr;
+  size_t size_ = 0;
+
+  uintptr_t cursor_ = 0;
+  uintptr_t data_end_ = 0;
+
+  DISALLOW_COPY_AND_ASSIGN(Buffer);
 };
 
 }  // namespace internal
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_LIB_BUFFER_H_