Add initial prototype for the GN meta-buildsystem.

tools/gn/item.h

+// Copyright (c) 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 TOOLS_GN_ITEM_H_
+#define TOOLS_GN_ITEM_H_
+
+#include <string>
+
+#include "tools/gn/label.h"
+
+class Config;
+class Target;
+class Toolchain;
+
+// A named item (target, config, etc.) that participates in the dependency
+// graph.
+class Item {
+ public:
+  Item(const Label& label);
+  virtual ~Item();
+
+  const Label& label() const { return label_; }
+
+  // Manual RTTI.
+  virtual Config* AsConfig();
+  virtual const Config* AsConfig() const;
+  virtual Target* AsTarget();
+  virtual const Target* AsTarget() const;
+  virtual Toolchain* AsToolchain();
+  virtual const Toolchain* AsToolchain() const;
+
+  // Returns a name like "target" or "config" for the type of item this is, to
+  // be used in logging and error messages.
+  std::string GetItemTypeName() const;
+
+  // Called when this item is resolved, meaning it and all of its dependents
+  // have no unresolved deps.
+  virtual void OnResolved() {}
+
+ private:
+  Label label_;
+};
+
+#endif  // TOOLS_GN_ITEM_H_