Make scoped dc objects smarter.

base/win/scoped_hdc.h

Index: base/win/scoped_hdc.h
===================================================================
--- base/win/scoped_hdc.h	(revision 121154)
+++ base/win/scoped_hdc.h	(working copy)
@@ -1,4 +1,4 @@
-// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// Copyright (c) 2012 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.
 
@@ -9,70 +9,92 @@
 #include <windows.h>
 
 #include "base/basictypes.h"
-#include "base/logging.h"
+#include "base/compiler_specific.h"
 
 namespace base {
 namespace win {
 
-// Like ScopedHandle but for HDC.  Only use this on HDCs returned from
-// GetDC.
-class ScopedGetDC {
+// The ScopedGetDC and ScopedCreateDC classes manage the default GDI objects
+// that are initially selected into a DC. They help you avoid the following
+// common mistake:
+//
+// HDC hdc = GetDC(NULL);
+// SelectObject(hdc, new_bitmap);
+// .. drawing code here ..
+// ReleaseDC(hdc);   <--- error: the DC has a custom object still selected!
+//
+// This code should be:
+//
+// HDC hdc = GetDC(NULL);
+// HGDIOBJ old_obj = SelectObject(hdc, new_bitmap);
+// .. drawing code here ..
+// SelectObject(hdc, old_obj);
+// ReleaseDC(hdc);    <--- ok to release now.
+//
+// But why work so hard? Use our handy classes:
+//
+// ScopedGetDC dc(NULL);
+// dc.SelectBitmap(hdc, new_bitmap);
+// .. drawing here
+// .. when dc goes out of scope it will select the original object before
+// .. being released.
+//
+class ScopedDC {
  public:
-  explicit ScopedGetDC(HWND hwnd)
-      : hwnd_(hwnd),
-        hdc_(GetDC(hwnd)) {
-    DCHECK(!hwnd_ || IsWindow(hwnd_));
-    DCHECK(hdc_);
-  }
+  virtual ~ScopedDC();
 
-  ~ScopedGetDC() {
-    if (hdc_)
-      ReleaseDC(hwnd_, hdc_);
-  }
+  virtual void DisposeDC(HDC hdc) = 0;
 
-  operator HDC() { return hdc_; }
+  HDC get() { return hdc_; }
 
+  void SelectBitmap(HBITMAP bitmap);
+  void SelectFont(HFONT font);
+  void SelectBrush(HBRUSH brush);
+  void SelectPen(HPEN pen);
+  void SelectRegion(HRGN region);
+
+ protected:
+  ScopedDC(HDC hdc);
+  void Close();
+  void Reset(HDC hdc);
+
  private:
-  HWND hwnd_;
+  void ResetObjects();
+  void Select(HGDIOBJ object, HGDIOBJ* holder);
+
   HDC hdc_;
+  HGDIOBJ bitmap_;
+  HGDIOBJ font_;
+  HGDIOBJ brush_;
+  HGDIOBJ pen_;
+  HGDIOBJ region_;
+};
 
+// Creates and manages an HDC obtained by GetDC.
+class ScopedGetDC : public ScopedDC  {
+ public:
+  explicit ScopedGetDC(HWND hwnd);
+  virtual ~ScopedGetDC();
+
+ private:
+  virtual void DisposeDC(HDC hdc) OVERRIDE;
+
+  HWND hwnd_;
   DISALLOW_COPY_AND_ASSIGN(ScopedGetDC);
 };
 
 // Like ScopedHandle but for HDC.  Only use this on HDCs returned from
 // CreateCompatibleDC, CreateDC and CreateIC.
-class ScopedCreateDC {
+class ScopedCreateDC : public ScopedDC {
  public:
-  ScopedCreateDC() : hdc_(NULL) { }
-  explicit ScopedCreateDC(HDC h) : hdc_(h) { }
+  ScopedCreateDC();
+  explicit ScopedCreateDC(HDC hdc);
+  virtual ~ScopedCreateDC();
+  void Set(HDC hdc);
 
-  ~ScopedCreateDC() {
-    Close();
-  }
-
-  HDC Get() {
-    return hdc_;
-  }
-
-  void Set(HDC h) {
-    Close();
-    hdc_ = h;
-  }
-
-  operator HDC() { return hdc_; }
-
  private:
-  void Close() {
-#ifdef NOGDI
-    assert(false);
-#else
-    if (hdc_)
-      DeleteDC(hdc_);
-#endif  // NOGDI
-  }
+  virtual void DisposeDC(HDC hdc) OVERRIDE;
 
-  HDC hdc_;
-
   DISALLOW_COPY_AND_ASSIGN(ScopedCreateDC);
 };