# HG changeset patch
# User Rubidium <rubidium@openttd.org>
# Date 2023-01-26 18:21:36
# Node ID cb65f12c003ba70f50204151ab96f58b0ee23468
# Parent  adbfc9f5a1fe1c745f9b640fb773ce0226c72ea8

Codechange: use RAII to automatically restore _cur_dpi after use

diff --git a/src/core/backup_type.hpp b/src/core/backup_type.hpp
--- a/src/core/backup_type.hpp
+++ b/src/core/backup_type.hpp
@@ -144,4 +144,46 @@ private:
 	const int line;
 };
 
+/**
+ * Class to backup a specific variable and restore it upon destruction of this object to prevent
+ * stack values going out of scope before resetting the global to its original value. Contrary to
+ * #Backup this restores the variable automatically and there is no manual option to restore.
+ */
+template <typename T>
+struct AutoRestoreBackup {
+	/*
+	 * There is explicitly no only original constructor version, as that would make it possible
+	 * for the new value to go out of scope before this object goes out of scope, thus defeating
+	 * the whole goal and reason for existing of this object.
+	 */
+
+	/**
+	 * Backup variable and switch to new value.
+	 * @param original Variable to backup.
+	 * @param new_value New value for variable.
+	 */
+	AutoRestoreBackup(T &original, T new_value) : original(original), original_value(original)
+	{
+		original = new_value;
+	}
+
+	/**
+	 * Restore the variable upon object destruction.
+	 */
+	~AutoRestoreBackup()
+	{
+		this->original = this->original_value;
+	}
+
+private:
+	T &original;
+	T original_value;
+
+	/* Prevent copy, assignment and allocation on stack. */
+	AutoRestoreBackup(const AutoRestoreBackup&) = delete;
+	AutoRestoreBackup& operator=(AutoRestoreBackup&) = delete;
+	static void *operator new(std::size_t) = delete;
+	static void *operator new[](std::size_t) = delete;
+};
+
 #endif /* BACKUP_TYPE_HPP */
diff --git a/src/dock_gui.cpp b/src/dock_gui.cpp
--- a/src/dock_gui.cpp
+++ b/src/dock_gui.cpp
@@ -547,15 +547,13 @@ public:
 				Axis axis = widget == WID_BDD_X ? AXIS_X : AXIS_Y;
 
 				if (FillDrawPixelInfo(&tmp_dpi, r.left, r.top, r.Width(), r.Height())) {
-					DrawPixelInfo *old_dpi = _cur_dpi;
-					_cur_dpi = &tmp_dpi;
+					AutoRestoreBackup dpi_backup(_cur_dpi, &tmp_dpi);
 					int x = (r.Width()  - ScaleSpriteTrad(96)) / 2;
 					int y = (r.Height() - ScaleSpriteTrad(64)) / 2;
 					int x1 = ScaleSpriteTrad(63);
 					int x2 = ScaleSpriteTrad(31);
 					DrawShipDepotSprite(x + (axis == AXIS_X ? x1 : x2), y + ScaleSpriteTrad(17), axis, DEPOT_PART_NORTH);
 					DrawShipDepotSprite(x + (axis == AXIS_X ? x2 : x1), y + ScaleSpriteTrad(33), axis, DEPOT_PART_SOUTH);
-					_cur_dpi = old_dpi;
 				}
 				break;
 			}
diff --git a/src/industry_gui.cpp b/src/industry_gui.cpp
--- a/src/industry_gui.cpp
+++ b/src/industry_gui.cpp
@@ -2920,10 +2920,9 @@ struct IndustryCargoesWindow : public Wi
 		if (widget != WID_IC_PANEL) return;
 
 		Rect ir = r.Shrink(WidgetDimensions::scaled.framerect);
-		DrawPixelInfo tmp_dpi, *old_dpi;
+		DrawPixelInfo tmp_dpi;
 		if (!FillDrawPixelInfo(&tmp_dpi, ir.left, ir.top, ir.Width(), ir.Height())) return;
-		old_dpi = _cur_dpi;
-		_cur_dpi = &tmp_dpi;
+		AutoRestoreBackup dpi_backup(_cur_dpi, &tmp_dpi);
 
 		int left_pos = ir.left;
 		if (this->ind_cargo >= NUM_INDUSTRYTYPES) left_pos += (CargoesField::industry_width + CargoesField::cargo_field_width) / 2;
@@ -2952,8 +2951,6 @@ struct IndustryCargoesWindow : public Wi
 			vpos += row_height;
 			if (vpos >= height) break;
 		}
-
-		_cur_dpi = old_dpi;
 	}
 
 	/**
diff --git a/src/misc_gui.cpp b/src/misc_gui.cpp
--- a/src/misc_gui.cpp
+++ b/src/misc_gui.cpp
@@ -820,8 +820,7 @@ void QueryString::DrawEditBox(const Wind
 	DrawPixelInfo dpi;
 	if (!FillDrawPixelInfo(&dpi, fr.left, fr.top, fr.Width(), fr.Height())) return;
 
-	DrawPixelInfo *old_dpi = _cur_dpi;
-	_cur_dpi = &dpi;
+	AutoRestoreBackup dpi_backup(_cur_dpi, &dpi);
 
 	/* We will take the current widget length as maximum width, with a small
 	 * space reserved at the end for the caret to show */
@@ -839,8 +838,6 @@ void QueryString::DrawEditBox(const Wind
 		int caret_width = GetStringBoundingBox("_").width;
 		DrawString(tb->caretxoffs + delta, tb->caretxoffs + delta + caret_width, 0, "_", TC_WHITE);
 	}
-
-	_cur_dpi = old_dpi;
 }
 
 /**
diff --git a/src/newgrf_debug_gui.cpp b/src/newgrf_debug_gui.cpp
--- a/src/newgrf_debug_gui.cpp
+++ b/src/newgrf_debug_gui.cpp
@@ -9,6 +9,7 @@
 
 #include "stdafx.h"
 #include <stdarg.h>
+#include "core/backup_type.hpp"
 #include "window_gui.h"
 #include "window_func.h"
 #include "random_access_file_type.h"
@@ -895,17 +896,13 @@ struct SpriteAlignerWindow : Window {
 
 				DrawPixelInfo new_dpi;
 				if (!FillDrawPixelInfo(&new_dpi, ir.left, ir.top, ir.Width(), ir.Height())) break;
-				DrawPixelInfo *old_dpi = _cur_dpi;
-				_cur_dpi = &new_dpi;
+				AutoRestoreBackup dpi_backup(_cur_dpi, &new_dpi);
 
 				DrawSprite(this->current_sprite, PAL_NONE, x, y, nullptr, ZOOM_LVL_GUI);
 				if (this->crosshair) {
 					GfxDrawLine(x, 0, x, ir.Height() - 1, PC_WHITE, 1, 1);
 					GfxDrawLine(0, y, ir.Width() - 1, y, PC_WHITE, 1, 1);
 				}
-
-				_cur_dpi = old_dpi;
-
 				break;
 			}
 
diff --git a/src/object_gui.cpp b/src/object_gui.cpp
--- a/src/object_gui.cpp
+++ b/src/object_gui.cpp
@@ -363,8 +363,7 @@ public:
 				DrawPixelInfo tmp_dpi;
 				/* Set up a clipping area for the preview. */
 				if (FillDrawPixelInfo(&tmp_dpi, r.left, r.top, r.Width(), r.Height())) {
-					DrawPixelInfo *old_dpi = _cur_dpi;
-					_cur_dpi = &tmp_dpi;
+					AutoRestoreBackup dpi_backup(_cur_dpi, &tmp_dpi);
 					if (spec->grf_prop.grffile == nullptr) {
 						extern const DrawTileSprites _objects[];
 						const DrawTileSprites *dts = &_objects[spec->grf_prop.local_id];
@@ -372,7 +371,6 @@ public:
 					} else {
 						DrawNewObjectTileInGUI(r.Width() / 2 - 1, (r.Height() + matrix_height / 2) / 2 - this->object_margin - ScaleSpriteTrad(TILE_PIXELS), spec, GB(widget, 16, 16));
 					}
-					_cur_dpi = old_dpi;
 				}
 				break;
 			}
@@ -390,8 +388,7 @@ public:
 				DrawPixelInfo tmp_dpi;
 				/* Set up a clipping area for the preview. */
 				if (FillDrawPixelInfo(&tmp_dpi, r.left, r.top, r.Width(), r.Height())) {
-					DrawPixelInfo *old_dpi = _cur_dpi;
-					_cur_dpi = &tmp_dpi;
+					AutoRestoreBackup dpi_backup(_cur_dpi, &tmp_dpi);
 					if (spec->grf_prop.grffile == nullptr) {
 						extern const DrawTileSprites _objects[];
 						const DrawTileSprites *dts = &_objects[spec->grf_prop.local_id];
@@ -400,7 +397,6 @@ public:
 						DrawNewObjectTileInGUI(r.Width() / 2 - 1, r.Height() - this->object_margin - ScaleSpriteTrad(TILE_PIXELS), spec,
 								std::min<int>(_selected_object_view, spec->views - 1));
 					}
-					_cur_dpi = old_dpi;
 				}
 				break;
 			}
diff --git a/src/rail_gui.cpp b/src/rail_gui.cpp
--- a/src/rail_gui.cpp
+++ b/src/rail_gui.cpp
@@ -1255,28 +1255,24 @@ public:
 			case WID_BRAS_PLATFORM_DIR_X:
 				/* Set up a clipping area for the '/' station preview */
 				if (FillDrawPixelInfo(&tmp_dpi, r.left, r.top, r.Width(), r.Height())) {
-					DrawPixelInfo *old_dpi = _cur_dpi;
-					_cur_dpi = &tmp_dpi;
+					AutoRestoreBackup dpi_backup(_cur_dpi, &tmp_dpi);
 					int x = (r.Width()  - ScaleSpriteTrad(64)) / 2 + ScaleSpriteTrad(31);
 					int y = (r.Height() + ScaleSpriteTrad(58)) / 2 - ScaleSpriteTrad(31);
 					if (!DrawStationTile(x, y, _cur_railtype, AXIS_X, _railstation.station_class, _railstation.station_type)) {
 						StationPickerDrawSprite(x, y, STATION_RAIL, _cur_railtype, INVALID_ROADTYPE, 2);
 					}
-					_cur_dpi = old_dpi;
 				}
 				break;
 
 			case WID_BRAS_PLATFORM_DIR_Y:
 				/* Set up a clipping area for the '\' station preview */
 				if (FillDrawPixelInfo(&tmp_dpi, r.left, r.top, r.Width(), r.Height())) {
-					DrawPixelInfo *old_dpi = _cur_dpi;
-					_cur_dpi = &tmp_dpi;
+					AutoRestoreBackup dpi_backup(_cur_dpi, &tmp_dpi);
 					int x = (r.Width()  - ScaleSpriteTrad(64)) / 2 + ScaleSpriteTrad(31);
 					int y = (r.Height() + ScaleSpriteTrad(58)) / 2 - ScaleSpriteTrad(31);
 					if (!DrawStationTile(x, y, _cur_railtype, AXIS_Y, _railstation.station_class, _railstation.station_type)) {
 						StationPickerDrawSprite(x, y, STATION_RAIL, _cur_railtype, INVALID_ROADTYPE, 3);
 					}
-					_cur_dpi = old_dpi;
 				}
 				break;
 
@@ -1306,14 +1302,12 @@ public:
 
 				/* Set up a clipping area for the station preview. */
 				if (FillDrawPixelInfo(&tmp_dpi, r.left, r.top, r.Width(), r.Height())) {
-					DrawPixelInfo *old_dpi = _cur_dpi;
-					_cur_dpi = &tmp_dpi;
+					AutoRestoreBackup dpi_backup(_cur_dpi, &tmp_dpi);
 					int x = (r.Width()  - ScaleSpriteTrad(64)) / 2 + ScaleSpriteTrad(31);
 					int y = (r.Height() + ScaleSpriteTrad(58)) / 2 - ScaleSpriteTrad(31);
 					if (!DrawStationTile(x, y, _cur_railtype, _railstation.orientation, _railstation.station_class, type)) {
 						StationPickerDrawSprite(x, y, STATION_RAIL, _cur_railtype, INVALID_ROADTYPE, 2 + _railstation.orientation);
 					}
-					_cur_dpi = old_dpi;
 				}
 				break;
 			}
@@ -1940,12 +1934,10 @@ struct BuildRailDepotWindow : public Pic
 
 		DrawPixelInfo tmp_dpi;
 		if (FillDrawPixelInfo(&tmp_dpi, r.left, r.top, r.Width(), r.Height())) {
-			DrawPixelInfo *old_dpi = _cur_dpi;
-			_cur_dpi = &tmp_dpi;
+			AutoRestoreBackup dpi_backup(_cur_dpi, &tmp_dpi);
 			int x = (r.Width()  - ScaleSpriteTrad(64)) / 2 + ScaleSpriteTrad(31);
 			int y = (r.Height() + ScaleSpriteTrad(48)) / 2 - ScaleSpriteTrad(31);
 			DrawTrainDepotSprite(x, y, widget - WID_BRAD_DEPOT_NE + DIAGDIR_NE, _cur_railtype);
-			_cur_dpi = old_dpi;
 		}
 	}
 
@@ -2052,12 +2044,10 @@ struct BuildRailWaypointWindow : PickerW
 
 				DrawPixelInfo tmp_dpi;
 				if (FillDrawPixelInfo(&tmp_dpi, r.left, r.top, r.Width(), r.Height())) {
-					DrawPixelInfo *old_dpi = _cur_dpi;
-					_cur_dpi = &tmp_dpi;
+					AutoRestoreBackup dpi_backup(_cur_dpi, &tmp_dpi);
 					int x = (r.Width()  - ScaleSpriteTrad(64)) / 2 + ScaleSpriteTrad(31);
 					int y = (r.Height() + ScaleSpriteTrad(58)) / 2 - ScaleSpriteTrad(31);
 					DrawWaypointSprite(x, y, type, _cur_railtype);
-					_cur_dpi = old_dpi;
 				}
 
 				if (!IsStationAvailable(statspec)) {
diff --git a/src/road_gui.cpp b/src/road_gui.cpp
--- a/src/road_gui.cpp
+++ b/src/road_gui.cpp
@@ -984,12 +984,10 @@ struct BuildRoadDepotWindow : public Pic
 
 		DrawPixelInfo tmp_dpi;
 		if (FillDrawPixelInfo(&tmp_dpi, r.left, r.top, r.Width(), r.Height())) {
-			DrawPixelInfo *old_dpi = _cur_dpi;
-			_cur_dpi = &tmp_dpi;
+			AutoRestoreBackup dpi_backup(_cur_dpi, &tmp_dpi);
 			int x = (r.Width()  - ScaleSpriteTrad(64)) / 2 + ScaleSpriteTrad(31);
 			int y = (r.Height() + ScaleSpriteTrad(48)) / 2 - ScaleSpriteTrad(31);
 			DrawRoadDepotSprite(x, y, (DiagDirection)(widget - WID_BROD_DEPOT_NE + DIAGDIR_NE), _cur_roadtype);
-			_cur_dpi = old_dpi;
 		}
 	}
 
@@ -1119,12 +1117,10 @@ struct BuildRoadStationWindow : public P
 
 		DrawPixelInfo tmp_dpi;
 		if (FillDrawPixelInfo(&tmp_dpi, r.left, r.top, r.Width(), r.Height())) {
-			DrawPixelInfo *old_dpi = _cur_dpi;
-			_cur_dpi = &tmp_dpi;
+			AutoRestoreBackup dpi_backup(_cur_dpi, &tmp_dpi);
 			int x = (r.Width()  - ScaleSpriteTrad(64)) / 2 + ScaleSpriteTrad(31);
 			int y = (r.Height() + ScaleSpriteTrad(48)) / 2 - ScaleSpriteTrad(31);
 			StationPickerDrawSprite(x, y, st, INVALID_RAILTYPE, _cur_roadtype, widget - WID_BROS_STATION_NE);
-			_cur_dpi = old_dpi;
 		}
 	}
 
diff --git a/src/roadveh_gui.cpp b/src/roadveh_gui.cpp
--- a/src/roadveh_gui.cpp
+++ b/src/roadveh_gui.cpp
@@ -8,6 +8,7 @@
 /** @file roadveh_gui.cpp GUI for road vehicles. */
 
 #include "stdafx.h"
+#include "core/backup_type.hpp"
 #include "roadveh.h"
 #include "window_gui.h"
 #include "strings_func.h"
@@ -130,13 +131,12 @@ void DrawRoadVehImage(const Vehicle *v, 
 	Direction dir = rtl ? DIR_E : DIR_W;
 	const RoadVehicle *u = RoadVehicle::From(v);
 
-	DrawPixelInfo tmp_dpi, *old_dpi;
+	DrawPixelInfo tmp_dpi;
 	int max_width = r.Width();
 
 	if (!FillDrawPixelInfo(&tmp_dpi, r.left, r.top, r.Width(), r.Height())) return;
 
-	old_dpi = _cur_dpi;
-	_cur_dpi = &tmp_dpi;
+	AutoRestoreBackup dpi_backup(_cur_dpi, &tmp_dpi);
 
 	int px = rtl ? max_width + skip : -skip;
 	int y = r.Height() / 2;
@@ -155,8 +155,6 @@ void DrawRoadVehImage(const Vehicle *v, 
 		px += rtl ? -width : width;
 	}
 
-	_cur_dpi = old_dpi;
-
 	if (v->index == selection) {
 		int height = ScaleSpriteTrad(12);
 		Rect hr = {(rtl ? px : 0), 0, (rtl ? max_width : px) - 1, height - 1};
diff --git a/src/screenshot.cpp b/src/screenshot.cpp
--- a/src/screenshot.cpp
+++ b/src/screenshot.cpp
@@ -8,6 +8,7 @@
 /** @file screenshot.cpp The creation of screenshots! */
 
 #include "stdafx.h"
+#include "core/backup_type.hpp"
 #include "fileio_func.h"
 #include "viewport_func.h"
 #include "gfx_func.h"
@@ -617,7 +618,7 @@ static void CurrentScreenCallback(void *
 static void LargeWorldCallback(void *userdata, void *buf, uint y, uint pitch, uint n)
 {
 	Viewport *vp = (Viewport *)userdata;
-	DrawPixelInfo dpi, *old_dpi;
+	DrawPixelInfo dpi;
 	int wx, left;
 
 	/* We are no longer rendering to the screen */
@@ -630,8 +631,7 @@ static void LargeWorldCallback(void *use
 	_screen.pitch = pitch;
 	_screen_disable_anim = true;
 
-	old_dpi = _cur_dpi;
-	_cur_dpi = &dpi;
+	AutoRestoreBackup dpi_backup(_cur_dpi, &dpi);
 
 	dpi.dst_ptr = buf;
 	dpi.height = n;
@@ -655,8 +655,6 @@ static void LargeWorldCallback(void *use
 		);
 	}
 
-	_cur_dpi = old_dpi;
-
 	/* Switch back to rendering to the screen */
 	_screen = old_screen;
 	_screen_disable_anim = old_disable_anim;
diff --git a/src/smallmap_gui.cpp b/src/smallmap_gui.cpp
--- a/src/smallmap_gui.cpp
+++ b/src/smallmap_gui.cpp
@@ -8,6 +8,7 @@
 /** @file smallmap_gui.cpp GUI that shows a small map of the world with metadata like owner or height. */
 
 #include "stdafx.h"
+#include "core/backup_type.hpp"
 #include "clear_map.h"
 #include "industry.h"
 #include "station_map.h"
@@ -973,10 +974,7 @@ void SmallMapWindow::DrawMapIndicators()
 void SmallMapWindow::DrawSmallMap(DrawPixelInfo *dpi) const
 {
 	Blitter *blitter = BlitterFactory::GetCurrentBlitter();
-	DrawPixelInfo *old_dpi;
-
-	old_dpi = _cur_dpi;
-	_cur_dpi = dpi;
+	AutoRestoreBackup dpi_backup(_cur_dpi, dpi);
 
 	/* Clear it */
 	GfxFillRect(dpi->left, dpi->top, dpi->left + dpi->width - 1, dpi->top + dpi->height - 1, PC_BLACK);
@@ -1027,8 +1025,6 @@ void SmallMapWindow::DrawSmallMap(DrawPi
 
 	/* Draw map indicators */
 	this->DrawMapIndicators();
-
-	_cur_dpi = old_dpi;
 }
 
 /**
diff --git a/src/statusbar_gui.cpp b/src/statusbar_gui.cpp
--- a/src/statusbar_gui.cpp
+++ b/src/statusbar_gui.cpp
@@ -8,6 +8,7 @@
 /** @file statusbar_gui.cpp The GUI for the bottom status bar. */
 
 #include "stdafx.h"
+#include "core/backup_type.hpp"
 #include "date_func.h"
 #include "gfx_func.h"
 #include "news_func.h"
@@ -68,10 +69,8 @@ static bool DrawScrollingStatusText(cons
 	int width = GetStringBoundingBox(buffer).width;
 	int pos = (_current_text_dir == TD_RTL) ? (scroll_pos - width) : (right - scroll_pos - left);
 
-	DrawPixelInfo *old_dpi = _cur_dpi;
-	_cur_dpi = &tmp_dpi;
+	AutoRestoreBackup dpi_backup(_cur_dpi, &tmp_dpi);
 	DrawString(pos, INT16_MAX, 0, buffer, TC_LIGHT_BLUE, SA_LEFT | SA_FORCE);
-	_cur_dpi = old_dpi;
 
 	return (_current_text_dir == TD_RTL) ? (pos < right - left) : (pos + width > 0);
 }
diff --git a/src/story_gui.cpp b/src/story_gui.cpp
--- a/src/story_gui.cpp
+++ b/src/story_gui.cpp
@@ -698,8 +698,7 @@ public:
 		DrawPixelInfo tmp_dpi;
 		if (!FillDrawPixelInfo(&tmp_dpi, fr.left, fr.top, fr.Width(), fr.Height())) return;
 
-		DrawPixelInfo *old_dpi = _cur_dpi;
-		_cur_dpi = &tmp_dpi;
+		AutoRestoreBackup dpi_backup(_cur_dpi, &tmp_dpi);
 
 		/* Draw content (now coordinates given to Draw** are local to the new clipping region). */
 		fr = fr.Translate(-fr.left, -fr.top);
@@ -758,9 +757,6 @@ public:
 				default: NOT_REACHED();
 			}
 		}
-
-		/* Restore clipping region. */
-		_cur_dpi = old_dpi;
 	}
 
 	void UpdateWidgetSize(int widget, Dimension *size, const Dimension &padding, Dimension *fill, Dimension *resize) override
diff --git a/src/textfile_gui.cpp b/src/textfile_gui.cpp
--- a/src/textfile_gui.cpp
+++ b/src/textfile_gui.cpp
@@ -8,6 +8,7 @@
 /** @file textfile_gui.cpp Implementation of textfile window. */
 
 #include "stdafx.h"
+#include "core/backup_type.hpp"
 #include "fileio_func.h"
 #include "fontcache.h"
 #include "gfx_type.h"
@@ -152,8 +153,7 @@ void TextfileWindow::SetupScrollbars(boo
 
 	DrawPixelInfo new_dpi;
 	if (!FillDrawPixelInfo(&new_dpi, fr.left, fr.top, fr.Width(), fr.Height())) return;
-	DrawPixelInfo *old_dpi = _cur_dpi;
-	_cur_dpi = &new_dpi;
+	AutoRestoreBackup dpi_backup(_cur_dpi, &new_dpi);
 
 	/* Draw content (now coordinates given to DrawString* are local to the new clipping region). */
 	fr = fr.Translate(-fr.left, -fr.top);
@@ -172,8 +172,6 @@ void TextfileWindow::SetupScrollbars(boo
 			DrawString(-this->hscroll->GetPosition(), fr.right, y_offset, line.text, TC_BLACK, SA_TOP | SA_LEFT, false, FS_MONO);
 		}
 	}
-
-	_cur_dpi = old_dpi;
 }
 
 /* virtual */ void TextfileWindow::OnResize()
diff --git a/src/train_gui.cpp b/src/train_gui.cpp
--- a/src/train_gui.cpp
+++ b/src/train_gui.cpp
@@ -96,7 +96,7 @@ void DrawTrainImage(const Train *v, cons
 	bool rtl = _current_text_dir == TD_RTL;
 	Direction dir = rtl ? DIR_E : DIR_W;
 
-	DrawPixelInfo tmp_dpi, *old_dpi;
+	DrawPixelInfo tmp_dpi;
 	/* Position of highlight box */
 	int highlight_l = 0;
 	int highlight_r = 0;
@@ -104,56 +104,55 @@ void DrawTrainImage(const Train *v, cons
 
 	if (!FillDrawPixelInfo(&tmp_dpi, r.left, r.top, r.Width(), r.Height())) return;
 
-	old_dpi = _cur_dpi;
-	_cur_dpi = &tmp_dpi;
+	{
+		AutoRestoreBackup dpi_backup(_cur_dpi, &tmp_dpi);
+
+		int px = rtl ? max_width + skip : -skip;
+		int y = r.Height() / 2;
+		bool sel_articulated = false;
+		bool dragging = (drag_dest != INVALID_VEHICLE);
+		bool drag_at_end_of_train = (drag_dest == v->index); // Head index is used to mark dragging at end of train.
+		for (; v != nullptr && (rtl ? px > 0 : px < max_width); v = v->Next()) {
+			if (dragging && !drag_at_end_of_train && drag_dest == v->index) {
+				/* Highlight the drag-and-drop destination inside the train. */
+				int drag_hlight_width = HighlightDragPosition(px, max_width, y, selection, _cursor.vehchain);
+				px += rtl ? -drag_hlight_width : drag_hlight_width;
+			}
+
+			Point offset;
+			int width = Train::From(v)->GetDisplayImageWidth(&offset);
 
-	int px = rtl ? max_width + skip : -skip;
-	int y = r.Height() / 2;
-	bool sel_articulated = false;
-	bool dragging = (drag_dest != INVALID_VEHICLE);
-	bool drag_at_end_of_train = (drag_dest == v->index); // Head index is used to mark dragging at end of train.
-	for (; v != nullptr && (rtl ? px > 0 : px < max_width); v = v->Next()) {
-		if (dragging && !drag_at_end_of_train && drag_dest == v->index) {
-			/* Highlight the drag-and-drop destination inside the train. */
-			int drag_hlight_width = HighlightDragPosition(px, max_width, y, selection, _cursor.vehchain);
-			px += rtl ? -drag_hlight_width : drag_hlight_width;
-		}
+			if (rtl ? px + width > 0 : px - width < max_width) {
+				PaletteID pal = (v->vehstatus & VS_CRASHED) ? PALETTE_CRASH : GetVehiclePalette(v);
+				VehicleSpriteSeq seq;
+				v->GetImage(dir, image_type, &seq);
+				seq.Draw(px + (rtl ? -offset.x : offset.x), y + offset.y, pal, (v->vehstatus & VS_CRASHED) != 0);
+			}
+
+			if (!v->IsArticulatedPart()) sel_articulated = false;
 
-		Point offset;
-		int width = Train::From(v)->GetDisplayImageWidth(&offset);
+			if (v->index == selection) {
+				/* Set the highlight position */
+				highlight_l = rtl ? px - width : px;
+				highlight_r = rtl ? px - 1 : px + width - 1;
+				sel_articulated = true;
+			} else if ((_cursor.vehchain && highlight_r != 0) || sel_articulated) {
+				if (rtl) {
+					highlight_l -= width;
+				} else {
+					highlight_r += width;
+				}
+			}
 
-		if (rtl ? px + width > 0 : px - width < max_width) {
-			PaletteID pal = (v->vehstatus & VS_CRASHED) ? PALETTE_CRASH : GetVehiclePalette(v);
-			VehicleSpriteSeq seq;
-			v->GetImage(dir, image_type, &seq);
-			seq.Draw(px + (rtl ? -offset.x : offset.x), y + offset.y, pal, (v->vehstatus & VS_CRASHED) != 0);
+			px += rtl ? -width : width;
 		}
 
-		if (!v->IsArticulatedPart()) sel_articulated = false;
-
-		if (v->index == selection) {
-			/* Set the highlight position */
-			highlight_l = rtl ? px - width : px;
-			highlight_r = rtl ? px - 1 : px + width - 1;
-			sel_articulated = true;
-		} else if ((_cursor.vehchain && highlight_r != 0) || sel_articulated) {
-			if (rtl) {
-				highlight_l -= width;
-			} else {
-				highlight_r += width;
-			}
+		if (dragging && drag_at_end_of_train) {
+			/* Highlight the drag-and-drop destination at the end of the train. */
+			HighlightDragPosition(px, max_width, y, selection, _cursor.vehchain);
 		}
-
-		px += rtl ? -width : width;
 	}
 
-	if (dragging && drag_at_end_of_train) {
-		/* Highlight the drag-and-drop destination at the end of the train. */
-		HighlightDragPosition(px, max_width, y, selection, _cursor.vehchain);
-	}
-
-	_cur_dpi = old_dpi;
-
 	if (highlight_l != highlight_r) {
 		/* Draw the highlight. Now done after drawing all the engines, as
 		 * the next engine after the highlight could overlap it. */
diff --git a/src/viewport.cpp b/src/viewport.cpp
--- a/src/viewport.cpp
+++ b/src/viewport.cpp
@@ -61,6 +61,7 @@
  */
 
 #include "stdafx.h"
+#include "core/backup_type.hpp"
 #include "landscape.h"
 #include "viewport_func.h"
 #include "station_base.h"
@@ -1726,9 +1727,6 @@ static void ViewportDrawStrings(ZoomLeve
 
 void ViewportDoDraw(const Viewport *vp, int left, int top, int right, int bottom)
 {
-	DrawPixelInfo *old_dpi = _cur_dpi;
-	_cur_dpi = &_vd.dpi;
-
 	_vd.dpi.zoom = vp->zoom;
 	int mask = ScaleByZoom(-1, vp->zoom);
 
@@ -1738,13 +1736,14 @@ void ViewportDoDraw(const Viewport *vp, 
 	_vd.dpi.height = (bottom - top) & mask;
 	_vd.dpi.left = left & mask;
 	_vd.dpi.top = top & mask;
-	_vd.dpi.pitch = old_dpi->pitch;
+	_vd.dpi.pitch = _cur_dpi->pitch;
 	_vd.last_child = nullptr;
 
 	int x = UnScaleByZoom(_vd.dpi.left - (vp->virtual_left & mask), vp->zoom) + vp->left;
 	int y = UnScaleByZoom(_vd.dpi.top - (vp->virtual_top & mask), vp->zoom) + vp->top;
 
-	_vd.dpi.dst_ptr = BlitterFactory::GetCurrentBlitter()->MoveTo(old_dpi->dst_ptr, x - old_dpi->left, y - old_dpi->top);
+	_vd.dpi.dst_ptr = BlitterFactory::GetCurrentBlitter()->MoveTo(_cur_dpi->dst_ptr, x - _cur_dpi->left, y - _cur_dpi->top);
+	AutoRestoreBackup dpi_backup(_cur_dpi, &_vd.dpi);
 
 	ViewportAddLandscape();
 	ViewportAddVehicles(&_vd.dpi);
@@ -1786,8 +1785,6 @@ void ViewportDoDraw(const Viewport *vp, 
 		ViewportDrawStrings(zoom, &_vd.string_sprites_to_draw);
 	}
 
-	_cur_dpi = old_dpi;
-
 	_vd.string_sprites_to_draw.clear();
 	_vd.tile_sprites_to_draw.clear();
 	_vd.parent_sprites_to_draw.clear();
diff --git a/src/widget.cpp b/src/widget.cpp
--- a/src/widget.cpp
+++ b/src/widget.cpp
@@ -8,6 +8,7 @@
 /** @file widget.cpp Handling of the default/simple widgets. */
 
 #include "stdafx.h"
+#include "core/backup_type.hpp"
 #include "company_func.h"
 #include "window_gui.h"
 #include "viewport_func.h"
@@ -2009,8 +2010,7 @@ NWidgetCore *NWidgetMatrix::GetWidgetFro
 	bool rtl = _current_text_dir == TD_RTL;
 	DrawPixelInfo tmp_dpi;
 	if (!FillDrawPixelInfo(&tmp_dpi, this->pos_x + (rtl ? this->pip_post : this->pip_pre), this->pos_y + this->pip_pre, this->current_x - this->pip_pre - this->pip_post, this->current_y - this->pip_pre - this->pip_post)) return;
-	DrawPixelInfo *old_dpi = _cur_dpi;
-	_cur_dpi = &tmp_dpi;
+	AutoRestoreBackup dpi_backup(_cur_dpi, &tmp_dpi);
 
 	/* Get the appropriate offsets so we can draw the right widgets. */
 	NWidgetCore *child = dynamic_cast<NWidgetCore *>(this->head);
@@ -2043,9 +2043,6 @@ NWidgetCore *NWidgetMatrix::GetWidgetFro
 			child->Draw(w);
 		}
 	}
-
-	/* Restore the clipping area. */
-	_cur_dpi = old_dpi;
 }
 
 /**
@@ -2847,8 +2844,7 @@ void NWidgetLeaf::Draw(const Window *w)
 	new_dpi.left += this->pos_x;
 	new_dpi.top += this->pos_y;
 
-	DrawPixelInfo *old_dpi = _cur_dpi;
-	_cur_dpi = &new_dpi;
+	AutoRestoreBackup dpi_backup(_cur_dpi, &new_dpi);
 
 	Rect r = this->GetCurrentRect();
 
@@ -2962,8 +2958,6 @@ void NWidgetLeaf::Draw(const Window *w)
 	if (this->IsDisabled()) {
 		GfxFillRect(r.Shrink(WidgetDimensions::scaled.bevel), _colour_gradient[this->colour & 0xF][2], FILLRECT_CHECKER);
 	}
-
-	_cur_dpi = old_dpi;
 }
 
 /**
diff --git a/src/window.cpp b/src/window.cpp
--- a/src/window.cpp
+++ b/src/window.cpp
@@ -980,9 +980,8 @@ static void DrawOverlappedWindow(Window 
  */
 void DrawOverlappedWindowForAll(int left, int top, int right, int bottom)
 {
-	DrawPixelInfo *old_dpi = _cur_dpi;
 	DrawPixelInfo bk;
-	_cur_dpi = &bk;
+	AutoRestoreBackup dpi_backup(_cur_dpi, &bk);
 
 	for (Window *w : Window::IterateFromBack()) {
 		if (MayBeShown(w) &&
@@ -994,7 +993,6 @@ void DrawOverlappedWindowForAll(int left
 			DrawOverlappedWindow(w, std::max(left, w->left), std::max(top, w->top), std::min(right, w->left + w->width), std::min(bottom, w->top + w->height));
 		}
 	}
-	_cur_dpi = old_dpi;
 }
 
 /**