Save/Restore main window size with a single function call

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Introduction

Saving the main window size at application exit and restoring it at startup time is not difficult, but inconvenient, because SDI,MDI and dialog based applications have to be handled differently. The class CMainWndPlacement described in this article reduces this coding task to a single function call.

How to use it

1. Add Subclass.h Subclass.cpp and MainWndPlacement.h to your project.
2. Add #include "MainWndPlacement.h" in the implementation file of your application class (MDI,SDI) or dialog class (dialog based application).
3. For MDI/SDI applications replace in InitInstance the call pMainFrame->ShowWindow(m_nCmdShow); by CMainWndPlacement::InitialShow(m_nCmdShow);. For dialog based applications add in OnInitDialog the call CMainWndPlacement::InitialShow();.

How it works

The magic, that a single function call is sufficient for the save/restore task which has to be done at program startup and and program exit time, is due to the use of a window hook. The call to CMainWndPlacement::InitialShow generates an object which manages it to get all window messages for the main window. As soon as this object gets the WM_DESTROY message, it saves the current window placement using the WriteProfileString function. Later on, when the WM_NCDESTROY message arrives, the object deletes itself. Hooking an object into the message loop of a window is quite tricky, but fortunately, the necessary code has already materialized in some MFC classes which can be obtained on this site. (see article CHookWnd v1.02 by PJ Naughter). For this utility, I used Paul DiLascias classes, which served as starting point for PJ Naughter's extensions. They are implemented in the files Subclass.h and Subclass.cpp.

Customization

You can edit the constructor of the class CMainWndPlacement to change the section and entry where the main window size is stored. Currently "Settings" is used as section and "MainWndPos" as entry.

Implementation Details

CMainWndPlacement is derived from CSubclassWnd which provides the method HookWindow to hook your object to a given window and the virtual method WindowProc which is called for every message which arrives at the hooked window. In WindowProc you can just listen to all the arriving messages, you can filter them out or you can even generate your own messages. CMainWndPlacement itself only listens to the messages WM_PAINT and WM_DESTROY.

One interesting implementation detail concerns the fact, that the class CMainWndPlacement has only one public method, namely InitialShow, even the constructor of CMainWndPlacement is private. Because of that, one can not allocate a CMainWndPlacement object on the stack or make it a member variable of another class. Generally such a technique restricts the flexibility of a class severely and should only be used for special cases (like this one).

Remarks

Just imagine what else can be done with a class like CSubclassWnd. Always consider the use of a window hook, when you want to add functionality to existing windows and when these windows belong to different C++ classes.