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Posted 27 Jan 2005
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Automatic resizing controls

, 27 Jan 2005
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Mechanism to automatically dock/anchor your controls in a window or dialog.

Sample Image - AutoResizingControls.jpg


In the following article, I will describe a mechanism that makes it possible to use anchoring and docking for your controls within dialogs and windows.

Docking and anchoring of controls means that some or all controls in a dialog or just a normal window adjust their size and position according to the size of the parent-window (a dialog or view for example). You've already seen this mechanism in many applications. The contact-form in MS Outlook is a good example. If you resize the form, the input-lines, memo-boxes and other controls will also shrink or increase their height and/or width to fit perfectly into the new area of the window.

You may also know this feature from the windows-forms designer in the .NET development environment. If you're a .NET developer by nature and will or have never used C or C++ to write Windows-applications, then this article will not help you very much. But for those who still write ATL/MFC-Applications or have to maintain such, the code presented herein may be a nice improvement for your applications.

The idea behind anchoring

The idea behind docking and anchoring is very simple. You give the user a resizable window or dialog and the elements within that window adjust their size and position to keep their logical position and size or adjust their size to fit into the area of the parent-window so that you don't have wasted space or have vanishing controls when the window becomes to small. With this mechanism, the user can adjust the size of the windows and controls and experiences more flexibility with your application.

If you'd write this code for every dialog and control, and if you have many of them, you'll surely need some time for this task. And in addition, it will become a boring task after the second or 3rd dialog.

The BPCtrlAnchorMap

The docking/anchoring-mechanism is solved by initializing a control-map with the initial size and position of every child-window. When the size of the parent-window changes, a handler-function is called which looks up the control-map and calculates the new size and position for the controls based on the relative size-change of the parent-window.

The BPAnchorControlMap (that's what this article is about) provides this control-map-mechanism and all required functions along with some useful macros. It provides a quick and easy way to implement anchoring/docking in your dialogs and reduces your code-writing efforts to a minimum. All you have to do is to call two functions and declare the so called anchor-map.

Implementing the BPCtrlAnchorMap

The BPCtrlAnchorMap can be implemented in any MFC or ATL-based application. With some minor modifications, it can also be used in pure win32 C or C++ applications that have no object-oriented framework.

The following example is based on an MFC-Dialog-Application and introduces these required steps to make it work.

  • Include the file bpctrlanchormap.h in the same header-file where your class is declared or include it in a common header-file of your application like stdafx.h.
  • Call the macro DECLARE_ANCHOR_MAP() within your class declaration. This macro produces the declarations for our two required functions named InitAnchors and HandleAnchors. We have to call these functions later in our code.
  • Call the InitAnchors() function within the OnInitDialog-Handler or the OnCreate-Handler of your window. It is important that, the child-windows of your dialog or window have already been created and have a valid window handle assigned to it when you call InitAnchors(). If this condition is met, you can call InitAnchors() from anywhere in your code, but the two functions mentioned above are the best place.
    BOOL CAnchorMapDemoDlg::OnInitDialog() {
      // some initialization code
      // ...
      // ...

    The InitAnchors() function can be called with a flags-parameter to specify additional functionality:

    ANIF_CALCSIZE (0x0001)

    If you specify this flag in the call to InitAnchors, the function will calculate the required size for the window by itself. This is useful for FormViews where the dialog-window is resized to fit into the area of the parent-window before the call to InitAnchors. (see also "Using the anchor-map with Form-Views")

    ANIF_SIZEGRIP (0x0002)

    This flag will tell the initialization function to add a sizing-grip into the bottom-right edge of the window. The sizing-grip will automatically be hidden when the window is maximized, and will be shown when the window is in "normal" or restored state.

  • Add an OnSize(WM_SIZE) message-handler to your class (claswizard will do this for you) and call HandleAnchors(&rcWnd) from it. The &rcWnd parameter is a pointer to a RECT-structure which must contain the actual window-coordinates of your dialog or window. You can get these coordinates by calling GetWindowRect(&rcWnd) before.
    void CAnchorMapDemoDlg::OnSize(UINT nType, int cx, int cy) {
      CDialog::OnSize(nType, cx, cy);
      CRect rcWnd;
      HandleAnchors(&rcWnd); // you can alternatively pass NULL for &rcWnd

    You can also call HandleAnchors(NULL) to let the function get the coordinates for you. In that case, your window-class must be derived directly or indirectly from CWnd (this is because m_hWnd is required).

  • Define the anchor-map within your C or CPP file. (You have to do this outside of your class). The anchor-map-definition looks like a message-map definition. We will discuss anchor-maps in the next section. A simple example of an anchor-map looks like this.
  • Compile and run your application

Defining anchor-maps

The anchor-map definition tells the code in BPCtrlAnchorMap.h which controls in your window should take part in docking/anchoring and how these controls should behave.

You begin the definition of an anchor-map with the macro BEGIN_ANCHOR_MAP(theclass) where you replace theclass with the name of your window-class (CMyDialog for example).

After this you can define one or more anchor-map entries with the macro ANCHOR_MAP_ENTRY(nIDCtrl, nFlags). The nIDCtrl-parameter is the control-id of the child-window that should be added to the docking/anchoring mechanism. The nFlags-Parameter is a combination of one or more of the following constants and specifies the docking-behaviour for the control.

The following docking-flags are used to dock a control to one border of a window. A docked control will be moved to the border-side of the window to which it is docked and will adjust its width or height to the same width or height of the border while the other dimension (width or height) of the control will stay constant. Note that the ANF_DOCK flags should not be combined together or combined with anchoring-flags.

ANF_DOCK_TOP (0x0001)

This flag docks the control to the top of the window.


This flag docks the control to the bottom of the window

ANF_DOCK_LEFT (0x0004)

This flag docks the control to the left-side of the window


This flag will dock the control to the right-side of the window


This flag will dock the control to all border-sides of the window

ANF_DOCK_TOP_EX (0x0200)

Docks the control to the top of the window but keeps its original width and height.


Docks the control to the bottom of the window but keeps its original with and height.


Docks the control to the left-side of the window but keeps its original width and height


Docks the control to the right-side of the window but keeps its original width and height.

The following are the anchoring-flags and they are used to define the borders of a control to have a constant distance to the border-sides of the parent-window. If the parent-window changes its size, an anchored control will move its edges along with the edges of the parent-window. The following flags can be combined together in any combination.

ANF_TOP (0x0010)

The distance of the control to the top of the parent-window will be constant

ANF_BOTTOM (0x0020)

The distance of the control to the bottom of the parent-window will be constant

ANF_LEFT (0x0040)

The distance of the control to the left-side of the parent-window will be constant

ANF_RIGHT (0x0080)

The distance of the control to the right-side of the parent-window will be constant


This is a special flag which lets the code determine the best anchoring-method. You should not combine this flag with any other flags.

Additionally, there are some special flags which you can combine with the docking/anchoring flags:

ANF_ERASE (0x02000)

This is a special flag which tells the EraseBackground() function to erase the area occupied by the control. The EraseBackground() function is a special function used to reduce the flickering in windows with many controls. For more information about this topic, see "Removing the flickering"

Special-case is NULL

Note that there is a special case when you pass NULL or 0 as Contro-ID to the ANCHOR_MAP_ENTRY macro. A value of 0 as Control-ID has the effect that all controls in the parent-window which are not already added to the control-map will now be added with the specified nFlags-parameter. The quickens way to define an anchor-map for a dialog is to add the entry ANCHOR_MAP_ENTRY(NULL, ANF_AUTOMATIC).

This will add all controls automatically and determine the best anchoring-method for each-control.


In the latest version, I have added the ANCHOR_MAP_ENTRY_RANGE macro which can be used to add a range of control-IDs to the control-map. This macro is used exactly the same way as ANCHOR_MAP_ENTRY, except that you have to pass two Control-IDs as parameters, one to specify the first ID in the range and one for the last ID. The the third parameter is the normal flags-value.


Now, after you have defined all the anchor-map entries, you end the anchor-map definition with a call to the macro END_ANCHOR_MAP().

You're now ready to test and run your application.

Using the anchor-map with Form-Views

The BPControlAnchorMap-mechanism was originally designed for dialogs where the dialog or parent-window and its controls, both have constant dimensions. A problem arises when you use the mechanism with CFormView. A CFormView resizes itself to fit into the area of the parent-window in which it is contained. Since this process happens before you can call InitAnchors(), the control-map will be empty and the controls will not be resized correctly.

A workaround for this is to use the flag ANIF_CALCSIZE when calling the InitAnchors-function. If you call InitAnchors(ANIF_CALCSIZE), then the InitAnchors-function will try to find the original size of the parent-window (your CFormView) by taking the bottom-right-most coordinate of all controls which are in the control-map.

Removing the flickering

Under certain circumstances or if you have large dialogs with many controls, the controls and the window may flicker when it is resized. This is because the default-implementation of the WM_ERASBKGND message-handler erases the whole dialog-window and therewith our controls. This works in standard dialogs where the controls do not move or change their size.

To get rid of the flickering and reduce it to a minimum, I have added a special EraseBackground-function that erases only the area which is not occupied by controls. The mechanism uses a Region (HRGN)-object which is initialized to the client-rectangle of the window. After the region-object has been initialized, the EraseBackground function cycles through all child-windows (not just the ones in the control-map) and removes their areas from the region. Finally, the region is filled with the background-color. This prevents the child-windows from being overpainted with the background-color.

A problem that I have experienced with this technique is when using Group-Boxes. Group-Boxes do not draw their inner background and because the whole area of the group-box is removed from the region, its background is not filled and the group-box becomes "transparent". To force the area of the group-box not to be removed from the region, use the ANF_ERASE-flag when you add the group-box control to the control-map.

If you want to use this custom EraseBackground function, add a WM_ERASEBKGND-Handler to your window and call m_bpfxAnchorMap.EraseBackground() instead of the default implementation.

BOOL CAnchorMapDemoDlg::OnEraseBkgnd(CDC* pDC) {
      // Here we call the EraseBackground-Handler from the
      // anchor-map which will reduce the flicker. 

Behind the scenes

The following lines will try to give you an insight into what all the macros do. If you're interested in how the code works, download it and have a look into bpctrlanchormap.h.

The whole mechanism takes place within a special class which will be embedded into your dialog-class when you call DECLARE_ANCHOR_MAP. The file bpctrlanchormap.h includes the declaration of this class (CBPCtrlAnchorMap) along with the code. The macro DECLARE_ANCHOR_MAP() which you call in your class declaration finally extends to the following C++ code:

CBPCtrlAnchorMap m_bpfxAnchorMap;
void InitAnchors(BOOL bFindCtrlEdges = 0);
void HandleAnchors(RECT *pRect);

Since you call DECLARE_ANCHOR_MAP within the declaration of your class, the member m_bpfxAnchorMap and the functions InitAnchors and HandleAnchors are added to your class.

The implementation (code) for the InitAnchors and HandleAnchors functions is generated with the anchor-map macros BEGIN_ANCHOR_MAP, ANCHOR_MAP_ENTRY and END_ANCHOR_MAP.

The BEGIN_ANCHOR_MAP(theclass) macro finally extends to the following code :

void theclass::HandleAnchors(RECT *pRect) {



void theclass::InitAnchors(BOOL bFindCtrlEdges) {

Note that the InitAnchors-function is not closed with a curly-brace. This is because the code for the function is still missing. It comes with the ANCHOR_MAP_ENTRY(nCtrlID, nFlags) which extends to the following code:

m_bpfxAnchorMap.AddControl(nIDCtrl, nFlags);

The END_ANCHOR_MAP macro contains some additional code for InitAnchors and finally outputs the closing-brace for the function:

   m_bpfxAnchorMap.Initialize(m_hWnd, bFindCtrlEdges);
   RECT rcWnd;
   ::GetWindowRect(m_hWnd, &rcWnd);


Now that we have our functions ready for use, we can call the InitAnchors and HandleAnchors-function from our class. The rest is done by the code of CBPCtrlAnchorMap.

Well, that´s it.

I hope you'll find this code useful and maybe someone can help me to reduce the flicker when large dialogs with many controls are resized.

bye, drice !


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Software Developer (Senior) BluePearl Software
Germany Germany
I started in the late 80´s with a VIC 20 and wrote my first computer programs. In the 1990´s I developed games for a published diskmag and also became active in the demo-scene. That was the time where I got a deep unserstanding about computers, Bits and Bytes and programemd alot of different things (Games, Compilers, Tools and Alot of other stuff - It was just pure Fun !). In that Time I was also reading alot of books. I´ve used Turbo Pascal, Assembler, C++ and finally arrived in the business and C# world. In 2001 I founded my own company "BluePearl Software" and I develop business and technical applications for small and mid-sized companies as a freelancer, both industrial and non-industrial and in a wide range of business sectors.

I am still interested in the nitty-gritty of PCs and software and how to code/design software in an elegant way. I like to face and solve problems and design good software.

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Comments and Discussions

GeneralMore simpler and more powerful way.. Pin
Mizan Rahman25-Feb-11 2:25
memberMizan Rahman25-Feb-11 2:25 
GeneralRe: More simpler and more powerful way.. Pin
Flaviu222-Dec-11 0:14
memberFlaviu222-Dec-11 0:14 
GeneralAuto-mode Pin
johnnyk42775-Feb-10 11:33
memberjohnnyk42775-Feb-10 11:33 
GeneralSimply Amazing Pin
bleubleu11-Oct-08 15:02
memberbleubleu11-Oct-08 15:02 
GeneralMinor enhancement code Pin
Mydraal15-Jan-07 11:15
memberMydraal15-Jan-07 11:15 
Generalreally perfect Pin
kalaveer19-Sep-06 19:14
memberkalaveer19-Sep-06 19:14 
GeneralControls getting vanished Pin
Raj Patil29-May-06 1:44
memberRaj Patil29-May-06 1:44 
GeneralComment Pin
vbachmut3-Feb-06 10:26
membervbachmut3-Feb-06 10:26 
GeneralRe: Comment Pin
vietdoorgroup26-Jul-06 21:58
membervietdoorgroup26-Jul-06 21:58 
QuestionAnother ResizableLib? Pin
Alexander Shevchenko31-Jan-05 18:01
memberAlexander Shevchenko31-Jan-05 18:01 
GeneralFlickering Pin
Stephane Rodriguez.30-Jan-05 3:46
memberStephane Rodriguez.30-Jan-05 3:46 
GeneralThe flickering problem Pin
W. Kleinschmit27-Jan-05 21:26
memberW. Kleinschmit27-Jan-05 21:26 
GeneralRe: The flickering problem Pin
prcarp28-Jan-05 3:02
memberprcarp28-Jan-05 3:02 
GeneralRe: The flickering problem Pin
drice28-Jan-05 11:01
memberdrice28-Jan-05 11:01 
GeneralRe: The flickering problem Pin
drice28-Jan-05 11:16
memberdrice28-Jan-05 11:16 
Generalquestion Pin
.dan.g.27-Jan-05 12:08
member.dan.g.27-Jan-05 12:08 
GeneralRe: question Pin
drice28-Jan-05 8:35
memberdrice28-Jan-05 8:35 

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