class CComboBox : public CWnd
The CComboBox class provides the functionality of a Windows combo box.
A combo box consists of a list box combined with either a static control or edit control. The list-box portion of the control may be displayed at all times or may only drop down when the user selects the drop-down arrow next to the control.
The currently selected item (if any) in the list box is displayed in the static or edit control. In addition, if the combo box has the drop-down list style, the user can type the initial character of one of the items in the list, and the list box, if visible, will highlight the next item with that initial character.
The following table compares the three combo-box styles.
|Style||When is list box visible?||Static or edit control?|
|Drop-down||When dropped down||Edit|
|Drop-down list||When dropped down||Static|
You can create a CComboBox object from either a dialog template or directly in your code. In both cases, first call the constructor CComboBox to construct the CComboBox object; then call the Create member function to create the control and attach it to the CComboBox object.
If you want to handle Windows notification messages sent by a combo box to its parent (usually a class derived from CDialog), add a message-map entry and message-handler member function to the parent class for each message.
Each message-map entry takes the following form:
ON_Notification( id, memberFxn )
where id specifies the child-window ID of the combo-box control sending the notification and memberFxn is the name of the parent member function you have written to handle the notification.
The parent's function prototype is as follows:
afx_msg void memberFxn( );
The order in which certain notifications will be sent cannot be predicted. In particular, a CBN_SELCHANGE notification may occur either before or after a CBN_CLOSEUP notification.
Potential message-map entries are the following:
- ON_CBN_CLOSEUP (Windows 3.1 and later.) The list box of a combo box has closed. This notification message is not sent for a combo box that has the CBS_SIMPLE style.
- ON_CBN_DBLCLK The user double-clicks a string in the list box of a combo box. This notification message is only sent for a combo box with the CBS_SIMPLE style. For a combo box with the CBS_DROPDOWN or CBS_DROPDOWNLIST style, a double-click cannot occur because a single click hides the list box.
- ON_CBN_DROPDOWN The list box of a combo box is about to drop down (be made visible). This notification message can occur only for a combo box with the CBS_DROPDOWN or CBS_DROPDOWNLIST style.
- ON_CBN_EDITCHANGE The user has taken an action that may have altered the text in the edit-control portion of a combo box. Unlike the CBN_EDITUPDATE message, this message is sent after Windows updates the screen. It is not sent if the combo box has the CBS_DROPDOWNLIST style.
- ON_CBN_EDITUPDATE The edit-control portion of a combo box is about to display altered text. This notification message is sent after the control has formatted the text but before it displays the text. It is not sent if the combo box has the CBS_DROPDOWNLIST style.
- ON_CBN_ERRSPACE The combo box cannot allocate enough memory to meet a specific request.
- ON_CBN_SELENDCANCEL (Windows 3.1 and later.) Indicates the user's selection should be canceled. The user clicks an item and then clicks another window or control to hide the list box of a combo box. This notification message is sent before the CBN_CLOSEUP notification message to indicate that the user's selection should be ignored. The CBN_SELENDCANCEL or CBN_SELENDOK notification message is sent even if the CBN_CLOSEUP notification message is not sent (as in the case of a combo box with the CBS_SIMPLE style).
- ON_CBN_SELENDOK The user selects an item and then either presses the ENTER key or clicks the DOWN ARROW key to hide the list box of a combo box. This notification message is sent before the CBN_CLOSEUP message to indicate that the user's selection should be considered valid. The CBN_SELENDCANCEL or CBN_SELENDOK notification message is sent even if the CBN_CLOSEUP notification message is not sent (as in the case of a combo box with the CBS_SIMPLE style).
- ON_CBN_KILLFOCUS The combo box is losing the input focus.
- ON_CBN_SELCHANGE The selection in the list box of a combo box is about to be changed as a result of the user either clicking in the list box or changing the selection by using the arrow keys. When processing this message, the text in the edit control of the combo box can only be retrieved via GetLBText or another similar function. GetWindowText cannot be used.
- ON_CBN_SETFOCUS The combo box receives the input focus.
If you create a CComboBox object within a dialog box (through a dialog resource), the CComboBox object is automatically destroyed when the user closes the dialog box.
If you embed a CComboBox object within another window object, you do not need to destroy it. If you create the CComboBox object on the stack, it is destroyed automatically. If you create the CComboBox object on the heap by using the new function, you must call delete on the object to destroy it when the Windows combo box is destroyed.
Note If you want to handle WM_KEYDOWN and WM_CHAR messages, you have to subclass the combo box's edit and list box controls, derive classes from CEdit and CListBox, and add handlers for those messages to the derived classes. For more information, see http://support.microsoft.com/default.aspx?scid=kb;en-us;Q174667 and CWnd::SubclassWindow.