The MessageBox() Function

The MessageBox() function is derived from Win32. Its syntax is:

int __fastcall MessageBox(const char * Message, const char * Caption, int Flags);

The MessageBox() function takes three arguments. The first argument, Message, is a null-terminated string representing the message that the user would read. The Text string could be a static sentence. It could be constructed from another control. Or it could be a combination of different strings appended using C/C++ string functions and operations.

You can create a simple message box similar to one implemented using the ShowMessage() function to display a simple message with an OK button. In this case, you would provide only the Message argument. Set the other two arguments as NULL. Here is an example:

//---------------------------------------------------------------------------
void __fastcall TForm1::Button1Click(TObject *Sender)
{
    Application->MessageBox("This operation can only be "
			    "performed by an administrator.",
                            NULL, NULL);
}
//---------------------------------------------------------------------------

The second argument, also a string, is the caption that would display on the title bar of the dialog box. You can also set it when creating the message box or you can build it from what would be available at runtime. If you do not have a caption, you can set the value of this argument as NULL. In that case the title bar would display Error. Therefore, to create a less boring message box, provide the Caption argument. Here is an example:

//---------------------------------------------------------------------------
void __fastcall TForm1::Button1Click(TObject *Sender)
{
    Application->MessageBox("Make sure the music is playing.",
                "CD PLayer Instructions", NULL);
}
//---------------------------------------------------------------------------

The third argument specifies the flags that would display on the dialog box: one or more buttons and an optional picture. You can create a simple message box with OK as the only button. In that case, set the third argument as MB_OK. Here is an example:

//---------------------------------------------------------------------------
void __fastcall TForm1::Button1Click(TObject *Sender)
{
    Application->MessageBox("Make sure the music is playing.",
                "CD PLayer Instructions", MB_OK);
}
//---------------------------------------------------------------------------

To display more than one button, use a constant integer that represents a group of the available buttons. Here are the constants and their buttons:

Constant Integer Button(s)
MB_OK
MB_OKCANCEL
MB_ABORTRETRYIGNORE
MB_YESNOCANCEL
MB_YESNO
MB_RETRYCANCEL
MB_CANCELTRYCONTINUE
MB_HELP
 

For example, to create a message box that displays the Yes and No buttons, you could write:

//---------------------------------------------------------------------------
void __fastcall TForm1::Button1Click(TObject *Sender)
{
    Application->MessageBox("Do you hear any music now or any sound at all?",
                "CD Player Instructions", MB_YESNO);
}
//---------------------------------------------------------------------------

If you provide the MB_HELP as the only button, the message box would display with an OK and a Help buttons.

The enhance your dialog and accentuate your message, you can display an icon using one of the Win32 defined integer constants. Although you can use any icon with any button, you should be tactful and make sure that the appearance is in accordance with the message. The values and icons are:

Value Icon Suited When
MB_ICONEXCLAMATION
MB_ICONWARNING
Warning the user of an action performed on the application.
MB_ICONINFORMATION
MB_ICONASTERISK
Informing the user of a non-critical situation.
MB_ICONQUESTION Asking a question that expects a Yes, No, or Cancel answers.
MB_ICONSTOP
MB_ICONERROR
MB_ICONHAND
A critical situation or error has occurred. This icon is appropriate when informing the user of a termination or deniability of an action.

The icons are used in conjunction with the buttons constant. To combine these two flags, use the bitwise OR operator |. Here is an example:

//---------------------------------------------------------------------------
void __fastcall TForm1::Button1Click(TObject *Sender)
{
    Application->MessageBox("Do you hear any music now or any sound at all?",
                "CD Player Instructions", MB_YESNOCANCEL | MB_ICONQUESTION);
}
//---------------------------------------------------------------------------

When a message box is configured to display more than one button, the operating system is set to decide which button is the default. The default button has a thick border that sets it apart from the other button(s). If the user presses Enter, the message box would behave as if the user had clicked the default button. Fortunately, if the message box has more than one button, you can decide what button would be the default. To specify the default button, use one of the following constants:

Constant Value If the message box has more than one button, the default button would be
MB_DEFBUTTON1 The first button
MB_DEFBUTTON2 The second button
MB_DEFBUTTON3 The third button
MB_DEFBUTTON4 The fourth button

To specify the default button, use the bitwise OR operator to combine the constant integer of the desired default button with the buttons constant and the icon. Here is an example:

 

//---------------------------------------------------------------------------
void __fastcall TForm1::Button1Click(TObject *Sender)
{
    Application->MessageBox("Do you hear any music now or any sound at all?",
                "CD Player Instructions",
                MB_YESNOCANCEL | MB_ICONQUESTION | MB_DEFBUTTON2);
}
//---------------------------------------------------------------------------

Since the combination of these buttons is using the OR bitwise operator to construct the Flags argument, it does not make a difference which constant appears first:

//---------------------------------------------------------------------------
void __fastcall TForm1::Button1Click(TObject *Sender)
{
    Application->MessageBox("Do you hear any music now or any sound at all?",
                "CD Player Instructions",
                MB_YESNOCANCEL | MB_DEFBUTTON3 | MB_ICONQUESTION);
}
//---------------------------------------------------------------------------

After reading the message displaying on the dialog box, the user would click one of the buttons and the dialog would be closed. Each one of the buttons has a constant integer number that is assigned and recognized by the compiler. You can use this number to find out what button the user had clicked. This means the MessageBox() function returns an integer value as in the following table:

The MessageBox() returns  If the user clicks
IDOK  OK
IDCANCEL  Cancel or presses Esc
IDABORT  Abort 
IDRETRY  Retry
IDIGNORE  Ignore
IDNO  No
IDYES  Yes
IDCONTINUE  Continue 
IDTRYAGAIN  Try Again

Therefore, you can use one of these integers to act depending on the button clicked:

 

 

//---------------------------------------------------------------------------
void __fastcall TForm1::Button1Click(TObject *Sender)
{
    if (Application->MessageBox(
        "Do you hear any music now or any sound at all?",
        "CD Player Instructions",
        MB_YESNOCANCEL | MB_ICONQUESTION) == IDNO)
    Panel1->Caption = "We will stop these tests now. Let the machine rest!";
}
//---------------------------------------------------------------------------


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