Determines whether a button control has a check mark next to it.
Specifies the integer identifier of the button control.
Nonzero if the given control is checked, and 0 if it is not checked. Only radio buttons and check boxes can be checked. For three-state buttons, the return value can be 2 if the button is indeterminate. This member function returns 0 for a pushbutton.
If the button is a three-state control, the member function determines whether it is dimmed, checked, or neither.