Constructors can use three-state statements to define

In some cases, you need to use different constructors in different situations, and you can only use constructors to generate objects at once, rather than setting a property of an object after the constructor has finished executing. I've been scratching my neck for this thing, because the object that is defined in the IF statement is not known.

In fact, this method can be used in three-state statements to be very simple to complete, see here:

MyClass *p = Usesubclass? New MyClass (): New MySubClass ();

Http://doc.qt.io/qt-5/qscopedpointer.html

Constructors can use three-state statements to define