QFlags Class Reference
[QtCore module]

Detailed Description

The QFlags class provides a type-safe way of storing OR-combinations of enum values.

The QFlags<Enum> class is a template class, where Enum is an enum type. QFlags is used throughout Qt for storing combinations of enum values.

The traditional C++ approach for storing OR-combinations of enum values is to use an int or uint variable. The inconvenience with this approach is that there's no type checking at all; any enum value can be OR'd with any other enum value and passed on to a function that takes an int or uint.

Qt uses QFlags to provide type safety. For example, the Qt::Alignment type is simply a typedef for QFlags<Qt::AlignmentFlag>. QLabel::setAlignment() takes a Qt::Alignment parameter, which means that any combination of Qt::AlignmentFlag values,or 0, is legal:

 label->setAlignment(Qt::AlignLeft | Qt::AlignTop);

If you try to pass a value from another enum or just a plain integer other than 0, the compiler will report an error. If you need to cast integer values to flags in a untyped fashion, you can use the explicit QFlags constructor as cast operator.

If you want to use QFlags for your own enum types, use the Q_DECLARE_FLAGS() and Q_DECLARE_OPERATORS_FOR_FLAGS().

Example:

 class MyClass
 {
 public:
     enum Option {
         NoOptions = 0x0,
         ShowTabs = 0x1,
         ShowAll = 0x2,
         SqueezeBlank = 0x4
     };
     Q_DECLARE_FLAGS(Options, Option)
     ...
 };

 Q_DECLARE_OPERATORS_FOR_FLAGS(MyClass::Options)

You can then use the MyClass::Options type to store combinations of MyClass::Option values.

 Q_FLAGS(Options)

 typedef QFlags<Enum> Flags;