QFlags Class▲
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Header: QFlags
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qmake: QT += core
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Group: QFlags is part of tools
Detailed Description▲
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.
Flags and the Meta-Object System▲
The Q_DECLARE_FLAGS() macro does not expose the flags to the meta-object system, so they cannot be used by Qt Script or edited in Qt Designer. To make the flags available for these purposes, the Q_FLAG() macro must be used:
Q_FLAG(Options)Naming Convention▲
A sensible naming convention for enum types and associated QFlags types is to give a singular name to the enum type (e.g., Option) and a plural name to the QFlags type (e.g., Options). When a singular name is desired for the QFlags type (e.g., Alignment), you can use Flag as the suffix for the enum type (e.g., AlignmentFlag).
See Also▲
See also QFlag
Member Type Documentation▲
[since 5.0] QFlags::Int▲
Typedef for the integer type used for storage as well as for implicit conversion. Either int or unsigned int, depending on whether the enum's underlying type is signed or unsigned.
This typedef was introduced in Qt 5.0.
QFlags::enum_type▲
Typedef for the Enum template type.