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.
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_FLAGS() macro must be used:
Q_FLAGS(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).
Member Function Documentation
QFlags::QFlags(const QFlags & other)
Constructs a copy of other.
QFlags::QFlags(Enum flag)
Constructs a QFlags object storing the given flag.
QFlags::QFlags(Zero zero = 0)
Constructs a QFlags object with no flags set. zero must be a literal 0 value.
QFlags::QFlags(QFlag value)
Constructs a QFlags object initialized with the given integer value.
The QFlag type is a helper type. By using it here instead of int, we effectively ensure that arbitrary enum values cannot be cast to a QFlags, whereas untyped enum values (i.e., int values) can.
bool QFlags::testFlag(Enum flag) const
Returns true if the flag is set, otherwise false.
This function was introduced in Qt 4.2.
QFlags::operator int() const
Returns the value stored in the QFlags object as an integer.
bool QFlags::operator!() const
Returns true if no flag is set (i.e., if the value stored by the QFlags object is 0); otherwise returns false.
QFlags QFlags::operator&(int mask) const
Returns a QFlags object containing the result of the bitwise AND operation on this object and mask.
See also operator&=(), operator|(), operator^(), and operator~().
QFlags QFlags::operator&(uint mask) const
This is an overloaded function.
QFlags QFlags::operator&(Enum mask) const
This is an overloaded function.
QFlags & QFlags::operator&=(int mask)
Performs a bitwise AND operation with mask and stores the result in this QFlags object. Returns a reference to this object.
See also operator&(), operator|=(), and operator^=().
QFlags & QFlags::operator&=(uint mask)
This is an overloaded function.
QFlags & QFlags::operator=(const QFlags & other)
Assigns other to this object and returns a reference to this object.
QFlags QFlags::operator^(QFlags other) const
Returns a QFlags object containing the result of the bitwise XOR operation on this object and other.
See also operator^=(), operator&(), operator|(), and operator~().
QFlags QFlags::operator^(Enum other) const
This is an overloaded function.
QFlags & QFlags::operator^=(QFlags other)
Performs a bitwise XOR operation with other and stores the result in this QFlags object. Returns a reference to this object.
See also operator^(), operator&=(), and operator|=().
QFlags & QFlags::operator^=(Enum other)
This is an overloaded function.
QFlags QFlags::operator|(QFlags other) const
Returns a QFlags object containing the result of the bitwise OR operation on this object and other.
See also operator|=(), operator^(), operator&(), and operator~().
QFlags QFlags::operator|(Enum other) const
This is an overloaded function.
QFlags & QFlags::operator|=(QFlags other)
Performs a bitwise OR operation with other and stores the result in this QFlags object. Returns a reference to this object.
See also operator|(), operator&=(), and operator^=().
QFlags & QFlags::operator|=(Enum other)
This is an overloaded function.
QFlags QFlags::operator~() const
Returns a QFlags object that contains the bitwise negation of this object.
See also operator&(), operator|(), and operator^().
Macro Documentation
Q_DECLARE_FLAGS( Flags, Enum)
The Q_DECLARE_FLAGS() macro expands to
typedef QFlags<Enum> Flags;
Enum is the name of an existing enum type, whereas Flags is the name of the QFlags<Enum> typedef.
See the QFlags documentation for details.
See also Q_DECLARE_OPERATORS_FOR_FLAGS().
Q_DECLARE_OPERATORS_FOR_FLAGS( Flags)
The Q_DECLARE_OPERATORS_FOR_FLAGS() macro declares global operator|() functions for Flags, which is of type QFlags<T>.
See the QFlags documentation for details.
See also Q_DECLARE_FLAGS().
