QFlags Class | Qt Core 5.15.19 (original) (raw)

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

Detailed Description

The QFlags 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 { }, is legal:

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

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:

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(int flags)

Constructs a QFlags object initialized with all flags combined using the bitwise OR operator.

This function was introduced in Qt 5.4.

See also operator|=() and operator|().

QFlags::QFlags(QFlag flag)

Constructs a QFlags object initialized with the integer flag.

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.

QFlags::QFlags(Enum flags)

Constructs a QFlags object storing the flags.

QFlags::QFlags()

Constructs a QFlags object with no flags set.

This function was introduced in Qt 5.15.

QFlags::QFlags(const QFlags<T> &other)

Constructs a copy of other.

QFlags<T> &QFlags::operator=(const QFlags<T> &other)

Assigns other to this object and returns a reference to this object.

QFlags<T> &QFlags::setFlag(Enum flag, bool on = true)

Sets the flag flag if on is true or unsets it if on is false. Returns a reference to this object.

This function was introduced in Qt 5.7.

bool QFlags::testFlag(Enum flag) const

Returns true if the flag flag is set, otherwise false.

Note: if flag contains multiple bits set to 1 (for instance, if it's an enumerator equal to the bitwise-OR of other enumerators) then this function will return true if and only if all the bits are set in this flags object. On the other hand, if flag contains no bits set to 1 (that is, its value as a integer is 0), then this function will return true if and only if this flags object also has no bits set to 1.

This function was introduced in Qt 4.2.

QFlags::Int QFlags::operator QFlags::Int() const

Returns the value stored in the QFlags object as an integer.

See also Int.

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<T> 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~().