Contents

QFlags Class

template <typename Enum> class QFlags

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

Header: #include <QFlags>
CMake: find_package(Qt6 REQUIRED COMPONENTS Core)
target_link_libraries(mytarget PRIVATE Qt6::Core)
qmake: QT += core

Public Types

Int
enum_type

Public Functions

QFlags()
QFlags(Enum flags)
QFlags(QFlag flag)
QFlags(std::initializer_list<Enum> flags)
QFlags(const QFlags<T> &other)
QFlags<T> &setFlag(Enum flag, bool on = true)
(since 6.2) bool testAnyFlag(Enum flag) const
(since 6.2) bool testAnyFlags(QFlags<T> flags) const
bool testFlag(Enum flag) const
(since 6.2) bool testFlags(QFlags<T> flags) const
(since 6.2) QFlags<T>::Int toInt() const
QFlags<T>::Int operator QFlags<T>::Int() const
bool operator!() const
QFlags<T> operator&(int mask) const
QFlags<T> operator&(Enum mask) const
(since 6.2) QFlags<T> operator&(QFlags<T> mask) const
QFlags<T> operator&(uint mask) const
QFlags<T> &operator&=(int mask)
QFlags<T> &operator&=(Enum mask)
(since 6.2) QFlags<T> &operator&=(QFlags<T> mask)
QFlags<T> &operator&=(uint mask)
int &operator=(const QFlags<T> &other)
QFlags<T> operator^(QFlags<T> other) const
QFlags<T> operator^(Enum other) const
QFlags<T> &operator^=(QFlags<T> other)
QFlags<T> &operator^=(Enum other)
QFlags<T> operator|(QFlags<T> other) const
QFlags<T> operator|(Enum other) const
QFlags<T> &operator|=(QFlags<T> other)
QFlags<T> &operator|=(Enum other)
QFlags<T> operator~() const

Static Public Members

(since 6.2) QFlags<T> fromInt(QFlags<T>::Int i)
(since 6.2) size_t qHash(QFlags<Enum> flags, size_t seed = 0)

Macros

Q_DECLARE_FLAGS(Flags, Enum)
Q_DECLARE_OPERATORS_FOR_FLAGS(Flags)

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 { }, 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 Widgets 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 QFlag.

Member Type Documentation

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.

QFlags::enum_type

Typedef for the Enum template type.

Member Function Documentation

[constexpr noexcept] QFlags::QFlags()

Constructs a QFlags object with no flags set.

[constexpr noexcept] QFlags::QFlags(Enum flags)

Constructs a QFlags object storing the flags.

[constexpr noexcept] 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.

[constexpr noexcept] QFlags::QFlags(std::initializer_list<Enum> flags)

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

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

[default] QFlags::QFlags(const QFlags<T> &other)

Constructs a copy of other.

[static constexpr noexcept, since 6.2] QFlags<T> QFlags::fromInt(QFlags<T>::Int i)

Constructs a QFlags object representing the integer value i.

This function was introduced in Qt 6.2.

[constexpr noexcept] 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.

[constexpr noexcept, since 6.2] bool QFlags::testAnyFlag(Enum flag) const

Returns true if any flag set in flag is also set in this flags object, otherwise false. If flag has no flags set, the return will always be false.

This function was introduced in Qt 6.2.

See also testFlag().

[constexpr noexcept, since 6.2] bool QFlags::testAnyFlags(QFlags<T> flags) const

Returns true if any flag set in flags is also set in this flags object, otherwise false. If flags has no flags set, the return will always be false.

This function was introduced in Qt 6.2.

See also testFlags().

[constexpr noexcept] 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.

See also testAnyFlag().

[constexpr noexcept, since 6.2] bool QFlags::testFlags(QFlags<T> flags) const

Returns true if this flags object matches the given flags.

If flags has any flags set, this flags object matches precisely if all flags set in flags are also set in this flags object. Otherwise, when flags has no flags set, this flags object only matches if it also has no flags set.

This function was introduced in Qt 6.2.

See also testAnyFlags().

[constexpr noexcept, since 6.2] QFlags<T>::Int QFlags::toInt() const

Returns the value stored in the QFlags object as an integer. Note that the returned integer may be signed or unsigned, depending on whether the enum's underlying type is signed or unsigned.

This function was introduced in Qt 6.2.

See also Int.

[constexpr noexcept] QFlags<T>::Int QFlags::operator QFlags<T>::Int() const

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

See also Int.

[constexpr noexcept] 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.

[constexpr noexcept] 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~().

[constexpr noexcept] QFlags<T> QFlags::operator&(Enum mask) const

This is an overloaded function.

[constexpr noexcept, since 6.2] QFlags<T> QFlags::operator&(QFlags<T> mask) const

This is an overloaded function.

This function was introduced in Qt 6.2.

[constexpr noexcept] QFlags<T> QFlags::operator&(uint mask) const

This is an overloaded function.

[constexpr noexcept] QFlags<T> &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^=().

[constexpr noexcept] QFlags<T> &QFlags::operator&=(Enum mask)

This is an overloaded function.

[constexpr noexcept, since 6.2] QFlags<T> &QFlags::operator&=(QFlags<T> mask)

This is an overloaded function.

This function was introduced in Qt 6.2.

[constexpr noexcept] QFlags<T> &QFlags::operator&=(uint mask)

This is an overloaded function.

[default] int &QFlags::operator=(const QFlags<T> &other)

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

[constexpr noexcept] QFlags<T> QFlags::operator^(QFlags<T> 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~().

[constexpr noexcept] QFlags<T> QFlags::operator^(Enum other) const

This is an overloaded function.

[constexpr noexcept] QFlags<T> &QFlags::operator^=(QFlags<T> 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|=().

[constexpr noexcept] QFlags<T> &QFlags::operator^=(Enum other)

This is an overloaded function.

[constexpr noexcept] QFlags<T> QFlags::operator|(QFlags<T> 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~().

[constexpr noexcept] QFlags<T> QFlags::operator|(Enum other) const

This is an overloaded function.

[constexpr noexcept] QFlags<T> &QFlags::operator|=(QFlags<T> 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^=().

[constexpr noexcept] QFlags<T> &QFlags::operator|=(Enum other)

This is an overloaded function.

[constexpr noexcept] QFlags<T> QFlags::operator~() const

Returns a QFlags object that contains the bitwise negation of this object.

See also operator&(), operator|(), and operator^().

Related Non-Members

[constexpr noexcept, since 6.2] template <typename Enum> size_t qHash(QFlags<Enum> flags, size_t seed = 0)

Calculates the hash for the flags flags, using seed to seed the calculation.

This function was introduced in Qt 6.2.

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().

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