QFlags Class | Qt Core 5.6

Qt 5.6 | Qt Core | C++ Classes | QFlags

Qt 5.6.2 Reference Documentation

# QFlags Class

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

Header: | #include <QFlags>
qmake: | QT += core

- List of all members, including inherited members

## Public Types

typedef | Int
typedef | enum_type

## Public Functions

| QFlags(const QFlags &other)
| QFlags(Enum flag)
| QFlags(Zero zero = Q_NULLPTR)
| QFlags(QFlag value)
| QFlags(std::initializer_list<Enum> flags)
bool | testFlag(Enum flag) const
| operator Int() const
bool | operator!() const
QFlags | operator&(int mask) const
QFlags | operator&(uint mask) const
QFlags | operator&(Enum mask) const
QFlags & | operator&=(int mask)
QFlags & | operator&=(uint mask)
QFlags & | operator&=(Enum mask)
QFlags & | operator=(const QFlags &other)
QFlags | operator^(QFlags other) const
QFlags | operator^(Enum other) const
QFlags & | operator^=(QFlags other)
QFlags & | operator^=(Enum other)
QFlags | operator|(QFlags other) const
QFlags | operator|(Enum other) const
QFlags & | operator|=(QFlags other)
QFlags & | operator|=(Enum other)
QFlags | operator~() const

## Macros

| Q_DECLARE_FLAGS(Flags, Enum)
| Q_DECLARE_OPERATORS_FOR_FLAGS(Flags)

## 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_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

### typedef 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.

### typedef QFlags::enum_type

Typedef for the Enum template type.

## 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 = Q_NULLPTR)

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.

### QFlags::QFlags(std::initializer_list<Enum> 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|().

### 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.

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 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&=(Enum 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().