QFlags tutorial

This article may require cleanup to meet the Qt Wiki's quality standards. Reason: Auto-imported from ExpressionEngine. Please improve this article if you can. Remove the {{cleanup}} tag and add this page to Updated pages list after it's clean.

Overview

First of all we should write about the macro:

Q_FLAGS

This macro registers one or several flags types to the meta-object system

Example:

class TestClass
 {
 public:
 enum Option {
 OptionA = 1, // 000001
 OptionB = 2, // 000010
 OptionC = 4, // 000100
 OptionD = 8, // 001000
 OptionE = 16, // 010000
 OptionF = 32 // 100000
 // … some more options with value which is a power of two
 };
 Q_DECLARE_FLAGS(Options, Option)
 };

Q_DECLARE_OPERATORS_FOR_FLAGS(TestClass::Options)

The

Q_DECLARE_FLAGS()

macro expands to

typedef QFlags<Enum> Flags;<code> In our case it expandes to <code>typedef QFlags<Option> Options;

where Option - is an enum name, and Options - is name for set of flags. The

Q_DECLARE_OPERATORS_FOR_FLAGS()

macro declares global

operator|()

functions for Flags, which is of type

QFlags<T>

. The

Q_DECLARE_FLAGS()

macro does not expose the flags to the meta-object system, so they cannot be used by Qt Script. To make the flags available for these purpose, the

Q_FLAGS()

macro must be used.

Usage sample

void test (TestClass::Options flag)
{
 if (flag.testFlag(TestClass::OptionA))
 qDebug() << "A";
 if (flag.testFlag(TestClass::OptionB))
 qDebug() << "B";
}

int main()
{
 test (TestClass::OptionA | TestClass::OptionB);
 test (0x1); // error
}

The

testFlag(flag)

method checks if flag is set in

QFlags

.

Some examples

TestClass::Options f1(TestClass::OptionA | TestClass::OptionB); // 000011
TestClass::Options f2(~f1); // 111100
TestClass::Options f3(Foo::OptionA | Foo::OptionC); // 000101
TestClass::Options f4(f1^f3); // 000110