QFlags tutorial

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; In our case it expands to 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 this 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
f4.setFlag(TestClass::OptionF).setFlag(TestClass::OptionB, false); //100100, introduced in Qt 5.7