QFlags tutorial

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Simple tutorial for safe-usage QFlags

Overview

First of all we should write about 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

}

<code>testFlag(flag)

method checks if flag is set in

QFlags

.

Some example

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