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QFlags tutorial: Difference between revisions

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[[Category:snippets]]
[[Category:Snippets::Misc]]
[[Category:Tutorial]]


[toc align_right="yes" depth="3"]
==Overview==


= Simple tutorial for safe-usage QFlags =
First of all we should write about the macro:
 
<code>Q_FLAGS</code>
== Overview ==
 
First of all we should write about macro <code>Q_FLAGS</code>
''This macro registers one or several flags types to the meta-object system''
''This macro registers one or several flags types to the meta-object system''


== Example: ==
==Example:==


<code>class TestClass
<code>class TestClass
Line 26: Line 24:
  Q_DECLARE_FLAGS(Options, Option)
  Q_DECLARE_FLAGS(Options, Option)
  };
  };


Q_DECLARE_OPERATORS_FOR_FLAGS(TestClass::Options)</code>
Q_DECLARE_OPERATORS_FOR_FLAGS(TestClass::Options)</code>


The <code>Q_DECLARE_FLAGS()</code> macro expands to <code>typedef QFlags<Enum> Flags;<code> In our case it expandes to <code>typedef QFlags<Option> Options;</code> where '''Option''' - is an enum name, and '''Options''' - is name for set of flags.
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 <code>Q_DECLARE_OPERATORS_FOR_FLAGS()</code> macro declares global <code>operator|()</code> functions for Flags, which is of type <code>QFlags<T></code>.
The '''Q_DECLARE_OPERATORS_FOR_FLAGS()''' macro declares global '''operator|()''' functions for Flags, which is of type '''QFlags<T>'''.


The <code>Q_DECLARE_FLAGS()</code> 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 <code>Q_FLAGS()</code> macro must be used.
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 ==
==Usage sample==


</code>void test (TestClass::Options flag)
<code>void test (TestClass::Options flag)
{
{
  if (flag.testFlag(TestClass::OptionA))
  if (flag.testFlag(TestClass::OptionA))
Line 49: Line 48:
  test (TestClass::OptionA | TestClass::OptionB);
  test (TestClass::OptionA | TestClass::OptionB);
  test (0x1); // error
  test (0x1); // error
}<code>


<code>testFlag(flag)</code> method checks if flag is set in <code>QFlags</code>.
}</code>
 
The '''testFlag(flag)''' method checks if flag is set in '''QFlags'''.


=== Some example ===
===Some examples===


</code>TestClass::Options f1(TestClass::OptionA | TestClass::OptionB); // 000011
<code>TestClass::Options f1(TestClass::OptionA | TestClass::OptionB); // 000011
TestClass::Options f2(~f1); // 111100
TestClass::Options f2(~f1); // 111100
TestClass::Options f3(Foo::OptionA | Foo::OptionC); // 000101
TestClass::Options f3(Foo::OptionA | Foo::OptionC); // 000101
TestClass::Options f4(f1^f3); // 000110
TestClass::Options f4(f1^f3); // 000110
f4.setFlag(TestClass::OptionF).setFlag(TestClass::OptionB, false); //100100, introduced in Qt 5.7
</code>

Latest revision as of 20:52, 27 October 2020


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