Use QFlags

[qt_developer]

Hi all,

How can I use QFlags?

I have always used C++ enum.

Can help me with an example?

Regards.

[ChrisW67]

At its heart it still is a C++ enum, just with a wrapper. You use it much like you use a simple integer except that it enforces type safety.

#include <QtCore>
#include <QDebug>
 
struct Test {
    enum MyFlag {
        A = 0x01,
        B = 0x02,
        C = 0x04,
        D = 0X80,
        // combinations
        AB = A | B
    };
    Q_DECLARE_FLAGS(MyFlags, MyFlag)
 
    enum AnotherEnum { Q = 0x01 };
};
Q_DECLARE_OPERATORS_FOR_FLAGS(Test::MyFlags)
 
 
int main(int argc, char **argv)
{
    QCoreApplication app(argc, argv);
 
    Test::MyFlags f;
    qDebug() << f;
 
    f |= Test::D;
    qDebug() << f;
 
    f = Test::A | Test::B;
    qDebug() << f;
    qDebug() << f.testFlag(Test::B) << f.testFlag(Test::C);
 
    f = Test::AB;
    qDebug() << f;
    qDebug() << f.testFlag(Test::B) << f.testFlag(Test::C);
 
    f &= ~Test::B;  // turn off B
    qDebug() << f;
 
    f = Test::B;
    Test::MyFlags g(Test::A | Test::C);
    f |= g;
    qDebug() << f;
 
    // These won't compile: type safety
    // f = 0x01;
    // f |= 0x08;
    // f  = Test::Q;
 
    // It is not perfect protection but you have to force it
    f |= QFlag(0x40); // invalid value
    qDebug() << f;
 
    return 0;
}

#include <QtCore>
#include <QDebug>

struct Test {
    enum MyFlag {
        A = 0x01,
        B = 0x02,
        C = 0x04,
        D = 0X80,
        // combinations
        AB = A | B
    };
    Q_DECLARE_FLAGS(MyFlags, MyFlag)

    enum AnotherEnum { Q = 0x01 };
};
Q_DECLARE_OPERATORS_FOR_FLAGS(Test::MyFlags)


int main(int argc, char **argv)
{
    QCoreApplication app(argc, argv);

    Test::MyFlags f;
    qDebug() << f;

    f |= Test::D;
    qDebug() << f;

    f = Test::A | Test::B;
    qDebug() << f;
    qDebug() << f.testFlag(Test::B) << f.testFlag(Test::C);

    f = Test::AB;
    qDebug() << f;
    qDebug() << f.testFlag(Test::B) << f.testFlag(Test::C);

    f &= ~Test::B;  // turn off B
    qDebug() << f;

    f = Test::B;
    Test::MyFlags g(Test::A | Test::C);
    f |= g;
    qDebug() << f;

    // These won't compile: type safety
    // f = 0x01;
    // f |= 0x08;
    // f  = Test::Q;

    // It is not perfect protection but you have to force it
    f |= QFlag(0x40); // invalid value
    qDebug() << f;

    return 0;
}

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[qt_developer]

Thanks!

I have written this code:

namespace Test
{
 
enum TState {
    ENone = 0x0,
    EPause = 0x1,
    EPlay = 0x2
};
 
Q_DECLARE_FLAGS(TStates, TState)
 
}
 
Q_DECLARE_OPERATORS_FOR_FLAGS( Test::TStates )
 
class myClass
{
...
Test::TStates getState(){return state;}
...
Test::TStates state;

namespace Test
{

enum TState {
    ENone = 0x0,
    EPause = 0x1,
    EPlay = 0x2
};

Q_DECLARE_FLAGS(TStates, TState)

}

Q_DECLARE_OPERATORS_FOR_FLAGS( Test::TStates )

class myClass
{
...
Test::TStates getState(){return state;}
...
Test::TStates state;

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¿How can I declare a QFlag inside the myClass class? Can you help me with an example?

Regards!

[ChrisW67]

Exactly as you have except the flag enum values should be powers of 2

namespace Test {
    enum MyFlag {
        A = 0x01,  // powers of two like this
        B = 0x02,
        C = 0x04,
        D = 0X08,
        E = 1 << 8,  // or you can do it like this
        F = 1 << 9,
        G = 1 << 10,
        H = 1 << 11
    };
    Q_DECLARE_FLAGS(MyFlags, MyFlag)
}
Q_DECLARE_OPERATORS_FOR_FLAGS(Test::MyFlags)
 
class MyClass
{
public:
    Test::MyFlags someFunc() { return flags; }
private:
    Test::MyFlags flags;
};

namespace Test {
    enum MyFlag {
        A = 0x01,  // powers of two like this
        B = 0x02,
        C = 0x04,
        D = 0X08,
        E = 1 << 8,  // or you can do it like this
        F = 1 << 9,
        G = 1 << 10,
        H = 1 << 11
    };
    Q_DECLARE_FLAGS(MyFlags, MyFlag)
}
Q_DECLARE_OPERATORS_FOR_FLAGS(Test::MyFlags)

class MyClass
{
public:
    Test::MyFlags someFunc() { return flags; }
private:
    Test::MyFlags flags;
};

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Compiles fine.