[solved]How to forward declare a Q_Flag for signal/slot connections?

[shavera]

// Foo.h
class Foo : public QObject{
    Q_OBJECT
    Q_PROPERTY(fooFlags flags READ flags WRITE setFlags NOTIFY flagsChanged)

public:
    enum Flag{
        fast = 0x01,
        far = 0x02,
        furious = 0x07
    };
    Q_DECLARE_FLAGS(Flags, Flag)
    Q_FLAG(Flag)
    Q_FLAG(Flags)

    Foo::Flags flags() const {return m_flags;}

signals:
     void flagsChanged(Flags)

public slots:
    void setFlags(const Flags _flags);

private:
    Flags m_flags;
};
Q_DECLARE_OPERATORS_FOR_FLAGS(Foo::Flags)

// bar.h
class Foo;

class Bar : public QObject{
    Q_OBJECT
public:
    Bar(Foo* _foo);

private slots:
    void respondToFooFlagChange(const Foo::Flags& flags);

private:
    Foo* m_foo;
};

//bar.cpp
#include "bar.h"
#include "foo.h"

Bar(Foo* _foo)  :  m_foo{ _foo } {
    connect(*_foo, &Foo::flagsChanged,
            this, &Bar::respondToFooFlagChange);
}

void Bar::respondToFooFlagChange(const Foo::Flags& flags){
// Do stuff
};

Suppose, for whatever design reasons, I prefer to keep Foo as a forward declared class for Bar. It won't compile because it doesn't know what "Foo::Flags" is. But at the same time, I don't know how forward declare it.

If enum was a straight c++11 enum class, forward declaration is pretty straightforward.

I could also hack it by downgrading to int and reconverting from int. But that seems even worse architecturally than just including the Foo header in the Bar header.

The examples I can see in the Qt Source code seem to include headers rather than using forward declarations, so they're not much help to me.

[SGaist]

Hi,

You can't, the compiler needs to know all the values of the enum in order setup the correct data storage for it.

[shavera]

@SGaist thanks, that answers that.