具有 QObject 继承的单例 - Qt

Question

Is it possible to create singleton class with QObject inheritance? I have an compilation error afer adding QObject inheritance. Maybe problem is in static singleton creation (should be dynamic)? Here is my approach

Header

#ifndef BLUETOOTHMANAGER_H
#define BLUETOOTHMANAGER_H

#include <QObject>

class BluetoothManager : public QObject
{
    Q_OBJECT
public:
    virtual ~BluetoothManager() {}

    /// Static getter
    static BluetoothManager & GetInstance()
    {
        return instance;
    }

private:
    /// static Bluetooth manager instance
    static BluetoothManager instance;

    explicit BluetoothManager(QObject * parent);
};

#endif // BLUETOOTHMANAGER_H

And cpp file

#include "BluetoothManager.h"

/// singleton creation
BluetoothManager BluetoothManager::instance = BluetoothManager(static_cast<QObject*>(nullptr));

BluetoothManager::BluetoothManager(QObject * parent)
    : QObject(parent)
{

}

During compilation i have an error

../QtHealthApp/network/bluetooth/BluetoothManager.cpp:4:94: error: use of deleted function ‘BluetoothManager::BluetoothManager(const BluetoothManager&)’  BluetoothManager BluetoothManager::instance = BluetoothManager(static_cast<QObject*>(nullptr));
                                                                                              ^ In file included from /opt/Qt5.12.LTS/5.12.6/gcc_64/include/QtCore/qnamespace.h:43:0,
                 from /opt/Qt5.12.LTS/5.12.6/gcc_64/include/QtCore/qobjectdefs.h:48,
                 from /opt/Qt5.12.LTS/5.12.6/gcc_64/include/QtCore/qobject.h:46,
                 from /opt/Qt5.12.LTS/5.12.6/gcc_64/include/QtCore/QObject:1,
                 from ../QtHealthApp/network/bluetooth/BluetoothManager.h:4,
                 from ../QtHealthApp/network/bluetooth/BluetoothManager.cpp:1: ../QtHealthApp/network/bluetooth/BluetoothManager.h:33:20: note: declared here
     Q_DISABLE_COPY(BluetoothManager)
                    ^ /opt/Qt5.12.LTS/5.12.6/gcc_64/include/QtCore/qglobal.h:372:5: note: in definition of macro ‘Q_DISABLE_COPY’
     Class(const Class &) Q_DECL_EQ_DELETE;\
     ^

Answer 1

First, you should make your constructor(s) private, consistently with the singleton pattern intent to ensure a class only has one instance. If you let everyone build their own instance using a public constructor, you can't call your class a singleton.

Then, in your implementation, you're initializing your instance using copy-construction:

BluetoothManager BluetoothManager::instance = BluetoothManager(static_cast<QObject*>(nullptr));

which you can't do because QObject has deleted copy constructors (which is good, in making a singleton, after all).

Just give a default value to the constructor argument:

explicit BluetoothManager(QObject * parent = nullptr);

so that your instance definition could be just:

BluetoothManager BluetoothManager::instance; 

and the issue should be gone.

Also, I suggest you a quite popular variant of singleton in c++, which is well known to avoid static initialization order 'fiasco' : move the static instance from the class scope to the GetInstance function scope:

class BluetoothManager : public QObject
{
    Q_OBJECT
public:

    virtual ~BluetoothManager() {}

    /// Static getter
    static BluetoothManager & GetInstance()
    {
        static BluetoothManager instance;
        return instance;
    }

private:
    explicit BluetoothManager(QObject * parent = nullptr){}
};

Answer 2

QObject is not copyable and you cannot call copy construction in this object, but in your static field initialization you are constructing your object and then assigning it to your static field which calls copy construction of your type.

Instead you can use a static pointer or smart pointer and initialize it by using new .

If your class is supposed to be a singleton class you have to define its construction private so no other code will be able to create instances of it or inherit it by using final keyword. (making the constructor private has already done it).

Answer 3

Yes, but the essence of Singleton is the constructor being private, so you can't create new instances of the class. Below is a quick and small example, but you can see a more complete one here: class PortableSettings .

bluetoothmanager.h

#ifndef BLUETOOTHMANAGER_H
#define BLUETOOTHMANAGER_H

#include <QObject>

class BluetoothManager : public QObject
{
    Q_OBJECT

public:
    static BluetoothManager* instance();

    void doSomething();

private:
    explicit BluetoothManager(QObject *parent = nullptr);

};

#endif // BLUETOOTHMANAGER_H

bluetoothmanager.cpp

#include "bluetoothmanager.h"

BluetoothManager *BluetoothManager::instance()
{
    static BluetoothManager inst;
    return &inst;
}

void BluetoothManager::doSomething()
{
    // ...
}

BluetoothManager::BluetoothManager(QObject *parent) : QObject(parent)
{
    // initialization
}

main.cpp

#include <QCoreApplication>
#include "bluetoothmanager.h"

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    BluetoothManager::instance()->doSomething();
    return 0;
}