[Interest] Doubt extending my app functions ability

[Bob Hood]

On 1/12/2017 4:33 PM, Ernesto wrote:
>
> Hello again, first, thanks to Bob Hood for his quickly reply.
>
> I understood your explanation about my second problem, but I have another 
> issue writing your solution in an example project. I have a MainApp (QT 
> Widgets project) and a Lib1 (C++ Static Library project) and here is the code:
>

If you are creating statically linked plug-ins, that requires a different 
approach.  However, you also mention "the Lib1.dll file" elsewhere, so I 
assume you are not.

I'm sure you have read the online docs about Qt plug-in, Ernesto, but probably 
missed a subtle point which is evident in your interface file:

> //This is header file of my interface, is included by both projects
>
> classPInterface:publicQObject
>
> {
>
> Q_OBJECT
>
> public:
>
> explicitPInterface(QObject*parent=0);
>
> virtualQString/Name/()const=0;
>
> signals:
>
> publicslots:
>
> };
>

You have to designate/declare the class that will be the plug-in interface.  
Try adding the following declarations to your interface file "PInterface", so 
it looks something like this:

     class PInterface : public QObject
     {
         Q_OBJECT
     public:
         explicit PInterface(QObject *parent = 0);
         virtual QString Name() const = 0;

     signals:
     public slots:
     };

*QT_BEGIN_NAMESPACE**
**    Q_DECLARE_INTERFACE(PInterface , "org.Ernesto.PInterface")**
**    QT_END_NAMESPACE*

And try again.

Also, a couple of observations:

 1. Add a virtual destructor to your interface class so destructors of
    subclasses are properly called.
 2. If you'll notice my original code example, I use QPluginLoader to load a
    factory instance.  The Qt plug-in system (QPluginLoader) *does not do
    multiple instancing of plug-ins*.  Therefore, if you will need multiple
    copies of any given plug-in you load, you'll need to use the Factory
    Pattern within each plug-in shared library to generate and return
    instances, and then designate the Factory class interface as the Qt
    plug-in interface instead; e.g.:

     class PInterfaceFactory : public QObject
     {
         Q_OBJECT
     public:
         virtual PInterface* newInstance() = 0;
     };

     QT_BEGIN_NAMESPACE
     Q_DECLARE_INTERFACE(PInterfaceFactory, "org.Ernesto.PInterfaceFactory")
     QT_END_NAMESPACE

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