[Interest] Doubt extending my app functions ability

Bob Hood bhood2 at comcast.net
Thu Jan 12 21:29:14 CET 2017

On 1/12/2017 6:23 AM, Ernesto wrote:
> Hello to everyone,
> My name is Ernesto. I am a beginner in Qt programming and I have an 
> specifics problems whit my first big project (desktop app). Let me explain 
> myself:
> I have to include plugins or add-ons to my application for new file format 
> types; I looked around QPluginLoader class, Dynamic and Static libraries 
> classes to achieve this goal, and here is my problem, in each one of this I 
> have to include the header file and the library (or plugins) in my project. 
> I have to develop other mini app to create those plugins or libraries. So 
> here is _my first problem_, I donĀ“t know how many plugins or libraries will 
> be development to include in my app.

Hello, Ernesto.

I will not address your entire problem (because I'm not clearly understanding 
the second one), but your first is easy enough to answer.  The answer is:  You 
don't /need/do know how many plug-ins will be in development.

Plug-ins, by definition, are external code to your main application.  You 
application needs to /discover/those pieces of external code when it runs.  
This way, any number of plug-ins can be included with your application--or 
even added later by the user to an existing installation of your 
application--without having to modify any of your main application's code.

For example, when your main application starts, it looks in a location--either 
hard-coded, or defined by the user--and discovers all the available shared 

     QDir plugins("ernestos_plugins");
     QStringList plugins_list = plugins.entryList(QStringList() << "*.dll" << 
"*.so", QDir::Files);

With each shared library it finds, it tests each to see if it is a plug-in 
designed for your application by coercing it to the interface contract, 
something like:

     foreach(const QString& filename, plugins_list)
         QString plugin_path = 
         QObject* instance = plugin.instance();
             IErnestoPlugin* iernestofactory = 

Each one that supports the contract of your interface ("IErnestoPlugin") can 
be safely used within your application as a plug-in designed for your main 

I hope I understood the first problem correctly.  Apologies if I didn't.
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