[Qt-interest] connect()ing multiple buttons to a receiver

[Brad Howes]

On Jan 8, 2009, at 3:17 AM, Oliver.Knoll at comit.ch wrote:

> While this works in practice it is dangerous and for good reason the  
> Qt docs discourage you to use the sender() method:
>
> http://doc.trolltech.com/4.4/qobject.html#sender
> "Warning: This function violates the object-oriented principle of  
> modularity. However, getting access to the sender might be useful  
> when many signals are connected to a single slot."

Discourage? Their 'however' clause covers the very scenario that was  
brought up.

> Imagine what would happen if anyone would connect any *other*  
> object's signal (other than from a QButton!) to your slot: Your code  
> above would miserably die with an invalid cast. In your slot you  
> *implicitly* assume that *every* sender is a QButton (again, in  
> practice for quick'n'dirty code this assumption is often okay).

The code should have used qobject_cast<> or dynamic_cast<> followed by  
a NULL check. Nothing scrary.

     QButton* b = dynamic_cast<QButton*>( sender() );
     if ( b ) {
         ...
     }

I use this idiom all the time for a "Recent File" menu that maintains  
a dynamic list of QAction items with file names/paths, all connected  
to just one openRecentFile() slot.

Brad

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Brad Howes
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