Re: [sigc] provided marshallers

[Martin Schulze <martin-ml hippogriff de>]

Am 2003.09.19 19:34 schrieb(en) Tim Flechtner:
> hi everybody,
>   i was trying to understand how to use a marshaller in sigc++-1.2, 
> and am a bit confused by the 'provided' ones.  specifically, how they 
> are usefull:
>
> reading this on the web page:
>
> The following marshallers are provided by default.
>
>   Marshal<void>
>   Marshal<T>
>   Marshal<R>  (untested, may not be portable)
>   FixedMarshal<T,V>
>   FastMarshal<T>
>
>
> 1. I can't find FixedMarshal or FastMarshal defined in any of the 
> headers.

Then the documentation is wrong. Please file a bug report at
bugzilla.gnome.org.

> 2. when i use the following code:
>
> bool marshalled_callback_1()
> {
>   cout << "marshalled_callback_1()" << endl;
>   return false;  // Not done handling signal.
> }
>
> bool marshalled_callback_2()
> {
>   cout << "marshalled_callback_2()" << endl;
>   return true;  // Done handling signal.
> }
>
> bool marshalled_callback_3()
> {
>   cout << "marshalled_callback_3()" << endl;
>   return false;  // Not done handling signal.
> }
>
> SigC::Signal0<bool, Marshal<bool> > signal_marshal_example;
> signal_marshal_example.connect(slot(&marshalled_callback_1));
> signal_marshal_example.connect(slot(&marshalled_callback_2));
> signal_marshal_example.connect(slot(&marshalled_callback_3));
> signal_marshal_example();
>
> i get the following ouput, when i wasn't expecting 
> marshalled_callback_3 to be called:
> marshalled_callback_1()
> marshalled_callback_2()
> marshalled_callback_3()

Well, indeed, your expectation is wrong. SigC::Marshal<T>,
the default marshaller, returns the value of the last callback
executed for any type T. Signal emission doesn't stop at a
special return value. Marshal<bool> doesn't make an exception.

> when i replace the Marshal<bool> parameter for signal_marshal_example 
> with my class BoolMarshal, defined below, marshalled_callback_3 is 
> not called.  can i use the provided Marshal<bool> in some way to 
> achieve the same thing as BoolMarshal?

You could write a template specialization for Marshal<bool> like so:

namespace SigC {

temlpate <>
class Marshal<bool> {
  [your code below]
};

}

Note, however, that Marshal<bool> is the default marshaller for
SigC::SignalX<bool,...>, i.e. your template specialization would
be used automatically for any signal that returns bool.
This is very intransparent, so I'd rather use BoolMarshal and
specify the marshaller explicitely.

Regards,

  Martin


> class BoolMarshal
> {
>   public:
>     typedef bool OutType;
>     typedef bool InType;
>
>     BoolMarshal() {}
>
>     OutType value() { return false; }
>
>     static OutType default_value() { return true; }
>
>     bool marshal(InType new_val)
>     {
>       return new_val;
>     }
>
>   private:
> };
>
>
> thanks!
> -tim
>
>
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