Re: Converting a Glib::ValueBase to a derived type

[José Alburquerque <jaalburquerque cox net>]

José Alburquerque wrote:
> Murray Cumming wrote:
>   
> > On Wed, 2008-01-30 at 14:48 -0500, José Alburquerque wrote:      
> >   
> >     
> > > > > Don't do that
> > > > > ( Glib::ValueBase get_something() )
> > > > >
> > > > > Do this:
> > > > >   void get_something(Glib::ValueBase& value);
> > > > >
> > > > > Then it's up to the caller to pass the correct type, and it's up to the
> > > > > get_something() to complain at runtime if it's not the correct type.
> > > > >
> > > > > See, for instance, Gtk::TreeModel
> > > > >   
> > > > >         
> > > > >           
> > > > I see; it works better returning value in reference.  Thanks.
> > > >   
> > > >       
> > > >         
> > > I forgot to ask:  How would the "get_something()" know if the value 
> > > returned is of the right type?
> > >     
> > >       
> > I'm not quite sure what you are asking, and the answer probably depends
> > on what you are doing? I guess you are trying to wrap a particular C
> > function. Which one?
> >
> >   
> >     
> I'm looking at the todo comments in lines 65 and 71 of the structure.hg 
> file (in gstreamermm).  I'm attempting to define the getter methods of 
> the Gst::Structure along with the id_get_value() method (commented 
> definition) correctly.
>
>   
More specifically, I modified the StructureValueProxy class defined in 
structure.hg to use Glib::ValueBase (just checked in to svn).  I'm still 
a bit unclear about working with Glib::ValueBase so I wrote the attached 
test (also checked in).  I'm stuck in the last if block where I'm trying 
to determine if the value obtained is of the right type.

-Jose

#include <gstreamermm.h>
#include <iostream>

int main (int argc, char* argv[])
{
  Gst::init(argc, argv);

  Glib::ustring fieldName = "field";
  Glib::RefPtr<Gst::Query> latencyQuery = Gst::QueryLatency::create();
  Gst::Structure structure = latencyQuery->get_structure();


  Glib::Value<Glib::ustring> stringValue;
  stringValue.init(Glib::Value<Glib::ustring>::value_type());
  stringValue.set("Hello; This is a ustring.");

  structure[fieldName] = stringValue;

  Gst::StructureValueProxy testProxy = structure[fieldName];

  if (!testProxy) {
    std::cout << "No field named '" << fieldName << "' found." << std::endl;
  };

  if (testProxy) {
    Glib::ValueBase value;
    testProxy.get(value);
    std::cout << "Value type id = '" << typeid(value).name() <<
      "'" << std::endl;
  };

  return 0;
}