On Thu, 31 Aug 2006 09:41:35 -0500, Gareth Foster
<[EMAIL PROTECTED]> wrote:
>> struct Derived: public Base { ... };
>>
>
> Yes, sorry.
>
>> it->member = <any pointer to a class/struct that derived from Base>;
>>
>
> But what about setting the data therein, using GTKmm, things like this
> are possible:
>
> Gtk::TreeModel::Row row = *(m_refTreeModel->append());
> row[m_Columns.m_col_id] = 1;
>
> I am trying to get my head around the magic that makes this possible.
>
> Hope that makes more sense.
>
> Gaz
Oh, now I see. You want similar behaviour as in GTKmm: the magic of
putting something into a row when columns are specified somewhere else. I
was thinking about the same yesterday /I am a newbie in GTKmm/ but here is
what I have found /it is beautiful implementation and use of
C++ templates/ :)
row[m_Columns.m_col_id] = 1
is equivalent to:
row.operator[]( mColumnst.m_col_id).operator=( 1);
[1] gtkmm/treeiter.h /TreeRow<A>::operator[](...) implementation/
template <class ColumnType> inline
TreeValueProxy<ColumnType> TreeRow::operator[](const
TreeModelColumn<ColumnType>& column) const
{
return TreeValueProxy<ColumnType>(*this, column);
}
this should be clear: operator=(...) is a member of Template:
TreeValueProxy.
[2] gtkmm/treeiter.h /TreeValueProxy<A>::operator =(...) implementation/
template <class ColumnType> inline
TreeValueProxy<ColumnType>& TreeValueProxy<ColumnType>::operator=(const
ColumnType& data)
{
row_.set_value(column_, data);
return *this;
}
data is your '1'
So, set_value for a given row is called and returned reference to a given
object to make nested '=' operators possible. Not a surprise.
[3] gtkmm/treeiter.h /TreeValueProxy declaration/
template <class ColumnType>
class TreeValueProxy
{
public:
#ifndef DOXYGEN_SHOULD_SKIP_THIS
inline TreeValueProxy(const TreeRow& row, const
TreeModelColumn<ColumnType>& column);
#endif
inline TreeValueProxy<ColumnType>& operator=(const ColumnType& data);
inline operator ColumnType() const;
private:
const TreeRow& row_;
const TreeModelColumn<ColumnType>& column_;
// no copy assignment
TreeValueProxy<ColumnType>& operator=(const TreeValueProxy<ColumnType>&);
};
row_ is a reference to our recently added row
column_ is a reference to our TreeModelColumn where you specified type of
a given column :)
Do you remember writing something like?:
class ModelColumns: public Gtk::TreeModelColumnRecord {
public:
ModelColumns();
virtual ~ModelColumns() {}
Gtk::TreeModelColumn<std::string> cFileName;
Gtk::TreeModelColumn<std::string> cDescription;
Gtk::TreeModelColumn< Glib::RefPtr<Gdk::Pixbuf> > oThumbnail;
// Next column will be hidden unless it is not added to a list of
// visible ones with TreeView::append_column(...)
Gtk::TreeModelColumn<std::string> cHiddenData;
};
/In your case it is a little bit different/
[4] gtkmm/treeiter.h /TreeRow::set_value implementation/
template <class ColumnType>
void TreeRow::set_value(const TreeModelColumn<ColumnType>& column, const
ColumnType& data) const
{
typedef typename Gtk::TreeModelColumn<ColumnType>::ValueType ValueType;
ValueType value;
value.init(column.type());
value.set(data);
this->set_value_impl(column.index(), value);
}
Now you see that column /TreeModelColumn<A>/ is not used at all except
grabbing a type of a given column. That's it. Very simple and very
beautiful. Here you create some object of given type, assign value to it
and that's it: you've got a cell in a given row/col.
So, if you want the same kind of flexibility in your code do something
similar just don't mess with Templates.
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