Thorsten Behrens wrote:
Hi *,
when one needs to convert between two interface types from the same
hierarchy (say, from awt::XWindow2 to awt::XWindow), copy-constructing
or assigning the corresponding uno::Reference types will fail:
Example 1:
void foo( uno::Reference< awt::XWindow > const& win );
uno::Reference< awt::XWindow2 > xWin;
foo( xWin ); -> ERROR
uno::Reference< awt::XWindow > xWinBase;
xWinBase = xWin; -> ERROR
Which is kind of surprising for the casual observer (and a common case
nowadays, with lots of 2er and 3er versions of interfaces), because
plain pointers of related types behave that way. Of course, you can
circumvent the problem like this:
Example 2:
foo( xWin.get() );
(this works, because now the implicit conversion to the base pointer
kicks in, and the uno::Reference<awt::XWindow> constructor is
automatically called).
My proposal now is, to add both a templatized copy constructor and
assignment operator to uno::Reference, such that example 1 works:
I vaguely remember having discussed this before with Daniel Bölzle, but
neither of us can remember whether there were any serious problems with
it. Whether or not the constructor should be explicit might be a
question of style, however.
-Stephan
--%<----------------------------------------------------------------
+ /* Constructor: copy-construct from derived interface
+
+ @param rRef
+ Interface reference that must be convertible to interface_type
+ (typically, this implies that interface_type is a basetype of
+ the passed type)
+ */
+ template< class Ifc > inline Reference( const Reference<Ifc>& rRef );
+ /* Assignment operator: assign from derived interface
+
+ @param rRef
+ Interface reference that must be convertible to interface_type
+ (typically, this implies that interface_type is a basetype of
+ the passed type)
+ */
+ template< class Ifc > inline Reference< interface_type >&
+ operator =( const Reference<Ifc>& rRef );
--%<----------------------------------------------------------------
What do you think?
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