conversion operators for UNO

[Michael Stahl]

On 25/04/13 10:35, Noel Grandin wrote:
> Hi
> 
> I would like to discuss the addition of conversion operators for UNO. 
> Let me start with an example.
> If we have IDL that looks like:
>     interface A {}
>     interface B : A {}
> 
> Then we typically have C++ code that looks like
>    void foo(Reference<A> a)
>    void bar() {
>        Reference<B> b = ....
>        methodThatTakesA( Reference<A>(b, UNO_QUERY_THROW) );
>    }
> 
> I want to change things so that I can write:
>    void bar() {
>        Reference<B> b = ....
>        methodThatTakesA( b );
>    }
> and have the code automagically perform the B to A conversion
> 
> Is there some reason this would not be desirable?

one reason may be that queryInterface calls are relatively expensive, so
it is better to have them explicit so you are aware that they happen.

e.g. this problem here caused a 3 % or so slowdown in ODF file load
and/or save (i forgot which), mainly due to 2 additional queryInterface
calls in a critical place:
https://issues.apache.org/ooo/show_bug.cgi?id=108161

with your proposal it would be possible to write code like:

	Reference<B> b = ...
	methodFooThatTakesA( b );
	methodBarThatTakesA( b );
	methodBazThatTakesA( b );

... and get 3 queryInterface calls instead of 1.

or let me reformulate that: is it possible to implement your
hypothetical operator such that it does not call queryInterface (i don't
know).