On 06.07.19 19:59, Adrien Ollier wrote:
So to close bug #74702, we require to keep the real type of any
OutputDevice* a function gets as argument.
If we are in a function g(OutputDevice*), we must rewind to its caller
f(OutputDevice*), and so on until a(OutputDevice*) which is called after
creation of an instance of a subclass of OutputDevice.
As you understand, it is very hard because there are many and many
functions to modify and to rewind to get the real type of the
OutputDevice* (OutputDevice subclass) that was passed as parameter.
An alternative is to use dynamic_cast and test the result. This solution
does not require so much changes but is less elegant and efficient.
Community, which solution do you prefer?
oh no, i stopped reading your other mail too early, you don't agree...
well there are some options:
1) keep tag enums
+ conditions are fast
- violates the purity of object-oriented dogma
2) use dynamic_cast instead
- slow
+- it is semi-object-oriented (at first glance you're making use
of subtyping but in reality it's not)
3) virtual operator subclass*() + overrides + conditionals
- if you think the other options are ugly, you haven't seen this one
in action
4) add more virtuals to OutputDevice, call them from g()
+ object oriented
- the logic may not "belong" to OutputDevice
- every time one of those virtuals changes you'll have to rebuild
~4000 objects and wait an hour
5) visitor pattern
+ very object oriented, straight from design patterns book
- lots of additional complexity, logic is distributed around multiple
functions (that's another way of writing "very object oriented"?)
6) multi-methods
+ rather simple & elegant
- haha, we dont use CLOS
I could try the first solution but to do so I need you to understand
that keeping the real type of an object is crucial, essential. And to
get this achevied, I need your cooperation for modifying functions that
do not return the real type of an newly created object into a function
that returns the real type of the object.
Example is OutputDevice*Application::GetDefaultDevice()which calls
ImplGetDefaultWindow()which returns a vcl::Window*.
Application::GetDefaultDevice should return a vcl::Window*so that
functions that act differently for Windows can be used instead of
functions that take any OutputDevicebut that are in trouble when they
require to do something special with Windows.
in this particular case it might be possible to change it, but i bet
you'll find other functions that may return several different subclasses
of OutputDevice, and overloading in C++ has the limitation that you
can't overload the return type, so you'd be out of luck then.
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