Idea: Lazy upcasting

[Mitja Slenc]

Aarti_pl wrote:

> Proposition has also nothing to do with implicit downcasting. It has 
> more with upcasting, but in opposite way - implicit upcasting should be 
> disallowed in some cases :-)
> 
> If my proposition is not clear in some aspects, feel free to ask 
> questions. I am not a native speaker, so parts of this proposition could 
> be unclear...

OK, let me try to restate the problem, to see if I get it - some methods 
always return this, so that several invocations on the same object can 
be chained together : foo.a().b().c(). But, if one defines an inherited 
class and doesn't override those methods to indicate the new return 
type, calling base class' methods results in losing information about 
the derived type, so neither can the object be assigned to references of 
the derived type, nor can new methods of the derived class be called 
after calling base methods.

The problem with your suggestion of 'lazy upcasting' is that it's quite 
impossible for the compiler to verify whether all the methods in fact 
return this (for example, the source file could've been compiled 
previously and its source is no longer available), so it can't tell 
whether to do the usual thing (use the declared return type) or not (use 
the derived type).

A better idea would be to use a special keyword for the return type to 
indicate what is going on:

class Base
{
	return_this foo(int i) { ... }
}

class Derived : Base
{
	return_this bar(int i) { ... }
}

void main()
{
	Derived d = (new Derived).foo(1).bar(2); // currently an error
}


Another idea would be to treat void methods as if they implicitly 
returned this, though I'm quite sure there are hidden dangers in doing 
so. But, method definitions could certainly be cleaner and more obvious 
(it's not always clear whether the object returned is supposed to be a 
new one, or the called one), and I guess relatively many temporary 
variables serve only the purpose of being able to call more than one 
method on a given object (without fetching it twice from somewhere), and 
those would often no longer be needed, as well.


xs0