On 2012-04-26 03:44:07 +0000, Walter Bright <[email protected]> said:
A subtle but nasty problem - are default arguments part of the type, or
part of the declaration?
See http://d.puremagic.com/issues/show_bug.cgi?id=3866
Currently, they are both, which leads to the nasty behavior in the bug report.
The problem centers around name mangling. If two types mangle the same,
then they are the same type. But default arguments are not part of the
mangled string. Hence the schizophrenic behavior.
But if we make default arguments solely a part of the function
declaration, then function pointers (and delegates) cannot have default
arguments. (And maybe this isn't a bad thing?)
Assuming we want to allow it, all you need is to treat the type having
the default parameters as a specialization of the type that has none.
In other words, the type with the default arugments is implicitly
castable to the type without.
Should it affect name mangling? Yes and not. It should affect name
mangling in the compiler since you're using the mangled name to check
for equality between types. But I think it should not affect name
mangling for the generated code: the base type without the default
arguments should give its name to the emitted symbols so that changing
the default argument does not change the ABI.
But is it desirable? I'm not convinced. I don't really see the point.
It looks like a poor substitute for overloading to me.
That said, there was some talk about adding support for named
parameters a year ago. For named parameters, I think it'd make sense to
have parameter names be part of the type, and little harm would result
in adding default parameters too into the mix. As suggested above, it
should be implicitly castable to a base type without parameter names or
default values. But for default parameters alone, I don't feel the
complication is justified.
--
Michel Fortin
[email protected]
http://michelf.com/
