John Fine escreveu:
In X::Y::Z when X has already been identified as a type (templated, so
its internals are unknown), I think Y might still be a namespace, but
I'm very unsure (haven't tested). Anyway, some of the compilers I use
insist on being told that Y is a type.
I don't think that Y might be a type, as there's nothing that I know of
that states that a namespace can be enclosed in a class declaration.
So, if X::Y names a namespace, X must be a namespace, and so on (X::Y::Z
is a namespace, X::Y also is).
Anyway, I thought your issue was with template rather than with
typename. I'd prefer if the standard made the compiler deduce template
in cases that can't be parsed any other way, rather than make the
programmer clutter the code with it. But for template, at least I
understand why deducing it would be hard on the structure of the
compiler and I understand when it is needed and what it qualifies.
I think it's good for the compiler to detect template declaration errors
earlier than only during instantiation, that's why there's all this fuss
about dependent names and non-dependent names. But it should use the
context in which the name is used to tell if it's dealing with a type or
something else. If it can't, well, the user should tell it what s/he
mean. I don't know how difficult it is for the compiler to handle this
issue, but with the rise of template metaprogramming, I find myself
writing 'typename' more and more, which is annoying when it's obvious
that I want a type.
Now, correcting what I said before, you really should use 'typename' in
variable declarations, according to Stroustrup (TC++PL, C13.5):
template <class T> void g(T &y)
{
T::x(y);
}
This may be a function call, calling T::x function with the parameter y,
or... surprise, declaring a variable 'y' of the type T::x, with
redundant parentheses around y. Why the parentheses are allowed is
beyond me. If they weren't allowed in the first place, typename
disambiguation shouldn't be necessary. So the committee don't allow
something useful (not needing typename) because of something redundant.
There's the intellectual purity over usability that you've mentioned.
Regards,
rod
