Re: [Concepts] Should any iterator have an operator->() in C++0x?
on Thu Mar 22 2007, noreply-AT-this.is.invalid (Niels Dekker - no return address) wrote:
When concepts become part of the language, should a user-defined
iterator be required to always have an operator->()?
ConceptGCC 4.1.1 alpha 5 tells me that a class does not meet the
requirements of an iterator concept, if it does not have an
operator->(). As a consequence, such a class would not be allowed as
a template argument for an std algorithm that required iterators as
arguments.
It seems to me that an operator-> is not necessary if the value type
of the iterator does not have any members. E.g., if the value type is
a built-in type.
According to the current iterator concept definitions, I believe
you're right, in the sense that the semantics of operator-> is defined
entirely in terms of member access on the value type. If the
value_type is not a UDT, operator-> is meaningless.
But even if the value type *does* have members, will an std
algorithm ever need to call the operator->() of the iterator?
None of the existing std:: algorithms require that value_type is a
UDT, but that doesn't really matter. Lots of code outside the
standard depends on the current iterator concepts.
I discussed this issue recently with Douglas Gregor, but we could not
really come to a conclusion. So your feedback is appreciated!
The C++98 iterator concepts are overburdened with redundancy
(http://boost.org/libs/iterator/doc/facade-and-adaptor.html#motivation).
operator-> is one of those things which, if you want to be faithful to
those concepts, you must represent in ConceptC++. There's plenty of
argument for stripping the iterator concepts down to something more
manageable in C++0x, but if you leave out operator-> (or anything else
that's redundant in the current concepts) then the new iterators will
not necessarily be compatible with old code, because old code is free
to use any bit of redundant interface it wants. Catch-22
--
Dave Abrahams
Boost Consulting
www.boost-consulting.com
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