Re: [boost] Iterator adaptors return a value from operator[]?

From: Anthony Williams (anthwil_at_[hidden])
Date: 2002-08-16 08:54:06

Peter Dimov writes:
 > From: "David Abrahams" <dave_at_[hidden]>
 > >
 > > From: "Peter Dimov" <pdimov_at_[hidden]>
 > >
 > > > From: "David Abrahams" <dave_at_[hidden]>
 > > > >
 > > > > Any iterator which stores the referent of its operator* within the
 > > > iterator
 > > > > object is an example that causes serious memory problems if operator[]
 > > > > returns a reference.
 > > > >
 > > > > That's a legal technique for random-access iterators.

 > > > Isn't this a defect report material?
 > >
 > > I don't know, what would the defect be?
 > 
 > Good question. There is no defect in the strict sense of the word. But do
 > you like the current situation, where you are forced to cripple all
 > iterators generated by the library just because someone _might_ do an
 > iterator that returns by value from []?

Can't we check whether [] returns a reference, somehow? It's not like we're
asking "what is the return value?", just is it T& or not, for known T,
something like:

template<typename T,typename IteratorType>
struct checkSubscriptReturnIsRefToT
{
    template<typename V>
    Big check(T& (IteratorType::*)(V));

    Small check(...);

    static const bool
    result=(sizeof(check(&IteratorType::operator[])==sizeof(Big));
};

Obviously, we need to cope with const, volatile and const volatile operator[],
and versions taking different types of parameter, but I'm sure you can fill in
the details.

Anthony