220 5858 <BC0A23AE-9034-4930-BC8B-C1E6D2394CBB@gmail.com> article
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From: Howard Hinnant <howard.hinnant@gmail.com>
Newsgroups: gmane.comp.lang.c++.isocpp.proposals
Subject: Re: Remove vector<bool>?
Date: Mon, 26 Aug 2013 20:56:45 -0400
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On Aug 26, 2013, at 8:16 PM, Geoffrey Romer <gromer@google.com> wrote:

> One problem this solves is that code like this is guaranteed to work:
> 
> template <typename iter_t>
> value_type& GetValue(iter_t iter) {
>   return *iter;
> }

Oops, I just realized this code is pretty dangerous even neglecting proxy iterators:

There's no enforced connection between iter_t and value_type.  So what happens when value_type is const int and:

double d;
double* dp = &d;
const int& cr = GetValue(dp);

You've got a dangling reference.  And I didn't even have to use proxies to crash this one.  To make this safer one could (use auto, or):

template <typename iter_t>
typename std::iterator_traits<iter_t>::value_type&
GetValue(iter_t iter)
{
   return *iter;
}

But by the time you've gone this far...

template <typename iter_t>
typename std::iterator_traits<iter_t>::reference
GetValue(iter_t iter)
{
   return *iter;
}

Howard

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