Re: [boost] proxies and const

From: David Abrahams (dave_at_[hidden])
Date: 2007-08-13 09:25:52

on Mon Aug 13 2007, "Martin Schulz" <Martin.Schulz-AT-synopsys.com> wrote:

> Giovanni, sorry for the late reply.
>
>> Consider this:
>> 
>>   std::vector<int> x = ....
>>   for_each(filter(x, some_filter_op),  do_something);
>> 
>> 'Filter' will obviously return a proxy for 'x' (actually 
>> wrapping x iterators with boost::filter iterators) as a 
>> temporary object.
>> for_each will bind the temporary to a const reference (at 
>> least until we get rvalue references).
>
> The fact that temporary objects cannot be passed in as non-const 
> references had been decided to avoid programming accidents in that 
> modifications to the temporary object were assumed to be 
> meaningfull and should be preserved. If you want this object as 
> non-const nevertheless, simply give a name to it.

That's less simple than you make it sound.  It could have a very
complicated type.

One good way to deal with this is to make the proxy object return a
constant object.  Then it will bind to a T& parameter.

Furthermore, the constness of the proxy should not affect the
constness of its access to elements of x.  That essentially means
returning a different proxy type depending on whether the incoming
container is const or not.

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com
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