[boost] Re: Re: Re: Zero Unnecessary Temporary Objects: A Revolution

From: Daniel Frey (d.frey_at_[hidden])
Date: 2002-10-13 16:21:46

On Sun, 13 Oct 2002 19:29:14 +0200, David Abrahams wrote:

> Daniel Frey <daniel.frey_at_[hidden]> writes:
> 
>> David Abrahams wrote:
>> > 
>> > Daniel Frey <daniel.frey_at_[hidden]> writes:
>> > 
>> > A const& is not a transparent replacement for a value in other ways.
>> > Consider:
>> > 
>> >    const iterator& x = c.end();
>> >    c.insert(start, finish);
>> >    // is x valid here, or dangling?
>> 
>> 'const iterator x = c.end();' won't be any better, would it? Maybe I
>> need some more explainations here :))
> 
> The question has to do with whether a const& _return_ _value_ is a
> transparent replacement for a by-value return. If c.end() returns a
> value, x binds to the temporary and keeps it alive for the duration of
> the scope. If c.end() returns a const reference, mutating functions on c
> may invalidate or change the object to which x is bound.

If the iterator is stored inside of the container, this is not a problem,
is it? And today, it's also a problem as iterators (even the objects) are
invalidated and you are not allowed to use them after an insert(). For
your example, I imagine something like this:

class my_container
{
   iterator end_it;

public
   const iterator& end() const { return end_it; }
};

so as long as the container exists, x is valid. Even better: x now
contains an end-value which will not be invalidated by insert(). (OK,
this won't be a good idea for a lot of other reasons).

Anyway, my current impression is, that we have a consensus about the NRVO
itself. I would suggest that we add it without the const return values
and discuss the const-topic separatly. Is this OK for everyone?

Regards, Daniel