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Given a const_iterator ci to an element of a container c, it is already
possible to obtain a non-const iterator with:

auto i=std::advance(c.begin(),std::distance(c.cbegin(),ci);

However, this is extremely inefficient for non-random-access containers. I
would like to add some mechanism to the standard containers so that given a
non-const reference to the container and a const_iterator, it is possible
to obtain a non-const iterator in constant time.
This would be useful in a scenario where an algorithm takes a const
reference to a container and finds some interesting element inside,
returning a const_iterator to it. Then, as long as you have a non-const
reference to the container, you should be able to modify the element you
found. It is already possible to erase it using a const_iterator.

Is there any reason why such a facility doesn't exist for the standard
containers?

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<div dir=3D"ltr">Given a const_iterator ci to an element of a container c, =
it is already possible to obtain a non-const iterator with:<div><br></div><=
div>auto i=3Dstd::advance(c.begin(),std::distance(c.cbegin(),ci);</div><div=
><br></div><div>However, this is extremely inefficient for non-random-acces=
s containers. I would like to add some mechanism to the standard containers=
 so that given a non-const reference to the container and a const_iterator,=
 it is possible to obtain a non-const iterator in constant time.</div><div>=
This would be useful in a scenario where an algorithm takes a const referen=
ce to a container and finds some interesting element inside, returning a co=
nst_iterator to it. Then, as long as you have a non-const reference to the =
container, you should be able to modify the element you found. It is alread=
y possible to erase it using a const_iterator.</div><div><br></div><div>Is =
there any reason why such a facility doesn't exist for the standard contain=
ers?</div><div><br></div></div>

<p></p>

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