STL set intersection question

[Phil]

Hello,

I'm trying to get the result of a set intersection and I'm sure that I've
made a very basic error.

A Google search has helped but I'm still not quite there. Can someone show
me the error?

set<int> firstSet;

secondSet.insert(1);
secondSet.insert(2);

set<int> secondSet;

secondSet.insert(2);

set<int> resultSet;

resultSet = set_intersection(firstSet.begin(), firstSet.end(),
secondSet.begin(), secondSet.end(), resultSet.begin());

 

[Pete Becker]

resultSet = set_intersection(firstSet.begin(), firstSet.end(),
secondSet.begin(), secondSet.end(), resultSet.begin());

First, set_intersection returns an iterator that points past the end of
the result sequence. Don't assign it to the container, both because it
doesn't make sense to do that and because set doesn't have an assignment
operator that takes an iterator.

More important, though: that last argument, resultSet.begin(), isn't
right. You need an output sequence, that is, a sequence whose elements
are writable. resultSet has no elements, and even if it did, writing
directly to them through that iterator isn't allowed. You need an
insertion iterator here: inserter(resultSet, resultSet.begin()) will do
the job. Assigning to the dereferenced iterator inserts a new element in
the container.