multimap::equal_range
returns a pair
whose second element in this case is an iterator to the past-the-end element ("which is the past-the-end value for the container" [container.requirements.general]/6).
I'll rewrite the code a bit to point something out:
auto iBeg = testMap.begin();
auto iEnd = testMap.end();
for(auto i = iBeg; i != iEnd; ++i)
{
testMap.insert( std::make_pair("b", i->second) );
}
Here, iEnd
contains a past-the-end iterator. The call to multimap::insert
doesn't invalidate this iterator; it stays a valid past-the-end iterator. Therefore the loop is equivalent to:
for(auto i = iBeg; i != testMap.end(); ++i)
Which is of course an infinite loop if you keep adding elements.