iterator acting weird while in for_each

Question

Doesn't an iterator have to be deferenced before using? I can't understand why

for_each(vecResult.begin(), vecResult.end(), [](auto counter) {cout << counter << endl;
});

is working (showing the contents of the vector) but

for_each(vecResult.begin(), vecResult.end(), [](auto counter) {cout << *counter << endl; });

is not.(My visual studio shows an error message

"'<<':illegal for class)

score 2 · Answer 1 · answered Aug 31 '19 at 09:03

A possible implementation of std::for_each is as follows:

template<class InputIt, class UnaryFunction>
constexpr UnaryFunction for_each(InputIt first, InputIt last, UnaryFunction f)
{
    for (; first != last; ++first) {
        f(*first);
    }
    return f; // implicit move since C++11
}

_{source: https://en.cppreference.com/w/cpp/algorithm/for_each}

As you can see above, what is passed to the f (in your case, the lambda) is already dereferenced. So, in your example counter is not an iterator, it's the value to which the iterator points.

@John If this answers your question, you should press the green tick on the left to mark your problem as solved. — HolyBlackCat, Aug 31 '19 at 13:11

iterator acting weird while in for_each

1 Answers1