unordered_set pass by address

Question

I have moved on to C++ from C and recently learning STL.

The last line gives pretty long error in STL style(Helpless) Or maybe I am new to templates thats why I find it helpless.

int insert(Forest *forest, int e1) {
    Forest::const_iterator te1;
    te1 = forest->begin();
    te1->insert(e1);
}
int main() {
    //some code here
    Forest forest = (5, myHash);
    insert(&forest, e1);
}

Forest is:

typedef unordered_set<unordered_set<int>, function<size_t(const unordered_set<int>)>> Forest;

EDIT: After trying as one of answers suggested

Forest::iterator te1 = forest->begin();
te1->insert(e1);

It still gives same errors.

Answer 1

In C++11, the items in a std::set or std::unordered_set are all const. You can't insert into one of them, you have to remove it and re-add.

This is because you're determining the position in the parent set by its hash value, which might change if you add a new item to it. It's analogous to the keys in a std::map being const.