Why unordered_set<string::iterator> does not work?

Question

I'm trying to get a list of iterators to erase from a string. I wanted to do it with unsorted_set because access time is constant and I'll be doing lookup often but it gives me a compile error. unordered_set<string::iterator> to_erase However if i try to define vector<string::iterator> to_erase it works.

But then when I try to do:

find(to_erase.begin(),to_erase.end(),it)

It doesn't work

Answer 1

I have not checked but it looks like the string iterator does not define a hash code. Anything stored in an unordered_* has to have a hash.

I am not sure how you would do it since the internal char* of the iterator is hidden from you.

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Mooing Duck contributed the following from http://coliru.stacked-crooked.com/a/a8d75d3ac153a799

#include <iostream>
#include <vector>
#include <string>
#include <unordered_set>

struct str_it_hasher {
    std::size_t operator()(std::string::const_iterator it) const {
        return std::hash<const char*>{}(&*it);
    }
};


int main() {
    std::unordered_set<std::string::iterator, str_it_hasher> set;
    std::string a = "apple";
    set.insert(a.begin());
    set.find(a.begin());
    set.erase(a.begin());
}

Answer 2

An iterator of a string can be easily converted back and forth to index:

auto idx = iter - str.begin();
auto iter = idx + str.begin();

Then you can store the indices.