Any way to trick std::transform into operating on the iterator themselves?

Question

So I wrote this code which won't compile. I think the reason is because std::transform, when given an iterator range such as this will operate on the type pointed to by the iterator, not the iterator itself. Is there any simple wrapper, standard lib tool, etc. to make this code work i.e. to store all the iterators of the original map into a new vector, with minimum changes required? Thanks!

#include <map>
#include <iostream>
#include <vector>

using MT = std::multimap<char, int>;
using MTI = MT::iterator;

int main()
{
    MT m;
    m.emplace('a', 1); m.emplace('a', 2); m.emplace('a', 3);
    m.emplace('b', 101);
        
    std::vector<MTI> itrs;
    std::transform(m.begin(), m.end(), std::back_inserter(itrs), [](MTI itr){
        return itr;
    });
}

EDIT 1: Failed to compile with gcc11 and clang13, C++17/20

EDIT 2: The purpose of the question is mostly out of curiosity. I want to see what's a good way to manipulate existing standard algorithm to work on the level that I want. The sample code and problem are entirely made up for demonstration but they are not related to any real problem that requires a solution

Answer 1

The function you pass to std::transform and algorithms in general are supposed to use elements not iterators. You could use the key to find the iterator in the map, though thats neither efficient nor simple. Instead use a plain loop:

for (auto it = m.begin(); it != m.end(); ++it) itrs.push_back(it);