Is there a function can modify map's value and return the iterator?

What I want is to insert a pair<key, value> into the map (create it if not existed, modify it otherwise), and got the value's pointer, to let others use.

here is a demo code:

#include <map>
using namespace std;
std::map<int, int> a = {{1, 1}};
int* set_and_return_value() {
  // option 1: a[1] = 2; return &(a[1]); i think it's ugly
  const auto [it, ok] = a.emplace(1, 2); 
  return &(it->second);
  // emplace is good, but it won't modify the existed key's value
}

So, is there a function, can return the iterator after real insert the pair?

Assigning via iterator does not invalidate the iterator, you can use that and the return value of emplace to conditionally assign if the insertion failed:

#include <map>

using namespace std;
std::map<int, int> a = {{1, 1}};
auto set_and_return_it() {
    const auto [it, inserted] = a.emplace(1, 2);
    if (!inserted) {
        it->second = 2;
    }
    return it;
}

If you want it more generic, use try_emplace to avoid constructing 2 twice.

template<typename K, typename T>
auto set_and_return_it(K&& key, T&& value) {
    const auto [it, inserted] = a.try_emplace(std::forward<K>(key), std::forward<T>(value));
    if (!inserted) {
        it->second = std::forward<T>(value);
    }
    return it;
}