c++ is it safe to read a map that may be written at the same time?

Question

I have two thread, thread 1 may add or delete to a map

//t1
std::map<int, int> my_map;
my_map[9] = 9;

And thread2 check if a certain key exists in the map,

//t2
void check_and_act() {
    if(my_map.find(9)==my_map.end()) {
       //do something
    }
}

check_and_act in t2 will be called repeatedly, so I do not care if operation in t1 is performed earlier or later than check done in t2, is there going to be any undefined behavoir in this case?