It should be

for (auto it = a.begin(); it != a.end(); ){
    //If elem must be deleted
        it = a erase(it);
    else
        ++it;
     continue;
}


The erase method returns the next valid iterator into the map.

An easier way to do this would be

std::erase_if(a, /*function obj to check condition for erasing*/); //C++20