This is safe as long as the element isn't removed from the map.

However, the second line is not quite safe :

object& obj = objmap.find(num)->second;

If there is no elements with key num in the map, find will return objmap.end(). This possibility should be tested before dereferencing the returned iterator :

const std::map<int, object>::iterator it = objmap.find(num);
if (it != objmap.end())
{
    object& obj = it->second;
    /* ... */
}

Now, if the goal isn't really to find but really to insert, calling operator[] is a possibility (although, as you already noticed, it requires the value to provide a parameterless constructor). But you have to understand that these are two very different things :

• find only finds : if the key isn't found, nothing gets inserted and the end iterator is returned
• operator[] always returns a reference to a value in the map : were the key absent, an insertion occurs (for a default constructed value : thus the constructor requirement)