When I should use std::map::at to retrieve map element

Contrary to most existing answers here, note that there are actually 4 methods related to finding an element in a map (ignoring lower_bound, upper_bound and equal_range, which are less precise):

• operator[] only exist in non-const version, as noted it will create the element if it does not exist
• at(), introduced in C++11, returns a reference to the element if it exists and throws an exception otherwise
• find() returns an iterator to the element if it exists or an iterator to map::end() if it does not
• count() returns the number of such elements, in a map, this is 0 or 1

Now that the semantics are clear, let us review when to use which:

• if you only wish to know whether an element is present in the map (or not), then use count().
• if you wish to access the element, and it shall be in the map, then use at().
• if you wish to access the element, and do not know whether it is in the map or not, then use find(); do not forget to check that the resulting iterator is not equal to the result of end().
• finally, if you wish to access the element if it exists or create it (and access it) if it does not, use operator[]; if you do not wish to call the type default constructor to create it, then use either insert or emplace appropriately