reading object from const unordered_map
The expression z[5]
calls a non-const member function of the map.
This is because a map's operator[]
will insert a new element if the key isn't found, so obviously it has to be non-const.
For a vector
nothing is inserted by operator[]
, the element must exist already (or you get undefined behaviour, so the equivalent code would access the 6th element of an empty vector, which is not fine!).
To lookup a key without adding it use:
int val = 0;
auto it = z.find(5);
if (it != z.end())
val = it->second;
As Jonathan already said, the operator[]
method is non-const because it might add a default value when the item being looked up is not found.
On the other hand, as highlighted from Benjamin in a comment, the at()
method is available for const as well.
const unordered_map<int, int> z;
int val = z.at(5); // Success!
The downside is that when the value being looked up is not in the map, a std::out_of_range
exception is raised, so it has to be managed.