at()
is a new method for std::map
in C++11.
Rather than insert a new default constructed element as operator[]
does if an element with the given key does not exist, it throws a std::out_of_range
exception. (This is similar to the behaviour of at()
for deque
and vector
.)
Because of this behaviour it makes sense for there to be a const
overload of at()
, unlike operator[]
which always has the potential to change the map.