Rui Maciel ha scritto:
In order to avoid wasting resources populating a map with useless
key:value pairs, is there a clean, unencumbered way to get the value
associated with a given key without being forced to insert new elements
or even resort to multiple lines of code?
But shouldn't you rather view it as an error to access an element which
does not exist? Perhaps what you should do is access elements with a
function like this:
template <class KeyType, class MappedType>
MappedType const &get(std::map<KeyType, MappedType> const &map, KeyType
const &key)
{
std::map<KeyType, MappedType>::const_iterator find_iter =
map.find(key);
assert(find_iter != map.end());
return find_iter->second;
}
(Note how this, in contrast to operator[], can be used with a const
std::map as well.)
Then make sure outside code accesses only those elements which exist. It
seems a more robust solution to me than relying on default values being
returned in case the key does not exist. Otherwise, what happens when
the default value happens to be one of the actual values in the map? You
won't be able to distinguish between legitimate access and error cases.
Hitting a value not stored in the table does not always indicate an error. I
recently had an interactive program that was mainly table driven. The key
and the default action was null_op. The main loop would just wait until the
user presses a key and execute the corresponding action. Only keys that