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In Python, there is a class called defaultdict which is essentially a dictionary that will construct elements on demand according to a function specified by the user at construction time..
Does a similar class already exists in C++, or would I have to create it myself by inheriting from map and overwriting the at method?
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A defaultdict works exactly like a normal dict, but it is initialized with a function (“default factory”) that takes no arguments and provides the default value for a nonexistent key. A defaultdict will never raise a KeyError. Any key that does not exist gets the value returned by the default factory.
Finally, using a defaultdict to handle missing keys can be faster than using dict.
It depends on the data; setdefault is faster and simpler with small data sets; defaultdict is faster for larger data sets with more homogenous key sets (ie, how short the dict is after adding elements);
This is not directly an answer to your question, but if you want the same behavior as defaultdict for aggregation, you could use map.emplace to assign a default value if the key does not exist, and return an iterator to the new or existing item (which avoids a second lookup):
unordered_map<int, size_t> map = {{1, 1}, {2, 3}};
// later...
for (int i = 1; i < 4; i++) {
auto emplace_pair = map.emplace(i, 0);
emplace_pair.first->second += 1;
}
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