Java HashSet equivalent in C++

c++, data-structures, lookup, stl

Solution

You can use `std::unordered_set<>` (standard § 23.5.6), its `find` method (to do a lookup) as an average complexity of O(1) :

#include <iostream>
#include <unordered_set>

int main()
{  
    std::unordered_set<int> example = {1, 2, 3, 4};

    auto search = example.find(2);
    if(search != example.end()) {
        std::cout << "Found " << (*search) << '\n';
    }
    else {
        std::cout << "Not found\n";
    }
}

EDIT:

As suggested by @Drew Dormann, you can alternatively use `count`, which also has a average complexity of O(1):

#include <iostream>
#include <unordered_set>

int main()
{  
    std::unordered_set<int> example = {1, 2, 3, 4};

    if(example.count(2)) {
        std::cout << "Found\n";
    }
    else {
        std::cout << "Not found\n";
    }
}

Problem

I was curious if there was something akin the Java HashSet in C++? I.e. a data structure with a fast look, as I will only be running `.contains(e)` on it. Likewise, if you could enlighten me on how to do a `.contains()` on whatever data structure you propose, I would be very appreciative. O, please do not post just look at the c++ docs as I have already done so and found them burdensome.

Original source