How do I customize hashing and equality with std::multiset?

In C++, `std::multiset` allows you to store multiple values in a sorted manner. By default, it uses the `std::less` comparator for ordering and the default equality operator for determining equality. However, you can customize both hashing and equality using a combination of templates and functors.

Here's an example of how to define your own hash function and equality comparison for a custom type, which you can then use with `std::multiset`.


#include <iostream>
#include <set>
#include <string>

// Custom struct
struct Person {
    std::string name;
    int age;

    // Constructor
    Person(std::string name, int age) : name(name), age(age) {}

    // Less-than operator for ordering
    bool operator<(const Person& other) const {
        return name < other.name;
    }

    // Equality operator
    bool operator==(const Person& other) const {
        return name == other.name && age == other.age;
    }
};

// Custom comparator
struct PersonComparator {
    bool operator()(const Person& lhs, const Person& rhs) const {
        return lhs.name < rhs.name; // You can extend this for more complex comparison
    }
};

int main() {
    std::multiset people;

    people.insert(Person("Alice", 30));
    people.insert(Person("Bob", 25));
    people.insert(Person("Alice", 22)); // Duplicate by custom logic

    for (const auto& p : people) {
        std::cout << p.name << " is " << p.age << " years old" << std::endl;
    }

    return 0;
}

How do I customize hashing and equality with std::multiset?

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How do I customize hashing and equality with std::multiset?

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