C++ set for user defined data type?

A set is a data structure that is used to store distinct values (meaning no two elements can have the same value) in ascending or descending order.

Set for User-defined Data Types

We can directly use sets for built-in data types, but for user-defined data types, values cannot be directly stored, because a set compares the elements to maintain order, but for user-defined data types such as array or struct, the compiler will not be able to perform a comparison between them. The set container is defined under the <set> header file.

So to use user-defined datatypes into a stack, we have to override < operator or custom comparator, which will compare two values of that type such as for set, map etc.

Syntax

Here is the following syntax of overloaded < operator inside the class/struct for set data structure.

struct structName {
    // member variables will be defined here

    bool operator<(const structName& other) const {
        // comparison logic will be implemented here and return the result in bool
    }
};

set<structName> mySet;

Here is the following syntax of using a custom comparator for struct for set data structure.

struct structName {
    // member variables will be defined here
};

struct Comparator {
    bool operator()(const structName& a, const structName& b) const {
        // comparison logic will be implemented here and return the result in bool
    }
};

set<structName, Comparator> mySet;

Example of Set for User-defined Data Types

Here is the following example program, showcasing how the set is used with struct to store a person object in a set with sorted elements, first by age and then by name using a custom comparator.

#include <iostream>
#include <set>
using namespace std;

struct person {
    string name;
    int age;
};

// here is the custom comparator to sort persons by age and then by name
struct personCompare {
    bool operator()(const person& a, const person& b) const {
        if (a.age != b.age)
            return a.age < b.age;
        return a.name < b.name;
    }
};

int main() {
    set<person, personCompare> people;
   
    // we can't add duplicates in set
    people.insert({"Aman", 30});
    people.insert({"Bob", 25});
    people.insert({"Alex", 30});
    people.insert({"Ashi", 30});  

    for (const auto& p : people) {
        cout << p.name << " (" << p.age << ")\n";
    }

    return 0;
}

Output

Bob (25)
Alex (30)
Aman (30)
Ashi (30)