What type of data or objects are permissible to store inside a std::set container in C++?

Any data types for which the less-than operator (<) is defined can be stored in a std::set container in C++, as std::set stores elements in sorted order and uses this operator for ordering.

These can include:

• Primitive types (int, double, char, etc.)
• User-defined classes and structures, if they have a defined < operator or a comparator is provided when creating the set

Example with a user-defined type:

#include <set>
#include <string>

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

    bool operator<(const Person& other) const {
        return age < other.age; // sorting by age
    }
};

int main() {
    std::set<Person> people;
    people.insert({"Alice", 30});
    people.insert({"Bob", 25});
}

If the < operator is not defined, compilation will fail.