std::sort with std::pairs

Here’s an example of sorting a std::vector of std::pair<string, int> (representing student names and IDs) using a custom comparator implemented in a class:

#include <iostream>
#include <vector>
#include <algorithm> // for std::sort
using namespace std;

// Custom comparator class
class CompareByName {
public:
    bool operator()(const pair<string, int> &a, const pair<string, int> &b) const {
        return a.first < b.first; // Compare based on the student name (first element of the pair)
    }
};

int main() {
    // Vector of student names and IDs
    vector<pair<string, int>> students = {
        {"Alice", 1002},
        {"Charlie", 1005},
        {"Bob", 1001},
        {"Eve", 1003}
    };

    // Sort the vector using the CompareByName comparator
    sort(students.begin(), students.end(), CompareByName());

    // Print the sorted list of students
    cout << "Sorted Students (by name):" << endl;
    for (const auto &student : students) {
        cout << "Name: " << student.first << ", ID: " << student.second << endl;
    }

    return 0;
}

Output:

Sorted Students (by name):
Name: Alice, ID: 1002
Name: Bob, ID: 1001
Name: Charlie, ID: 1005
Name: Eve, ID: 1003

Explanation:

1. Comparator Class:
   • The CompareByName class defines an operator() to act as a functor (function object).
   • This allows the class to behave like a callable function, making it usable in std::sort.
2. Sorting Logic:
   • The comparison is done on the first element of the pairs (student names), which determines the sort order.
3. Usage in std::sort:
   • An instance of CompareByName is passed to std::sort as the comparator.
4. Result:
   • The students vector is sorted alphabetically by the student name while preserving the associated IDs.