Custom Object Sorting in C++

Introduction

When working with primitive data types such as int, float, or string, the C++ sort() function can sort them automatically.

However, for custom objects (classes or structures), sort() does not know which property should be used for comparison.

To solve this problem, we use a Custom Comparator Function.

A comparator function tells sort():

• When two objects are already in the correct order.
• When two objects need to be swapped.

---

Student Class

class Student
{
public:
    string name;
    int roll;
    int marks;
};

Each student contains:

Property	Description
name	Student Name
roll	Roll Number
marks	Obtained Marks

---

Sorting by Marks

Comparator Function

bool cmp(Student a, Student b)
{
    return a.marks < b.marks;
}

Explanation

The comparator receives two student objects.

cmp(a, b)

If:

a.marks < b.marks

returns true, then a should appear before b.

Otherwise, sort() will rearrange them.

---

Sorting Call

sort(a, a + n, cmp);

Parameters

Parameter	Meaning
a	Starting address
a + n	Ending address
cmp	Comparator function

---

Multi-Level Sorting

Sometimes multiple students may have the same marks.

In that case:

1. Sort by Marks (Ascending)
2. If Marks are Equal → Sort by Roll (Ascending)

---

Comparator

bool cmp(Student a, Student b)
{
    if (a.marks != b.marks)
        return a.marks < b.marks;

    return a.roll < b.roll;
}

---

Comparison Flow

Compare Marks
      │
      ├── Different
      │      │
      │      └── Smaller Marks First
      │
      └── Equal
             │
             └── Compare Roll Numbers
                        │
                        └── Smaller Roll First

---

Example

Input

4

Rahim 10 85
Karim 5 90
Naim 2 85
Hasan 1 70

Output

Hasan 1 70
Naim 2 85
Rahim 10 85
Karim 5 90

Why?

• 70 comes before 85
• Two students have 85
• Roll 2 comes before Roll 10

---

Time Complexity

Operation	Complexity
sort()	O(N log N)

---

Best Practice

❌ Avoid

return a.marks <= b.marks;

✅ Use

return a.marks < b.marks;

std::sort() expects a strict comparison rule. Using <= may lead to undefined behavior.

---

Key Takeaways

• Use custom comparators to sort objects.
• Pass the comparator as the third parameter of sort().
• Return true when the current order is correct.
• Multiple conditions can be applied inside a comparator.
• std::sort() works in O(N log N) time.
• Always use strict comparison (<) whenever possible.

---

Author Notes

This example demonstrates how custom comparator functions allow powerful sorting logic for user-defined objects in C++.