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In C++, sorting is a common operation that can be customized by providing a comparator function. When working with classes, we might need to sort objects based on their member variables. Using a member function as a comparator is a good technique to achieve this.
In this article, we will learn how to sort using a member function as a comparator in C++.
Example:
Consider an example, where we have a Student class with name and age as its attributes and we want to sort a vector of Student objects based on the age attribute.
Input:
vector<Student> students = {
Student("Alice", 23),
Student("Bob", 20),
Student("Charlie", 22)
};
Output:
Sorted students by age:
Bob 20
Charlie 22
Alice 23 Ways to Sort Using Member Function as ComparatorThere are two efficient approaches to sort using a member function as a comparator in C++:
1. Sort Using a Static Member FunctionIn C++, the static member function is defined within the class and can be used directly as a comparator in the std::sort function. This function compares two objects of the class and determines their order based on a specific member variable, although it can access only static members of the class, but can be used as a comparator.
Example:
C++
// C++ Program to illustrate how to sort using a static
// member function
#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;
// define a class with a compare member function
class Student {
public:
string name;
int age;
// Constructor
Student(string n, int a)
: name(n)
, age(a)
{
}
// Static member function to compare students by age
static bool compareByAge(const Student& a,
const Student& b)
{
return a.age < b.age;
}
};
int main()
{
// Initialize the vector of Student objects
vector<Student> students
= { Student("Alice", 23), Student("Bob", 20),
Student("Charlie", 22) };
// Sort the vector using the static member function as
// the comparator
sort(students.begin(), students.end(),
Student::compareByAge);
// Output the sorted list of students
cout << "Sorted students by age:" << endl;
for (const Student& student : students) {
cout << student.name << " " << student.age << endl;
}
return 0;
}
OutputSorted students by age:
Bob 20
Charlie 22
Alice 23
Time Complexity: O(n log n), where n is the number of elements
Auxiliary Space: O(1)
2. Sort Using a Lambda Function with Member Function CallAnother approach is to use a lambda function that can be used to call the member function as the comparator, when sorting objects. This allows more flexibility and can access non-static members.
Example:
C++
// C++ Program to illustrate how to sort using a lambda
// function calling a member function
#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;
// define a class student with a member function to compare
class Student {
public:
string name;
int age;
// Constructor
Student(string n, int a)
: name(n)
, age(a)
{
}
// Member function to compare students by age
bool isYoungerThan(const Student& other) const
{
return age < other.age;
}
};
int main()
{
// Initialize the vector of Student objects
vector<Student> students
= { Student("Alice", 23), Student("Bob", 20),
Student("Charlie", 22) };
// Sort the vector using a lambda function as the
// comparator
sort(students.begin(), students.end(),
[](const Student& a, const Student& b) {
return a.isYoungerThan(b);
});
// Output the sorted list of students
cout << "Sorted students by age:" << endl;
for (const Student& student : students) {
cout << student.name << " " << student.age << endl;
}
return 0;
}
OutputSorted students by age:
Bob 20
Charlie 22
Alice 23
Time Complexity: O(n log n), where n is the number of elements
Auxiliary Space: O(1)
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