std::not_equal_to in C++ with Examples

Last Updated : 12 Jul, 2025

The std::not_equal_to is a functional object class for non-equality comparison and binary function object class. It returns a boolean value depending upon the condition whether the two arguments are not equal or not.
Header File:

#include <functional.h>

Template Class:

template struct not_equal_to :
            binary_function  {

  // Declaration of the
  // not equal to operation 
  bool operator() (const T& x,
                   const T& y) 
       const 
  {
     return x!=y;
  }

  // Type of first parameter
  typedef T first_argument_type;

  // Type of second parameter
  typedef T second_argument_type;

  // The result is returned
  // as bool type
  typedef bool result_type;
}

Syntax:

std::not_equal_to <int> ()

Parameter: This function accepts the type of the arguments T, as the parameter, to be compared by the functional call.
Return Type: It return a boolean value depending upon condition(let a & b are 2 element):

True: If a is not equals to b.
False: If a is equals to b.

Below is the illustration of std::not_equal_to in C++:
Program 1:

CPP

// C++ code to illustrate std::not_equal_to

#include <algorithm>
#include <functional>
#include <iostream>
#include <vector>
using namespace std;

// Driver Code
int main()
{
    // Initialise vectors
    vector<int> v1 = { 50, 55, 60,
                       65, 70 };
    vector<int> v2 = { 50, 55, 85,
                       65, 70 };

    // Declaring pointer of pairs
    pair<vector<int>::iterator,
         vector<int>::iterator>
        pairs1;

    // Use mismatch() function to
    // search first match between 
    // v1 and v2
    pairs1 = mismatch(v1.begin(), v1.end(),
                      v2.begin(), 
                      not_equal_to<int>());

    // Print the match pair
    cout << "The 1st match element"
         << " of 1st container : ";
    cout << *pairs1.first << endl;

    cout << "The 1st match element "
         << "of 2nd container : ";
    cout << *pairs1.second << endl;

    return 0;
}

Output:

The 1st match element of 1st container : 50
The 1st match element of 2nd container : 50

Program 2:

CPP

// C++ program to illustrate 
// std::not_equals_to
#include <algorithm>
#include <functional>
#include <iostream>
using namespace std;

// Template
template <typename A, typename B, 
          typename U = not_equal_to<int> >

// Function to check if a != b or not
bool f(A a, B b, U u = U())
{
    return u(a, b);
}

// Driver Code
int main()
{
    int X = 1, Y = 2;

    // If X is not equals to Y or not
    cout << boolalpha;
    cout << f(X, Y) << '\n';

    X = -1, Y = -1;

    // If X is not equals to Y or not
    cout << f(X, Y) << '\n';

    return 0;
}

Output:

true
false

Reference: https://cplusplus.com/reference/functional/not_equal_to/

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