What are the Operators that Can be and Cannot be Overloaded in C++?

Difficulty Level : Easy
Last Updated : 02 Dec, 2021

There are various ways to overload Operators in C++ by implementing any of the following types of functions:

1) Member Function

2) Non-Member Function

3) Friend Function

List of operators that can be overloaded are:

+	–	*	⁄	%	‸	&	\|	~
!	=	<	>	+=	-=	*=	⁄=	%=
‸=	&=	\|=	<<	>>	<<=	>>=	==	!=
<=	>=	&&	\|\|	++	—	,	->*	->
( )	[ ]	new	delete	new[]	delete[]

Example 1: Overloading ++ Operator

CPP

// CPP program to illustrate
// operators that can be overloaded
#include <iostream>
using namespace std;
  
class overload {
private:
    int count;
  
public:
    overload()
        : count(4)
    {
    }
  
    void operator++() { count = count + 1; }
    void Display() { cout << "Count: " << count; }
};
  
int main()
{
    overload i;
    // this calls "function void operator ++()" function
    ++i;
    i.Display();
    return 0;
}

Output

Count: 5

This function is called when the ++ operator operates on the object of the overload class (object i in this case). In the program, void operator ++ () operator function is defined (inside overload class). This function increments the value of count by 1 for i object.

Example 2: Overloading ++ operator i.e. pre and post increment operator

CPP

// CPP program to demonstrate the
// Difference between pre increment 
// and post increment overload operator
#include <iostream>
using namespace std;
  
class overload {
private:
    int count;
  
public:
    overload(int i)
        : count(i)
    {
    }
  
    overload operator++(int) { return (count++); }
    overload operator++()
    {
        count = count + 1;
        return count;
    }
    void Display() { cout << "Count: " << count << endl; }
};
// Driver code
int main()
{
    overload i(5);
    overload post(5);
    overload pre(5);
  
    // this calls "function overload operator ++()" function
    pre = ++i;
    cout << "results of I   =   ";
    i.Display();
    cout << "results of preincrement   =  ";
    pre.Display();
    // this call "function overload operator ++()"function
    i++; // just to show diff
    i++; // just to show diff
    post = i++;
    cout << "Results of post increment   =   ";
    post.Display();
    cout << "And results of i , here we see difference   : "
            "  ";
    i.Display();
    return 0;
}

Output

results of I   =   Count: 6
results of preincrement   =  Count: 6
Results of post increment   =   Count: 8
And results of i , here we see difference   :   Count: 9

Example 3: Overloading [ ] operator

CPP

// CPP program to illustrate overloading the
// [ ] operator
  
#include <iostream>
using namespace std;
class overload {
    int a[3];
  
public:
    overload(int i, int j, int k)
    {
        a[0] = i;
        a[1] = j;
        a[2] = k;
    }
    int operator[](int i) { return a[i]; }
};
int main()
{
    overload ob(1, 2, 3);
    cout << ob[1]; // displays 2
    return (0);
}

Output

Example 4 : Overloading -> operator

CPP

// CPP program to illustrate
// operators that can be overloaded
#include <bits/stdc++.h>
using namespace std;
  
class GFG {
public:
    int num;
    GFG(int j) { num = j; }
    GFG* operator->(void) { return this; }
};
  
// Driver code
int main()
{
    GFG T(5);
    GFG* Ptr = &T;
  
    // Accessing num normally
    cout << "T.num = " << T.num << endl;
  
    // Accessing num using normal object pointer
    cout << "Ptr->num = " << Ptr->num << endl;
  
    // Accessing num using -> operator
    cout << "T->num = " << T->num << endl;
  
    return 0;
}

Output

T.num = 5
Ptr->num = 5
T->num = 5

List of operators that cannot be overloaded

1) Scope Resolution Operator (::)

2) Ternary or Conditional Operator (?:)

3) Member Access or Dot operator (.)

4) Pointer-to-member Operator (.*)

5) Object size Operator (sizeof)

6) Object type Operator(typeid)

Example 5: Overloading this .(dot) operator

Dot (.) operator can’t be overloaded, so it will generate an error.

CPP

// C++ program to illustrate
// Overloading this .(dot) operator
#include <iostream>
using namespace std;
  
class cantover {
public:
    void fun();
};
class X {
    cantover* p;
    cantover& operator.() { return *p; }
    void fun();
};
void g(X& x)
{
    x.fun(); // X::fun or cantover::fun or error?
}

Output: Error

prog.cpp:12:23: error: expected type-specifier before ‘.’ token
    cantover& operator.() { return *p; }

This program will generate an error. Similarly, the above operators will also generate an error, if overloaded.

This article is contributed by Shivani Ghughtyal. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

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