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

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

2

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) 

7) static_cast (casting operator)

8) const_cast (casting operator)

9) reinterpret_cast (casting operator)

10) dynamic_cast (casting operator)

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.