What are the operators that can be and cannot be overloaded in C++?

Difficulty Level : Easy
Last Updated : 12 Jun, 2019

List of operators that can be overloaded are:

+    -    *    /      %        ^
&    |    ~    !,        =
    =      ++        --
    ==    !=      &&        ||
+=    -=    /=    %=      ^=        &=
|=    *=    =      []        ()
->    ->*    new    new []      delete    delete []

Example 1 : Overloading ++ operator

// 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 ++ operator operates on the object of 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 and overloading postincrement operator

// this program will demonstrate  
// Difference between pre increment and post 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 this [ ] 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 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; 
} 
// Thank you Rohan Agarwal for suggesting this example. 

Output :

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

List of operators that cannot be overloaded

1> Scope Resolution Operator  (::)    
2> Pointer-to-member Operator (.*)    
3> Member Access or Dot operator  (.)    
4> Ternary or Conditional 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 cause error.

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

Output

prog.cpp:8:27: error: expected type-specifier before '.' token
         cantover& operator.() 
                           ^
prog.cpp:8:19: error: expected ';' at end of member declaration
         cantover& operator.() 
                   ^
prog.cpp:8:27: error: expected unqualified-id before '.' token
         cantover& operator.() 
                           ^
prog.cpp: In function 'void g(X&)':
prog.cpp:13:14: error: 'void X::fun()' is private
         void fun();
              ^
prog.cpp:18:15: error: within this context
         x.fun();              // X::fun or cantover::fun or error?
               ^

This problem can be solved in several ways. At the time of standardization, it was not obvious which way would be best.

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