Call by Reference and Call by Value

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Call by Value and Call by Reference are two ways of passing parameters to a function.

Call by Reference    -> modifies the passed value in the main function or the calling function
                                 -> actual arguments are passed to function
                                 -> can be done using pointers or references

Call by Value         -> does not modify value in the main function
                                -> copy of arguments is passed to function

Example, 
 void modref( int & x ) { x = 0; } // call by reference using reference  
 void modp (int * x ) { *x = 1; } // call by reference using pointer  
 void nomod (int x ) { x = 30; } // call by value  
 int main()  
 {    
   int y = 20;  
   cout<<y<<endl; // 20  
   modref(y);  
   cout<<y<<endl; // 0  
   modp(&y);  
   cout<<y<<endl; // 1  
   nomod(y);  
   cout<<y<<endl; // 1  
 }

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