C++ LANGUAGE CALL BY REFERENCE PAGE 74

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C++ LANGUAGE CALL BY REFERENCE

Call by Reference 


The call by reference method of passing arguments to 
a function copies the reference of an argument into 
the formal parameter. Inside the function, the reference 
is used to access the actual argument used in the call. 
This means that changes made to the parameter affect the 
passed argument. 

To pass the value by reference, argument reference is 
passed to the functions just like any other value. 
So accordingly you need to declare the function 
parameters as reference types as in the following 
function swap(), which exchanges the values of the 
two integer variables pointed to by its arguments.
 
// function definition to swap the values.
 
void swap(int &x, int &y) 
{ 
   int temp; 
   temp = x; /* save the value at address x */ 
   x = y;    /* put y into x */ 
   y = temp; /* put x into y */ 
   
   return; 
} 
For now, let us call the function swap() by passing 
values by reference as in the following example: 

#include <iostream> 
using namespace std; 
 
// function declaration 
void swap(int &x, int &y); 
 
int main () 
{ 
   // local variable declaration: 
   int a = 100; 
   int b = 200; 
  
   cout << "Before swap, value of a :" << a << endl; 
   cout << "Before swap, value of b :" << b << endl; 
 
   /* calling a function to swap the values using variable reference.*/ 
   swap(a, b); 
 
   cout << "After swap, value of a :" << a << endl; 
   cout << "After swap, value of b :" << b << endl; 
  
   return 0; 
} 
When the above code is put together in a file, 
compiled and executed, it produces the following result: 

Before swap, value of a :100 
Before swap, value of b :200 
After swap, value of a :200 
After swap, value of b :100 
 
By default, C++ uses call by value to pass arguments. In general, 
this means that code within a function cannot alter the arguments 
used to call the function and above mentioned example while calling max() 
function used the same method.