Passing Arguments by Value and by Reference

I. The general problem

A. Sometimes it might be useful if a function could modify the function arguments

int a = 2, b = 3;
Swap(a, b);
// a and b have swapped values
cout << a << b; // 3 and 2

B. Suppose this function was written like so:

void Swap(int x, int y)
{
     int temp = x;
     x = y;
     y = temp;
}

int a = 2, b = 3;
Swap(a, b); // a and b have NOT swapped values!
cout << a << b;// 2 and 3

II. Pass by value

A. By default, arguments are passed by value to a function which means a copy is made of the argument

B. Any changes made to a parameter are only changing the copy, not the actual argument

C. It's like handing someone a photocopy of a paper you wrote; any changes the person makes to the photocopy doesn't change the original paper

III. Pass by reference

void Swap(int &x;, int &y;)
{
     int temp = x;
     x = y;
     y = temp;
}

B. This means the parameter is just another name for the argument

C. Therefore any changes made to the reference parameter will also change the argument (modifying  x will change  a, and modifying  y will change  b)

D. It's like handing someone the original paper you wrote; any changes the person makes to the original will affect the original.

E. When using pass by reference, a variable must be used as the argument

// Won't work because literals cannot change value or be passed by reference!
Swap(2, 3);
// This works because variables are used as arguments
Swap(a, b);

IV. Illustration showing how RAM is affected

void AddOne(int x, int &y;)
{
  x++;
  y++;
}

void main()
{
  int a = 2, b = 2;
  AddOne(a, b); 

  cout << a << b; // 2 and 3
}

B. RAM before the call to AddOne:

C. RAM during the call to  AddOne:

D. RAM after adding one to  x and  y in  AddOne:

E. RAM after the call to  AddOne:

F. Note how  a's value did not change because it was passed by value, but  b's value changed because it was passed by reference

V. Example of using pass-by-reference in a function in the Pepsi Challenge

// This function adds one to coke_votes if the incoming vote is C or c,
// or it adds one to pepsi_votes if the vote is P or p
void CountVote(char vote, int &coke;_votes, int &pepsi;_votes)
{
    vote = toupper(vote);
    if (vote == 'C')
        coke_votes++;
    else if (vote == 'P')
        pepsi_votes++;
}
 
void main()
{
    int coke_votes = 0, pepsi_votes = 0;
    char vote;
     
    cout << "Enter a vote for Coke or Pepsi (C or P): ";
    cin >> vote;
     
    CountVote(vote, coke_votes, pepsi_votes);
}

Source: Franc McCown and Harding University, http://web.archive.org/web/20161110160528/http://www.harding.edu/fmccown/classes/comp151-s14/notes/notes19.html
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