4.1 Pass-by-value

``Pass-by-value'' refers to the mechanism of ``give a subroutine a copy [of a value] to play with''. Let us consider the following example:

void sub1(int x)
{
  printf("%d\n",x);
  // cout << x << eoln;
}

This is a fairly simple example that prints the value of parameter x to the standard output. Now, let us consider an invocation (call) of the subroutine:

sub1(z);

In this invocation, the expression z is evaluated, then a copy of this value is given to subroutine sub1. Because we are only working with a copy of the expression, we can use any expression that evaluates to an integer value, such as the following:

sub1(z*z+10*y);

You can consider pass-by-value as a safe method to provide extra information to a subroutine. This is because the subroutine cannot change the value of any variable or object that belongs to the caller. Let us consider another example:

void inc(int x)
{
  x = x + 1;
}

A good compiler should give you a warning. This is because what we do to parameter x remains local to the subroutine. We can incrementing a copy of whatever expression is used to specify the argument. For example, we may have the following invocation:

inc(z);

What is provided to subroutine inc is not a method to find variable z, but rather just the current value of z. Parameter x in subroutine sub1 works with a copy of the value of z. The incremented value is not related to variable z, at all.

In other words, if you have an object or variable to be altered by a subroutine, pass-by-value should not be used.