Concepts of Programming Languages

414 Chapter 9 Subprograms

Contemporary software-engineering principles dictate that access by sub-

program code to data outside the subprogram should be minimized. With this

goal in mind, in-mode parameters should be used whenever no data are to be

returned through parameters to the caller. Out-mode parameters should be

used when no data are transferred to the called subprogram but the subprogram

must transmit data back to the caller. Finally, inout-mode parameters should

be used only when data must move in both directions between the caller and

the called subprogram.

There is a practical consideration that is in conflict with this principle. Some-

times it is justifiable to pass access paths for one-way parameter transmission.

For example, when a large array is to be passed to a subprogram that does not

modify it, a one-way method may be preferred. However, pass-by-value would

require that the entire array be moved to a local storage area of the subprogram.

This would be costly in both time and space. Because of this, large arrays are

often passed by reference. This is precisely the reason why the Ada 83 defini-

tion allowed implementors to choose between the two methods for structured

parameters. C++ constant reference parameters offer another solution. Another

alternative approach would be to allow the user to choose between the methods.

The choice of a parameter-passing method for functions is related to another

design issue: functional side effects. This issue is discussed in Section 9.10.

9.5.8 Examples of Parameter Passing

Consider the following C function:

void swap1(int a, int b) {

int temp = a;

a = b;

b = temp;

}

Suppose this function is called with

swap1(c, d);

Recall that C uses pass-by-value. The actions of swap1 can be described by

the following pseudocode:

a = c — Move first parameter value in

b = d — Move second parameter value in

temp = a

a = b

b = temp

Although a ends up with d’s value and b ends up with c’s value, the values of c

and d are unchanged because nothing is transmitted back to the caller.