Chapter 5: Programming Practices - TOC - 5.2 PARAMETER LISTS

5.2.3 Default Parameters

guideline

Provide default parameters to allow for occasional, special use of widely used subprograms or entries.

Place default parameters at the end of the formal parameter list.

Consider providing default values to new parameters added to an existing subprogram.

example

Annex A of the Ada Reference Manual (1995) contains many examples of this practice.

rationale

Often, the majority of uses of a subprogram or entry need the same value for a given parameter. Providing that value, as the default for the parameter, makes the parameter optional on the majority of calls. It also allows the remaining calls to customize the subprogram or entry by providing different values for that parameter.

Placing default parameters at the end of the formal parameter list allows the caller to use positional association on the call; otherwise, defaults are available only when named association is used.

Often during maintenance activities, you increase the functionality of a subprogram or entry. This requires more parameters than the original form for some calls. New parameters may be required to control this new functionality. Give the new parameters default values that specify the old functionality. Calls needing the old functionality need not be changed; they take the defaults. This is true if the new parameters are added to the end of the parameter list, or if named association is used on all calls. New calls needing the new functionality can specify that by providing other values for the new parameters.

This enhances maintainability in that the places that use the modified routines do not themselves have to be modified, while the previous functionality levels of the routines are allowed to be "reused."

exceptions

Do not go overboard. If the changes in functionality are truly radical, you should be preparing a separate routine rather than modifying an existing one. One indicator of this situation would be that it is difficult to determine value combinations for the defaults that uniquely and naturally require the more restrictive of the two functions. In such cases, it is better to go ahead with creation of a separate routine.