Quick start manual

Procedures and functions 6-19
Parameters
Default parameters
You can specify default parameter values in a procedure or function heading. Default
values are allowed only for typed const and value parameters. To provide a default
value, end the parameter declaration with the = symbol followed by a constant
expression that is assignment-compatible with the parameter’s type.
For example, given the declaration
procedure FillArray(A: array of Integer; Value: Integer = 0);
the following procedure calls are equivalent.
FillArray(MyArray);
FillArray(MyArray, 0);
A multiple-parameter declaration cannot specify a default value. Thus, while
function MyFunction(X: Real = 3.5; Y: Real = 3.5): Real;
is legal,
function MyFunction(X, Y: Real = 3.5): Real; // syntax error
is not.
Parameters with default values must occur at the end of the parameter list. That is, all
parameters following the first declared default value must also have default values.
So the following declaration is illegal.
procedure MyProcedure(I: Integer = 1; S: string); // syntax error
Default values specified in a procedural type override those specified in an actual
routine. Thus, given the declarations
type TResizer = function(X: Real; Y: Real = 1.0): Real;
function Resizer(X: Real; Y: Real = 2.0): Real;
var
F: TResizer;
N: Real;
the statements
F := Resizer;
F(N);
result in the values (N, 1.0) being passed to Resizer.
Default parameters are limited to values that can be specified by a constant
expression. (See “Constant expressions” on page 5-44.) Hence parameters of a
dynamic-array, procedural, class, class-reference, or interface type can have no value
other than nil as their default. Parameters of a record, variant, file, static-array, or
object type cannot have default values at all.
For information about calling routines with default parameter values, see “Calling
procedures and functions.”.