Section 12: Generic Units
{generic unit}
A
generic unit is a program unit that is either
a generic subprogram or a generic package.
{template}
A generic unit is a
template[, which can be
parameterized, and from which corresponding (nongeneric) subprograms
or packages can be obtained]. The resulting program units are said to
be
instances of the original generic unit.
{template:
See generic unit} {macro:
See generic unit} {parameter:
See generic formal parameter}
Glossary entry: {
Generic unit}
A generic unit is a template for a (nongeneric) program unit; the template
can be parameterized by objects, types, subprograms, and packages. An
instance of a generic unit is created by a
generic_instantiation.
The rules of the language are enforced when a generic unit is compiled,
using a generic contract model; additional checks are performed upon
instantiation to verify the contract is met. That is, the declaration
of a generic unit represents a contract between the body of the generic
and instances of the generic. Generic units can be used to perform the
role that macros sometimes play in other languages.
[A generic unit is declared by a
generic_declaration.
This form of declaration has a
generic_formal_part
declaring any generic formal parameters. An instance of a generic unit
is obtained as the result of a
generic_instantiation
with appropriate generic actual parameters for the generic formal parameters.
An instance of a generic subprogram is a subprogram. An instance of a
generic package is a package.
Generic units are templates. As templates they do
not have the properties that are specific to their nongeneric counterparts.
For example, a generic subprogram can be instantiated but it cannot be
called. In contrast, an instance of a generic subprogram is a (nongeneric)
subprogram; hence, this instance can be called but it cannot be used
to produce further instances.]
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