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<H1>Section 12: Generic Units</H1>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> <A NAME="I4155"></A>A <I>generic unit</I> is a
program unit that is either a generic subprogram or a generic package.
<A NAME="I4156"></A>A generic unit is a <I>template</I>[, which can be
parameterized, and from which corresponding (nongeneric) subprograms
or packages can be obtained]. The resulting program units are said to
be <I>instances</I> of the original generic unit. <A NAME="I4157"></A><A NAME="I4158"></A><A NAME="I4159"></A></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>1.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Glossary entry: </B>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 <FONT FACE="Arial, Helvetica">generic_instantiation</FONT>.
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.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="Normal"> [A generic unit is declared by a <FONT FACE="Arial, Helvetica">generic_declaration</FONT>.
This form of declaration has a <FONT FACE="Arial, Helvetica">generic_formal_part</FONT>
declaring any generic formal parameters. An instance of a generic unit
is obtained as the result of a <FONT FACE="Arial, Helvetica">generic_instantiation</FONT>
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.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="Normal"> 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.] </DIV>
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