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<H1> 11.1 Exception Declarations</H1>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> <A NAME="I4022"></A>An <FONT FACE="Arial, Helvetica">exception_declaration</FONT>
declares a name for an exception. </DIV>
<H4 ALIGN=CENTER>Syntax</H4>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">exception_declaration<A NAME="I4023"></A>
::= </FONT><A NAME="I4024"></A><FONT FACE="Arial, Helvetica">defining_identifier_list</FONT> : <B>exception</B>;</DIV>
<H4 ALIGN=CENTER>Static Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="Normal"> Each single <FONT FACE="Arial, Helvetica">exception_declaration</FONT>
declares a name for a different exception. If a generic unit includes
an <FONT FACE="Arial, Helvetica">exception_declaration</FONT>, the <FONT FACE="Arial, Helvetica">exception_declaration</FONT>s
implicitly generated by different instantiations of the generic unit
refer to distinct exceptions (but all have the same <FONT FACE="Arial, Helvetica">defining_identifier</FONT>).
The particular exception denoted by an exception name is determined at
compilation time and is the same regardless of how many times the <FONT FACE="Arial, Helvetica">exception_declaration</FONT>
is elaborated. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B>We considered removing
this requirement inside generic bodies, because it is an implementation
burden for implementations that wish to share code among several instances.
In the end, it was decided that it would introduce too much implementation
dependence. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>Hence, if
an <FONT FACE="Arial, Helvetica">exception_declaration</FONT> occurs
in a recursive subprogram, the exception name denotes the same exception
for all invocations of the recursive subprogram. The reason for this
rule is that we allow an exception occurrence to propagate out of its
declaration's innermost containing master; if exceptions were created
by their declarations like other entities, they would presumably be destroyed
upon leaving the master; we would have to do something special to prevent
them from propagating to places where they no longer exist. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3.c</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>Exception
identities are unique across all partitions of a program. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="Normal"> <A NAME="I4025"></A><A NAME="I4026"></A><A NAME="I4027"></A><A NAME="I4028"></A><A NAME="I4029"></A>The
<I>predefined</I> exceptions are the ones declared in the declaration
of package Standard: Constraint_Error, Program_Error, Storage_Error,
and Tasking_Error[; one of them is raised when a language-defined check
fails.] </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>The exceptions
declared in the language-defined package IO_Exceptions, for example,
are not predefined. </FONT></DIV>
<H4 ALIGN=CENTER>Dynamic Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="Normal"> <A NAME="I4030"></A>The elaboration of an <FONT FACE="Arial, Helvetica">exception_declaration</FONT>
has no effect.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="Normal"> <A NAME="I4031"></A><A NAME="I4032"></A><A NAME="I4033"></A>The
execution of any construct raises Storage_Error if there is insufficient
storage for that execution. <A NAME="I4034"></A>The amount of storage
needed for the execution of constructs is unspecified. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>Note that
any execution whatsoever can raise Storage_Error. This allows much implementation
freedom in storage management. </FONT></DIV>
<H4 ALIGN=CENTER>Examples</H4>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> <I>Examples of user-defined
exception declarations:</I> </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<DIV Class="Examples"><TT>Singular : <B>exception</B>;<BR>
Error : <B>exception</B>;<BR>
Overflow, Underflow : <B>exception</B>;</TT></DIV>
<H4 ALIGN=CENTER>Inconsistencies With Ada 83</H4>
<DIV Class="Paranum"><FONT SIZE=-2>8.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><A NAME="I4035"></A>The exception
Numeric_Error is now defined in the Obsolescent features Annex, as a
rename of Constraint_Error. All checks that raise Numeric_Error in Ada
83 instead raise Constraint_Error in Ada 95. To increase upward compatibility,
we also changed the rules to allow the same exception to be named more
than once by a given handler. Thus, ``<B>when</B> Constraint_Error |
Numeric_Error =>'' will remain legal in Ada 95, even though Constraint_Error
and Numeric_Error now denote the same exception. However, it will not
be legal to have separate handlers for Constraint_Error and Numeric_Error.
This change is inconsistent in the rare case that an existing program
explicitly raises Numeric_Error at a point where there is a handler for
Constraint_Error; the exception will now be caught by that handler. </FONT></DIV>
<H4 ALIGN=CENTER>Wording Changes from Ada 83</H4>
<DIV Class="Paranum"><FONT SIZE=-2>8.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>We explicitly define elaboration
for <FONT FACE="Arial, Helvetica">exception_declaration</FONT>s. </FONT></DIV>
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