11.1 Exception Declarations
1
{exception}
An
exception_declaration
declares a name for an exception.
Syntax
2
exception_declaration ::= defining_identifier_list :
exception;
Static Semantics
3
Each single exception_declaration
declares a name for a different exception. If a generic unit includes
an exception_declaration, the exception_declarations
implicitly generated by different instantiations of the generic unit
refer to distinct exceptions (but all have the same defining_identifier).
The particular exception denoted by an exception name is determined at
compilation time and is the same regardless of how many times the exception_declaration
is elaborated.
3.a
Reason: 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.
3.b
Ramification: Hence, if an exception_declaration
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.
3.c
Ramification: Exception identities are
unique across all partitions of a program.
4
{predefined exception}
{Constraint_Error
(raised by failure of run-time check)} {Program_Error
(raised by failure of run-time check)} {Storage_Error
(raised by failure of run-time check)} {Tasking_Error
(raised by failure of run-time check)} The
predefined 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.]
4.a
Ramification: The exceptions declared
in the language-defined package IO_Exceptions, for example, are not predefined.
Dynamic Semantics
5
{elaboration (exception_declaration)
[partial]} The elaboration of an
exception_declaration
has no effect.
6
{Storage_Check
[partial]} {check,
language-defined (Storage_Check)} {Storage_Error
(raised by failure of run-time check)} The
execution of any construct raises Storage_Error if there is insufficient
storage for that execution.
{unspecified
[partial]} The amount of storage needed for
the execution of constructs is unspecified.
6.a
Ramification: Note that any execution
whatsoever can raise Storage_Error. This allows much implementation freedom
in storage management.
Examples
7
Examples of user-defined
exception declarations:
8
Singular : exception;
Error : exception;
Overflow, Underflow : exception;
Inconsistencies With Ada 83
8.a
{
inconsistencies with Ada 83}
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, “
when
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.
Wording Changes from Ada 83
8.b
We explicitly define elaboration for exception_declarations.
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