Section 11: Exceptions
1 This section defines the facilities for dealing with errors or other
exceptional situations that arise during program execution. An exception
represents a kind of exceptional situation; an occurrence of such a situation
(at run time) is called an exception occurrence. To raise an exception is to
abandon normal program execution so as to draw attention to the fact that the
corresponding situation has arisen. Performing some actions in response to the
arising of an exception is called handling the exception.
2 An exception_declaration declares a name for an exception. An exception is
raised initially either by a raise_statement or by the failure of a
language-defined check. When an exception arises, control can be transferred
to a user-provided exception_handler at the end of a handled_sequence_of_-
statements, or it can be propagated to a dynamically enclosing execution.
11.1 Exception Declarations
1 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.
4 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.
Dynamic Semantics
5 The elaboration of an exception_declaration has no effect.
6 The execution of any construct raises Storage_Error if there is
insufficient storage for that execution. The amount of storage needed for the
execution of constructs is unspecified.
Examples
7 Examples of user-defined exception declarations:
8 Singular : exception;
Error : exception;
Overflow, Underflow : exception;
11.2 Exception Handlers
1 The response to one or more exceptions is specified by an
exception_handler.
Syntax
2 handled_sequence_of_statements ::=
sequence_of_statements
[exception
exception_handler
{exception_handler}]
3 exception_handler ::=
when [choice_parameter_specification:] exception_choice
{| exception_choice} =>
sequence_of_statements
4 choice_parameter_specification ::= defining_identifier
5 exception_choice ::= exception_name | others
Legality Rules
6 A choice with an exception_name covers the named exception. A choice with
others covers all exceptions not named by previous choices of the same handled_-
sequence_of_statements. Two choices in different exception_handlers of the
same handled_sequence_of_statements shall not cover the same exception.
7 A choice with others is allowed only for the last handler of a
handled_sequence_of_statements and as the only choice of that handler.
8 An exception_name of a choice shall not denote an exception declared in a
generic formal package.
Static Semantics
9 A choice_parameter_specification declares a choice parameter, which is a
constant object of type Exception_Occurrence (see 11.4.1). During the handling
of an exception occurrence, the choice parameter, if any, of the handler
represents the exception occurrence that is being handled.
Dynamic Semantics
10 The execution of a handled_sequence_of_statements consists of the
execution of the sequence_of_statements. The optional handlers are used to
handle any exceptions that are propagated by the sequence_of_statements.
Examples
11 Example of an exception handler:
12 begin
Open(File, In_File, "input.txt"); -- see A.8.2
exception
when E : Name_Error =>
Put("Cannot open input file : ");
Put_Line(Exception_Message(E)); -- see 11.4.1
raise;
end;
11.3 Raise Statements
1 A raise_statement raises an exception.
Syntax
2/2 raise_statement ::= raise;
| raise exception_name [with string_expression];
Legality Rules
3 The name, if any, in a raise_statement shall denote an exception. A
raise_statement with no exception_name (that is, a re-raise statement) shall
be within a handler, but not within a body enclosed by that handler.
Name Resolution Rules
3.1/2 The expression, if any, in a raise_statement, is expected to be of type
String.
Dynamic Semantics
4/2 To raise an exception is to raise a new occurrence of that exception, as
explained in 11.4. For the execution of a raise_statement with an
exception_name, the named exception is raised. If a string_expression is present, the
expression is evaluated and its value is associated with the exception
occurrence. For the execution of a re-raise statement, the exception
occurrence that caused transfer of control to the innermost enclosing handler
is raised again.
Examples
5 Examples of raise statements:
6/2 raise Ada.IO_Exceptions.Name_Error; -- see A.13
raise Queue_Error with "Buffer Full"; -- see 9.11
7 raise; -- re-raise the current exception
11.4 Exception Handling
1 When an exception occurrence is raised, normal program execution is
abandoned and control is transferred to an applicable exception_handler, if
any. To handle an exception occurrence is to respond to the exceptional event.
To propagate an exception occurrence is to raise it again in another context;
that is, to fail to respond to the exceptional event in the present context.
Dynamic Semantics
2 Within a given task, if the execution of construct a is defined by this
International Standard to consist (in part) of the execution of construct b,
then while b is executing, the execution of a is said to dynamically enclose
the execution of b. The innermost dynamically enclosing execution of a given
execution is the dynamically enclosing execution that started most recently.
3 When an exception occurrence is raised by the execution of a given
construct, the rest of the execution of that construct is abandoned; that is,
any portions of the execution that have not yet taken place are not performed.
The construct is first completed, and then left, as explained in 7.6.1. Then:
4 * If the construct is a task_body, the exception does not propagate
further;
5 * If the construct is the sequence_of_statements of a
handled_sequence_of_statements that has a handler with a choice
covering the exception, the occurrence is handled by that handler;
6 * Otherwise, the occurrence is propagated to the innermost dynamically
enclosing execution, which means that the occurrence is raised again
in that context.
7 When an occurrence is handled by a given handler, the
choice_parameter_specification, if any, is first elaborated, which creates the
choice parameter and initializes it to the occurrence. Then, the
sequence_of_statements of the handler is executed; this execution replaces the
abandoned portion of the execution of the sequence_of_statements.
NOTES
8 1 Note that exceptions raised in a declarative_part of a body are not
handled by the handlers of the handled_sequence_of_statements of that
body.
11.4.1 The Package Exceptions
Static Semantics
1 The following language-defined library package exists:
2/2 with Ada.Streams;
package Ada.Exceptions is
pragma Preelaborate(Exceptions);
type Exception_Id is private;
pragma Preelaborable_Initialization(Exception_Id);
Null_Id : constant Exception_Id;
function Exception_Name(Id : Exception_Id) return String;
function Wide_Exception_Name
(Id : Exception_Id) return Wide_String;
function Wide_Wide_Exception_Name(Id : Exception_Id)
return Wide_Wide_String;
3/2 type Exception_Occurrence is limited private;
pragma Preelaborable_Initialization(Exception_Occurrence);
type Exception_Occurrence_Access
is access all Exception_Occurrence;
Null_Occurrence : constant Exception_Occurrence;
4/2 procedure Raise_Exception(E : in Exception_Id;
Message : in String := "");
pragma No_Return(Raise_Exception);
function Exception_Message
(X : Exception_Occurrence) return String;
procedure Reraise_Occurrence(X : in Exception_Occurrence);
5/2 function Exception_Identity(X : Exception_Occurrence)
return Exception_Id;
function Exception_Name(X : Exception_Occurrence) return String;
-- Same as Exception_Name(Exception_Identity(X)).
function Wide_Exception_Name(X : Exception_Occurrence)
return Wide_String;
-- Same as Wide_Exception_Name(Exception_Identity(X)).
function Wide_Wide_Exception_Name(X : Exception_Occurrence)
return Wide_Wide_String;
-- Same as Wide_Wide_Exception_Name(Exception_Identity(X)).
function Exception_Information
(X : Exception_Occurrence) return String;
6/2 procedure Save_Occurrence(Target : out Exception_Occurrence;
Source : in Exception_Occurrence);
function Save_Occurrence(Source : Exception_Occurrence)
return Exception_Occurrence_Access;
6.1/2 procedure Read_Exception_Occurrence
(Stream : not null access Ada.Streams.Root_Stream_Type'Class;
Item : out Exception_Occurrence);
procedure Write_Exception_Occurrence
(Stream : not null access Ada.Streams.Root_Stream_Type'Class;
Item : in Exception_Occurrence);
6.2/2 for Exception_Occurrence'Read use Read_Exception_Occurrence;
for Exception_Occurrence'Write use Write_Exception_Occurrence;
6.3/2 private
... -- not specified by the language
end Ada.Exceptions;
7 Each distinct exception is represented by a distinct value of type
Exception_Id. Null_Id does not represent any exception, and is the default
initial value of type Exception_Id. Each occurrence of an exception is
represented by a value of type Exception_Occurrence. Null_Occurrence does not
represent any exception occurrence, and is the default initial value of type
Exception_Occurrence.
8/1 For a prefix E that denotes an exception, the following attribute is
defined:
9 E'Identity E'Identity returns the unique identity of the exception. The
type of this attribute is Exception_Id.
10/2 Raise_Exception raises a new occurrence of the identified exception.
10.1/2 Exception_Message returns the message associated with the given
Exception_Occurrence. For an occurrence raised by a call to Raise_Exception,
the message is the Message parameter passed to Raise_Exception. For the
occurrence raised by a raise_statement with an exception_name and a
string_expression, the message is the string_expression. For the occurrence raised by
a raise_statement with an exception_name but without a string_expression, the
message is a string giving implementation-defined information about the
exception occurrence. In all cases, Exception_Message returns a string with
lower bound 1.
10.2/2 Reraise_Occurrence reraises the specified exception occurrence.
11 Exception_Identity returns the identity of the exception of the occurrence.
12/2 The Wide_Wide_Exception_Name functions return the full expanded name of
the exception, in upper case, starting with a root library unit. For an
exception declared immediately within package Standard, the
defining_identifier is returned. The result is implementation defined if the
exception is declared within an unnamed block_statement.
12.1/2 The Exception_Name functions (respectively, Wide_Exception_Name) return
the same sequence of graphic characters as that defined for
Wide_Wide_Exception_Name, if all the graphic characters are defined in
Character (respectively, Wide_Character); otherwise, the sequence of
characters is implementation defined, but no shorter than that returned by
Wide_Wide_Exception_Name for the same value of the argument.
12.2/2 The string returned by the Exception_Name, Wide_Exception_Name, and
Wide_Wide_Exception_Name functions has lower bound 1.
13/2 Exception_Information returns implementation-defined information about
the exception occurrence. The returned string has lower bound 1.
14/2 Reraise_Occurrence has no effect in the case of Null_Occurrence.
Raise_Exception and Exception_Name raise Constraint_Error for a Null_Id.
Exception_Message, Exception_Name, and Exception_Information raise
Constraint_Error for a Null_Occurrence. Exception_Identity applied to
Null_Occurrence returns Null_Id.
15 The Save_Occurrence procedure copies the Source to the Target. The
Save_Occurrence function uses an allocator of type Exception_Occurrence_Access
to create a new object, copies the Source to this new object, and returns an
access value designating this new object; the result may be deallocated using
an instance of Unchecked_Deallocation.
15.1/2 Write_Exception_Occurrence writes a representation of an exception
occurrence to a stream; Read_Exception_Occurrence reconstructs an exception
occurrence from a stream (including one written in a different partition).
Implementation Requirements
16/2 This paragraph was deleted.
Implementation Permissions
17 An implementation of Exception_Name in a space-constrained environment may
return the defining_identifier instead of the full expanded name.
18 The string returned by Exception_Message may be truncated (to no less than
200 characters) by the Save_Occurrence procedure (not the function), the
Reraise_Occurrence procedure, and the re-raise statement.
Implementation Advice
19 Exception_Message (by default) and Exception_Information should produce
information useful for debugging. Exception_Message should be short (about one
line), whereas Exception_Information can be long. Exception_Message should not
include the Exception_Name. Exception_Information should include both the
Exception_Name and the Exception_Message.
11.4.2 Pragmas Assert and Assertion_Policy
1/2 Pragma Assert is used to assert the truth of a Boolean expression at any
point within a sequence of declarations or statements. Pragma Assertion_Policy
is used to control whether such assertions are to be ignored by the
implementation, checked at run-time, or handled in some implementation-defined
manner.
Syntax
2/2 The form of a pragma Assert is as follows:
3/2 pragma Assert([Check =>] boolean_expression[, [Message =>]
string_expression]);
4/2 A pragma Assert is allowed at the place where a declarative_item or a
statement is allowed.
5/2 The form of a pragma Assertion_Policy is as follows:
6/2 pragma Assertion_Policy(policy_identifier);
7/2 A pragma Assertion_Policy is a configuration pragma.
Name Resolution Rules
8/2 The expected type for the boolean_expression of a pragma Assert is any
boolean type. The expected type for the string_expression of a pragma Assert
is type String.
Legality Rules
9/2 The policy_identifier of a pragma Assertion_Policy shall be either Check,
Ignore, or an implementation-defined identifier.
Static Semantics
10/2 A pragma Assertion_Policy is a configuration pragma that specifies the
assertion policy in effect for the compilation units to which it applies.
Different policies may apply to different compilation units within the same
partition. The default assertion policy is implementation-defined.
11/2 The following language-defined library package exists:
12/2 package Ada.Assertions is
pragma Pure(Assertions);
13/2 Assertion_Error : exception;
14/2 procedure Assert(Check : in Boolean);
procedure Assert(Check : in Boolean; Message : in String);
15/2 end Ada.Assertions;
16/2 A compilation unit containing a pragma Assert has a semantic dependence
on the Assertions library unit.
17/2 The assertion policy that applies to a generic unit also applies to all
its instances.
Dynamic Semantics
18/2 An assertion policy specifies how a pragma Assert is interpreted by the
implementation. If the assertion policy is Ignore at the point of a pragma
Assert, the pragma is ignored. If the assertion policy is Check at the point
of a pragma Assert, the elaboration of the pragma consists of evaluating the
boolean expression, and if the result is False, evaluating the Message
argument, if any, and raising the exception Assertions.Assertion_Error, with a
message if the Message argument is provided.
19/2 Calling the procedure Assertions.Assert without a Message parameter is
equivalent to:
20/2 if Check = False then
raise Ada.Assertions.Assertion_Error;
end if;
21/2 Calling the procedure Assertions.Assert with a Message parameter is
equivalent to:
22/2 if Check = False then
raise Ada.Assertions.Assertion_Error with Message;
end if;
23/2 The procedures Assertions.Assert have these effects independently of the
assertion policy in effect.
Implementation Permissions
24/2 Assertion_Error may be declared by renaming an implementation-defined
exception from another package.
25/2 Implementations may define their own assertion policies.
NOTES
26/2 2 Normally, the boolean expression in a pragma Assert should not call
functions that have significant side-effects when the result of the
expression is True, so that the particular assertion policy in effect
will not affect normal operation of the program.
11.4.3 Example of Exception Handling
Examples
1 Exception handling may be used to separate the detection of an error from
the response to that error:
2/2 package File_System is
type File_Handle is limited private;
3 File_Not_Found : exception;
procedure Open(F : in out File_Handle; Name : String);
-- raises File_Not_Found if named file does not exist
4 End_Of_File : exception;
procedure Read(F : in out File_Handle; Data : out Data_Type);
-- raises End_Of_File if the file is not open
5 ...
end File_System;
6/2 package body File_System is
procedure Open(F : in out File_Handle; Name : String) is
begin
if File_Exists(Name) then
...
else
raise File_Not_Found with "File not found: " & Name & ".";
end if;
end Open;
7 procedure Read(F : in out File_Handle; Data : out Data_Type) is
begin
if F.Current_Position <= F.Last_Position then
...
else
raise End_Of_File;
end if;
end Read;
8 ...
9 end File_System;
10 with Ada.Text_IO;
with Ada.Exceptions;
with File_System; use File_System;
use Ada;
procedure Main is
begin
... -- call operations in File_System
exception
when End_Of_File =>
Close(Some_File);
when Not_Found_Error : File_Not_Found =>
Text_IO.Put_Line(Exceptions.Exception_Message(Not_Found_Error));
when The_Error : others =>
Text_IO.Put_Line("Unknown error:");
if Verbosity_Desired then
Text_IO.Put_Line(Exceptions.Exception_Information(The_Error));
else
Text_IO.Put_Line(Exceptions.Exception_Name(The_Error));
Text_IO.Put_Line(Exceptions.Exception_Message(The_Error));
end if;
raise;
end Main;
11 In the above example, the File_System package contains information about
detecting certain exceptional situations, but it does not specify how to
handle those situations. Procedure Main specifies how to handle them; other
clients of File_System might have different handlers, even though the
exceptional situations arise from the same basic causes.
11.5 Suppressing Checks
1/2 Checking pragmas give instructions to an implementation on handling
language-defined checks. A pragma Suppress gives permission to an
implementation to omit certain language-defined checks, while a pragma
Unsuppress revokes the permission to omit checks..
2 A language-defined check (or simply, a "check") is one of the situations
defined by this International Standard that requires a check to be made at run
time to determine whether some condition is true. A check fails when the
condition being checked is false, causing an exception to be raised.
Syntax
3/2 The forms of checking pragmas are as follows:
4/2 pragma Suppress(identifier);
4.1/2 pragma Unsuppress(identifier);
5/2 A checking pragma is allowed only immediately within a
declarative_part, immediately within a package_specification, or as a
configuration pragma.
Legality Rules
6/2 The identifier shall be the name of a check.
7/2 This paragraph was deleted.
Static Semantics
7.1/2 A checking pragma applies to the named check in a specific region, and
applies to all entities in that region. A checking pragma given in a
declarative_part or immediately within a package_specification applies from
the place of the pragma to the end of the innermost enclosing declarative
region. The region for a checking pragma given as a configuration pragma is
the declarative region for the entire compilation unit (or units) to which it
applies.
7.2/2 If a checking pragma applies to a generic instantiation, then the
checking pragma also applies to the instance. If a checking pragma applies to
a call to a subprogram that has a pragma Inline applied to it, then the
checking pragma also applies to the inlined subprogram body.
8/2 A pragma Suppress gives permission to an implementation to omit the named
check (or every check in the case of All_Checks) for any entities to which it
applies. If permission has been given to suppress a given check, the check is
said to be suppressed.
8.1/2 A pragma Unsuppress revokes the permission to omit the named check (or
every check in the case of All_Checks) given by any pragma Suppress that
applies at the point of the pragma Unsuppress. The permission is revoked for
the region to which the pragma Unsuppress applies. If there is no such
permission at the point of a pragma Unsuppress, then the pragma has no effect.
A later pragma Suppress can renew the permission.
9 The following are the language-defined checks:
10 * The following checks correspond to situations in which the exception
Constraint_Error is raised upon failure.
11/2 Access_Check
When evaluating a dereference (explicit or implicit),
check that the value of the name is not null. When
converting to a subtype that excludes null, check that the
converted value is not null.
12 Discriminant_Check
Check that the discriminants of a composite value have the
values imposed by a discriminant constraint. Also, when
accessing a record component, check that it exists for the
current discriminant values.
13/2 Division_Check
Check that the second operand is not zero for the
operations /, rem and mod.
14 Index_Check Check that the bounds of an array value are equal to the
corresponding bounds of an index constraint. Also, when
accessing a component of an array object, check for each
dimension that the given index value belongs to the range
defined by the bounds of the array object. Also, when
accessing a slice of an array object, check that the given
discrete range is compatible with the range defined by the
bounds of the array object.
15 Length_Check
Check that two arrays have matching components, in the
case of array subtype conversions, and logical operators
for arrays of boolean components.
16 Overflow_Check
Check that a scalar value is within the base range of its
type, in cases where the implementation chooses to raise
an exception instead of returning the correct mathematical
result.
17 Range_Check Check that a scalar value satisfies a range constraint.
Also, for the elaboration of a subtype_indication, check
that the constraint (if present) is compatible with the
subtype denoted by the subtype_mark. Also, for an
aggregate, check that an index or discriminant value
belongs to the corresponding subtype. Also, check that
when the result of an operation yields an array, the value
of each component belongs to the component subtype.
18 Tag_Check Check that operand tags in a dispatching call are all
equal. Check for the correct tag on tagged type
conversions, for an assignment_statement, and when
returning a tagged limited object from a function.
19 * The following checks correspond to situations in which the exception
Program_Error is raised upon failure.
19.1/2 Accessibility_Check
Check the accessibility level of an entity or view.
19.2/2 Allocation_Check
For an allocator, check that the master of any tasks to be
created by the allocator is not yet completed or some
dependents have not yet terminated, and that the
finalization of the collection has not started.
20 Elaboration_Check
When a subprogram or protected entry is called, a task
activation is accomplished, or a generic instantiation is
elaborated, check that the body of the corresponding unit
has already been elaborated.
21/2 This paragraph was deleted.
22 * The following check corresponds to situations in which the exception
Storage_Error is raised upon failure.
23 Storage_Check
Check that evaluation of an allocator does not require
more space than is available for a storage pool. Check
that the space available for a task or subprogram has not
been exceeded.
24 * The following check corresponds to all situations in which any
predefined exception is raised.
25 All_Checks Represents the union of all checks; suppressing All_Checks
suppresses all checks.
Erroneous Execution
26 If a given check has been suppressed, and the corresponding error
situation occurs, the execution of the program is erroneous.
Implementation Permissions
27/2 An implementation is allowed to place restrictions on checking pragmas,
subject only to the requirement that pragma Unsuppress shall allow any check
names supported by pragma Suppress. An implementation is allowed to add
additional check names, with implementation-defined semantics. When
Overflow_Check has been suppressed, an implementation may also suppress an
unspecified subset of the Range_Checks.
27.1/2 An implementation may support an additional parameter on pragma
Unsuppress similar to the one allowed for pragma Suppress (see J.10). The
meaning of such a parameter is implementation-defined.
Implementation Advice
28 The implementation should minimize the code executed for checks that have
been suppressed.
NOTES
29 3 There is no guarantee that a suppressed check is actually removed;
hence a pragma Suppress should be used only for efficiency reasons.
29.1/2 4 It is possible to give both a pragma Suppress and Unsuppress for
the same check immediately within the same declarative_part. In that
case, the last pragma given determines whether or not the check is
suppressed. Similarly, it is possible to resuppress a check which has
been unsuppressed by giving a pragma Suppress in an inner declarative
region.
Examples
30/2 Examples of suppressing and unsuppressing checks:
31/2 pragma Suppress(Index_Check);
pragma Unsuppress(Overflow_Check);
11.6 Exceptions and Optimization
1 This clause gives permission to the implementation to perform certain "
optimizations" that do not necessarily preserve the canonical semantics.
Dynamic Semantics
2 The rest of this International Standard (outside this clause) defines the
canonical semantics of the language. The canonical semantics of a given
(legal) program determines a set of possible external effects that can result
from the execution of the program with given inputs.
3 As explained in 1.1.3, "
Conformity of an Implementation with the Standard", the external effect of a
program is defined in terms of its interactions with its external environment.
Hence, the implementation can perform any internal actions whatsoever, in any
order or in parallel, so long as the external effect of the execution of the
program is one that is allowed by the canonical semantics, or by the rules of
this clause.
Implementation Permissions
4 The following additional permissions are granted to the implementation:
5 * An implementation need not always raise an exception when a
language-defined check fails. Instead, the operation that failed the
check can simply yield an undefined result. The exception need be
raised by the implementation only if, in the absence of raising it,
the value of this undefined result would have some effect on the
external interactions of the program. In determining this, the
implementation shall not presume that an undefined result has a value
that belongs to its subtype, nor even to the base range of its type,
if scalar. Having removed the raise of the exception, the canonical
semantics will in general allow the implementation to omit the code
for the check, and some or all of the operation itself.
6 * If an exception is raised due to the failure of a language-defined
check, then upon reaching the corresponding exception_handler (or the
termination of the task, if none), the external interactions that have
occurred need reflect only that the exception was raised somewhere
within the execution of the sequence_of_statements with the handler
(or the task_body), possibly earlier (or later if the interactions are
independent of the result of the checked operation) than that defined
by the canonical semantics, but not within the execution of some
abort-deferred operation or independent subprogram that does not
dynamically enclose the execution of the construct whose check failed.
An independent subprogram is one that is defined outside the library
unit containing the construct whose check failed, and has no Inline
pragma applied to it. Any assignment that occurred outside of such
abort-deferred operations or independent subprograms can be disrupted
by the raising of the exception, causing the object or its parts to
become abnormal, and certain subsequent uses of the object to be
erroneous, as explained in 13.9.1.
NOTES
7 5 The permissions granted by this clause can have an effect on the
semantics of a program only if the program fails a language-defined
check.
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