11.4.1 The Package Exceptions

Ramification: In a distributed program, the identity is unique across an entire program, not just across a single partition. Exception propagation works properly across RPC's. An exception can be propagated from one partition to another, and then back to the first, where its identity is known. 

Implementation defined: The information returned by Exception_Message.

Ramification: Given an exception E, the raise_statement:

is equivalent to this call to Raise_Exception: 

{AI95-00361-01} Similarly, the raise_statement:

is equivalent to this call to Raise_Exception: 

Ramification: The following handler: 

is equivalent to this one: 

Ramification: See the Implementation Permission below. 

To be honest: This name, as well as each prefix of it, does not denote a renaming_declaration.

Implementation defined: The result of Exceptions.Wide_Wide_Exception_Name for exceptions declared within an unnamed block_statement.

Ramification: Note that we're talking about the name of the exception, not the name of the occurrence. 

Implementation defined: The sequence of characters of the value returned by Exceptions.Exception_Name (respectively, Exceptions.Wide_Exception_Name) when some of the graphic characters of Exceptions.Wide_Wide_Exception_Name are not defined in Character (respectively, Wide_Character).

Implementation defined: The information returned by Exception_Information.

Ramification: {AI95-00241-01} Null_Occurrence can be tested for by comparing Exception_Identity(Occurrence) to Null_Id. 

Discussion: {AI95-00446-01} Raise_Exception was changed so that it always raises an exception and thus can be a No_Return procedure. A similar change was not made for Reraise_Occurrence, as doing so was determined to be a significant incompatibility. It is not unusual to pass an Exception_Occurrence to other code to delay raising it. If there was no exception, passing Null_Occurrence works fine (nothing is raised). Moreover, as there is no test for Null_Occurrence in Ada 95, this is the only way to write such code without using additional flags. Breaking this sort of code is unacceptable. 

Ramification: It's OK to pass Null_Occurrence to the Save_Occurrence subprograms; they don't raise an exception, but simply save the Null_Occurrence. 

Ramification: This routines are used to define the stream attributes (see 13.13.2) for Exception_Occurrence.

The identity of the exception, as well as the Exception_Name and Exception_Message, have to be preserved across partitions.

The string returned by Exception_Name or Exception_Message on the result of calling the Read attribute on a given stream has to be the same as the value returned by calling the corresponding function on the exception occurrence that was written into the stream with the Write attribute. The string returned by Exception_Information need not be the same, since it is implementation defined anyway. 

Reason: This is important for supporting writing exception occurrences to external files for post-mortem analysis, as well as propagating exceptions across remote subprogram calls in a distributed system (see E.4). 

Reason: The reason for allowing truncation is to ease implementations. The reason for choosing the number 200 is that this is the minimum source line length that implementations have to support, and this feature seems vaguely related since it's usually a “one-liner”. Note that an implementation is allowed to do this truncation even if it supports arbitrarily long lines. 

Implementation Advice: Exception_Information should provide information useful for debugging, and should include the Exception_Name and Exception_Message.

Implementation Advice: Exception_Message by default should be short, provide information useful for debugging, and should not include the Exception_Name.

Reason: It may seem strange to define two subprograms whose semantics is implementation defined. The idea is that a program can print out debugging/error-logging information in a portable way. The program is portable in the sense that it will work in any implementation; it might print out different information, but the presumption is that the information printed out is appropriate for debugging/error analysis on that system. 

Implementation Note: As an example, Exception_Information might include information identifying the location where the exception occurred, and, for predefined exceptions, the specific kind of language-defined check that failed. There is an implementation trade-off here, between how much information is represented in an Exception_Occurrence, and how much can be passed through a re-raise.

The string returned should be in a form suitable for printing to an error log file. This means that it might need to contain line-termination control characters with implementation-defined I/O semantics. The string should neither start nor end with a newline.

If an implementation chooses to provide additional functionality related to exceptions and their occurrences, it should do so by providing one or more children of Ada.Exceptions.

Note that exceptions behave as if declared at library level; there is no “natural scope” for an exception; an exception always exists. Hence, there is no harm in saving an exception occurrence in a data structure, and reraising it later. The reraise has to occur as part of the same program execution, so saving an exception occurrence in a file, reading it back in from a different program execution, and then reraising it is not required to work. This is similar to I/O of access types. Note that it is possible to use RPC to propagate exceptions across partitions.

Here's one way to implement Exception_Occurrence in the private part of the package. Using this method, an implementation need store only the actual number of characters in exception messages. If the user always uses small messages, then exception occurrences can be small. If the user never uses messages, then exception occurrences can be smaller still: 

At the point where an exception is raised, an Exception_Occurrence can be allocated on the stack with exactly the right amount of space for the message — none for an empty message. This is just like declaring a constrained object of the type: 

After finding the appropriate handler, the stack can be cut back, and the Temp copied to the right place. This is similar to returning an unknown-sized object from a function. It is not necessary to allocate the maximum possible size for every Exception_Occurrence. If, however, the user declares an Exception_Occurrence object, the discriminant will be permanently set to 200. The Save_Occurrence procedure would then truncate the Exception_Message. Thus, nothing is lost until the user tries to save the occurrence. If the user is willing to pay the cost of heap allocation, the Save_Occurrence function can be used instead.

Note that any arbitrary-sized implementation-defined Exception_Information can be handled in a similar way. For example, if the Exception_Occurrence includes a stack traceback, a discriminant can control the number of stack frames stored. The traceback would be truncated or entirely deleted by the Save_Occurrence procedure — as the implementation sees fit.

If the internal representation involves pointers to data structures that might disappear, it would behoove the implementation to implement it as a controlled type, so that assignment can either copy the data structures or else null out the pointers. Alternatively, if the data structures being pointed at are in a task control block, the implementation could keep a unique sequence number for each task, so it could tell when a task's data structures no longer exist.

Using the above method, heap space is never allocated unless the user calls the Save_Occurrence function.

An alternative implementation would be to store the message strings on the heap when the exception is raised. (It could be the global heap, or it could be a special heap just for this purpose — it doesn't matter.) This representation would be used only for choice parameters. For normal user-defined exception occurrences, the Save_Occurrence procedure would copy the message string into the occurrence itself, truncating as necessary. Thus, in this implementation, Exception_Occurrence would be implemented as a variant record: 

Exception_Occurrences created by the run-time system during exception raising would be As_Choice_Param. User-declared ones would be Normal — the user cannot see the discriminant, and so cannot set it to As_Choice_Param. The strings in the heap would be freed upon completion of the handler.

This alternative implementation corresponds to a heap-based implementation of functions returning unknown-sized results.

One possible implementation of Reraise_Occurrence is as follows: 

However, some implementations may wish to retain more information across a re-raise — a stack traceback, for example.

Ramification: Note that Exception_Occurrence is a definite subtype. Hence, values of type Exception_Occurrence may be written to an error log for later analysis, or may be passed to subprograms for immediate error analysis. 

This paragraph was deleted.{AI95-00400-01}

{extensions to Ada 83} The Identity attribute of exceptions is new, as is the package Exceptions.

{AI95-00241-01} {inconsistencies with Ada 95} Amendment Correction: Exception_Identity of an Exception_Occurrence now is defined to return Null_Id for Null_Occurrence, rather than raising Constraint_Error. This provides a simple way to test for Null_Occurrence. We expect that programs that need Constraint_Error raised will be very rare; they can be easily fixed by explicitly testing for Null_Id or by using Exception_Name instead.

{AI95-00378-01} {AI95-00417-01} Amendment Correction: We now define the lower bound of the string returned from [[Wide_]Wide_]Exception_Name, Exception_Message, and Exception_Information. This makes working with the returned string easier, and is consistent with many other string-returning functions in Ada. This is technically an inconsistency; if a program depended on some other lower bound for the string returned from one of these functions, it could fail when compiled with Ada 2005. Such code is not portable even between Ada 95 implementations, so it should be very rare.

{AI95-00446-01} Amendment Correction: Raise_Exception now raises Constraint_Error if passed Null_Id. This means that it always raises an exception, and thus we can apply pragma No_Return to it. We expect that programs that call Raise_Exception with Null_Id will be rare, and programs that do that and expect no exception to be raised will be rarer; such programs can be easily fixed by explicitly testing for Null_Id before calling Raise_Exception. 

{AI95-00400-01} {AI95-00438-01} {incompatibilities with Ada 95} Functions Wide_Exception_Name and Wide_Wide_Exception_Name, and procedures Read_Exception_Occurrence and Write_Exception_Occurrence are newly added to Exceptions. If Exceptions is referenced in a use_clause, and an entity E with the same defining_identifier as a new entity in Exceptions is defined in a package that is also referenced in a use_clause, the entity E may no longer be use-visible, resulting in errors. This should be rare and is easily fixed if it does occur. 

{AI95-00362-01} {extensions to Ada 95} The package Exceptions is preelaborated, and types Exception_Id and Exception_Occurrence have preelaborable initialization, allowing this package to be used in preelaborated units. 

{AI95-00361-01} The meaning of Exception_Message is reworded to reflect that the string can come from a raise_statement as well as a call of Raise_Exception.

{AI95-00400-01} Added Wide_Exception_Name and Wide_Wide_Exception_Name because identifiers can now contain characters outside of Latin-1.