In the LED_Demo STM32 example project I have the following code (which compiles in GPS):
procedure Does_Nothing is
begin
Null;
exception
when others =>
Null;
end Does_Nothing;
If I modify the same code to the code below, it does not compile. I get the error "violation of restriction No_Exception_Propagation".
procedure Does_Nothing is
begin
Null;
exception
when Error: others =>
UART.Put(Exception_Information(Error));
end Does_Nothing;
Could someone explain why this is the case?
This is explained in the documentation.
https://gcc.gnu.org/onlinedocs/gnat_rm/No_005fException_005fPropagation.html
(emphasis added)
5.1.28 No_Exception_Propagation
[GNAT] This restriction guarantees that exceptions are never
propagated to an outer subprogram scope. The only case in which an
exception may be raised is when the handler is statically in the same
subprogram, so that the effect of a raise is essentially like a goto
statement. Any other raise statement (implicit or explicit) will be
considered unhandled. Exception handlers are allowed, but may not
contain an exception occurrence identifier (exception choice). In
addition, use of the package GNAT.Current_Exception is not permitted,
and reraise statements (raise with no operand) are not permitted.
The identifier Error violates the restriction.
Related
I am using Ada83 (It's a course requirement to use this version), I am using multiple procedures. I don't know how to come out of the entire program. Some thing like Exit in C program which closes entire procedure. Where is the exit is called from?
If your program does not use tasks, you can define an exception that signifies an "emergency exit"; perhaps in some package:
package Emergency is
Emergency_Exit : exception;
end Emergency;
In your main procedure, catch this exception:
procedure Your_Main_Procedure is
begin
... whatever the main procedure does
exception
when Emergency.Emergency_Exit =>
null;
end Your_Main_Procedure;
So that whenever you raise the exception somewhere in your program:
raise Emergency_Exit;
it will transfer control to the null statement, which will then reach the end of the main procedure and exit the program.
Doing it this way means you can add cleanup code to other procedures:
procedure Something_Else is
...
begin
Direct_IO_Instance.Open (Some_File, Direct_IO_Instance.Out_File, "filename");
begin
... do your work
exception
when Emergency.Emergency_Exit =>
-- cleanup
Finish_Writing (Some_File);
Direct_IO_Instance.Close (Some_File);
-- now reraise, which will eventually reach the end of the program
raise;
end;
end Something_Else;
So when Emergency_Exit is raised, it will eventually transfer control to the end of the main procedure, but it might stop off in other exception handlers along the way to do any needed cleanup.
I don't think this works if there are other tasks running, because the main procedure will wait for those other tasks to complete before the program exits. In that case, in Ada 83, you'll need to coordinate the exit with the other tasks; perhaps you can define a global Boolean that those tasks check periodically to get them to exit, or perhaps you can structure the program in a way so that the Emergency_Exit exception handler knows what tasks to abort, or can call a task entry to get those tasks to terminate. The best solution would depend on the actual program structure.
On this page there is some explanation on how to do it, and what the risks are:
http://rosettacode.org/wiki/Program_termination#Ada
Let's say I have a method which looks like:
commentsInsert: (comment) ->
Comments.insert comment, (err) ->
throw err if err
updateCommentCounts()
The goal here is to avoid calling updateCommentCounts if the insert failed. If this code runs only on the server could I skip the callback and the throw? For example:
commentsInsert: (comment) ->
Comments.insert comment
updateCommentCounts()
The meteor docs say:
On the server, if you don't provide a callback, then insert blocks until the database acknowledges the write, or throws an exception if something went wrong.
I'm assuming this means it will bail out of the function and return an error code to the caller. Is that right?
So to test this I added a unique index to the comments collection and did two identical inserts. The second insert threw an error (as expected), sent a 500 to the client, and bailed out of the method. So it seems as if the answer to my question is: yes, I can skip the callback.
Below, why doesn't Throw maintain my originating line number?
If I run the DerivedPage, my log file lists the error as line 7.
Shouldn't it say line 4? It would make sense if I had Throw ex, but I don't.
Isn't Throw by itself just supposed to rethrow and bubble up the error?
If I remove the Try...Catch block entirely in DerivedPage, then my log file correctly lists 3 as the error line, but I am not able to log any info in case of an error.
What can I do to maintain my DerivedPage, and still have my log keep the correct line number?
Public Class DerivedPage Inherits BasePage
Page_Load(o,e)
Try
Dim a = 3 -"a"
Catch ex As Exception
log.Info(...)
Throw
End Try
End Class
base page:
Public Class BasePage
Protected Overrides Sub OnError(e)
MyBase.OnError(e)
log.Error(Me.GetType(), Server.GetLastError)
End Sub
End Class
EDIT: log.Error does output the InnerException if it exists. It does in this case. However, the stack trace for the InnerException doesn't contain a line number, just the Exception details.
When an exception is rethrown, the original exception details are stored in the InnerException property of the object. You should have all of your details there.
There is a method that you can implement that will give you the correct line number in your stack trace:
Rethrowing exceptions and preserving the full call stack trace - Fabrice's weblog
The code is in C#, but the code for the PreserveStackTrace method should be relatively easy to port over to VB.NET.
SOLUTION: Two solutions per Wrong line number on stack trace are throw a new exception with the current exception as the inner, or use a helper method.
I'm going to go with throwing a new exception.
Java has the finalize block which allows to execute some statements after a block
is left (executed even if an exception is raised). Example:
try {
...
} catch (Exception e) {
...
} finally {
... // any code here
}
Ada has the controlled objects which allows to implement a Finalize operation
but there is no finalize block equivalent as in java. This is useful for logging,
closing files, transactions and so on (without having to create a specific tagged type for each possible block).
How would you implement such finalize block in Ada 2005 (while keeping the code readable)?
Are there plans in Ada 2012 to allow executing any finalization code easily?
I believe this code will do what you ask; it successfully prints out 42 with the present raise or with return. It's an implementation of T.E.D's suggestion.
Tested with GCC 4.5.0 on Mac OS X, Darwin 10.6.0.
with Ada.Finalization;
package Finally is
-- Calls Callee on deletion.
type Caller (Callee : not null access procedure)
is new Ada.Finalization.Limited_Controlled with private;
private
type Caller (Callee : not null access procedure)
is new Ada.Finalization.Limited_Controlled with null record;
procedure Finalize (Object : in out Caller);
end Finally;
package body Finally is
procedure Finalize (Object : in out Caller)
is
begin
Object.Callee.all;
end Finalize;
end Finally;
with Ada.Text_IO; use Ada.Text_IO;
with Finally;
procedure Finally_Demo is
begin
declare
X : Integer := 21;
-- The cleanup procedure, to be executed when this block is left
procedure F
is
begin
Put_Line ("X is " & Integer'Image (X));
end F;
-- The controlled object, whose deletion will execute F
F_Caller : Finally.Caller (F'Access);
begin
X := 42;
raise Constraint_Error;
end;
end Finally_Demo;
As Adrien mentions in the comment, Finalize is more analogous to a destructor.
To get something approximating an exception/final sequence you can do something along these lines (WARNING, not compiled, just typed--we'll work out any errors together :-) See also the Exceptions section of the Ada RM.
with Ada.Exceptions; use Ada.Exceptions;
procedure Do_Something is
-- Variables and what-not...
-- In case you have an exception and want to reraise it after you've done
-- the 'final' processing.
Exception_Caught : Exception_Occurrence := Null_Occurrence;
begin
-- You can have some statements, like initializations, here that will not
-- raise exceptions. But you don't have to, it can all just go in the
-- following block. However you want to do it...
declare
-- If you need to declare some entities local to a block, put those here.
-- If not, just omit this declare section. Be aware, though, that if
-- you initialize something in here and it raises an exception, the
-- block's exception handler will not catch it. Such an exception will
-- propagate out of the whole procedure (unless it has an outermost
-- exception handler) because you're _not_ in the block's scope yet.
begin
-- Main processing that might raise an exception
...
exception
when E : others =>
-- Handle any exception that's raised. If there are specific
-- exceptions that can be raised, they should be explicitly
-- handled prior to this catch-all 'others' one.
-- Save the exception occurrence, i.e. make a copy of it that can
-- be reraised in the 'Final' section if needed. (If you want to
-- reraise for a specific exception, do this in those handlers as
-- well.
Save_Occurrence(Exception_Caught, E);
end;
-- Final processing. Everything from here to the end of the procedure is
-- executed regardless of whether an exception was raised in the above
-- block. By it including an others handler, it ensured that no exception
-- will propagate out of this procedure without hitting this 'Final' code.
-- If an exception was raised and needs to be propagated:
if Exception_Caught /= Null_Occurrence then
Reraise_Exception(Exception_Caught);
end if;
end Do_Something;
Assuming you have understood the difference between ada.finalization and a finalize block in java, i would do something similar to the following, which should have the same effect.
procedure x is
begin
-- some code
begin
-- more code (your try)
exception
-- handle exception if necessary (caught exception)
end;
-- yet more code which is executed regardless of any exception handling.
end x;
Marc C has the right approach for trying to emulate that in straight-line procedural code.
However, IMHO that structure is mostly a way to hack around Java's OO system, for those who want one of the structural benifits of OO in old-fashioned procedural programming. Even in Java you are almost always better off creating a proper class instead.
So I don't think it is too much of a stretch to say that the proper way to get that functionality in Ada would be to make a proper object, and make your object a child of Ada.Finalization.Controlled.
If you don't want to bother with creating an actual object, you could just create a dummy one, put your finalization code in it, and declare it on the stack at the top of the block you want it run for. The drawback to that is that controlled types themselves (as least the last time I used them) have to be declared at package-level scope. When that's the case, you'd be unable to put direct references to lower-declared objects in them. They claimed they were going to fix that in future language revision, but I haven't tried it recently to see if they did.
Just thought of another answer. Its a bit heavy (and perhaps more trouble than it is worth). But it would give you something that looks a bit like your old finalize block
The idea would be to put your "finalizable" code in a task. You cannot leave the scope a task is declared in until the task terminates. So you could put your work code in the task and your "finally" code just outside of the scope the task is defined in. The parent task would sit there and wait for the work task to end (one way or another), and then it would run the "finally" code no matter how it ended. The drawback is that if the task throws an exception, it will stop at the task. So you still don't quite get the behavior that you can throw an exception and it will propagate out automatically while the "finalize" code gets run. You could maybe get that behavior back by adding a rendezvous and a second task (that's the problem with tasks. They are like potato chips...you always need one more).
procedure Finalized is
begin
declare
task Worker is end Worker;
task body Worker is begin
--// Working code in here. Can throw exceptions or whatever.
--// Does not matter.
end Worker;
begin
end;
--// If we get here, we know the task finished somehow (for good or ill)
--// so the finalization code goes here.
end Finalized;
It seems to me there might be a way to do something like this with protected objects too. I'll leave that one for others to figure out.
Lets put this problem into perspective.
In programming theory there exists concepts of an object's create & destroy and a procedure's try & finally. Both ultimately deal with resource management but they pivot on different things.
With objects we pivot on the potentially long-living tree of objects and variables referenced by the object pointer.
With procedures we pivot on the usually temporary objects and variables existing in the scope of the procedure call (the notable exception is main)
Now, depending on the procedure we might need to spawn a series of resources which, if the procedure is interrupted by a fatal error, must rollback all spawned resources in reverse order. Quite often this is easiest achieved by creating objects dedicated to managing their respective resource. Ada.Finalization becomes quite useful here.
But this can be overkill, and there can be arguments made against this technique on a case-by-case basis. So if we are interested in self-containing resource management to a procedure and making use of a C++-style finally keyword, consider the following.
I have often initially been pleased by the promise of convenience using finally only to be disappointed by how complicated the code can turn out after making use of it. Nesting is required for complex rollback operations, and many times the process is not linear, requiring logic to decide exactly how to roll back depending on how far we made it through the initialization. The nested block structure corners you to use linear logic. Thus when you need something more complicated then you are seriously considering making use of goto.
I've gone through this enough times to realize that, as appetizing as OOP-style finalize can be with what I mentioned, a true try & finally can be achieved in Ada without all the headaches as demonstrated in the following example. Note, it's far from perfect, but I think the good outweighs the bad with this strategy. What I especially like about this strategy is how explicit the finalize process becomes, all steps labeled and checked against Ada's type system, well organized and easy to manage. The C++-style try & finally scatters this kind of code, and Ada's Finalize hide's it and makes it harder to control the higher-level order of fail-over logic.
procedure Proc is
type Init_Stages_All is (Do_Null, Do_Place, Do_Pour, Do_Drink);
subtype Init_Stages is Init_Stages_All range Do_Place .. Do_Drink;
Init_Stage : Init_Stages_All := Do_Null;
procedure Initialize_Next is
begin
Init_Stage := Init_Stages_All'Succ(Init_Stage);
case Init_Stage is
when Do_Place => ...
when Do_Pour => ...
when Do_Drink => ...
when Do_Null => null;
end case;
end Initialize_Next;
procedure Finally is
begin
for Deinit_Stage in reverse Init_Stage .. Init_Stages'Last loop
case Deinit_Stage is
when Do_Place => ...
when Do_Pour => ...
when Do_Drink => ...
end case;
end loop;
end Finally;
begin
Initialize_Next; -- Do_Place
...
Initialize_Next; -- Do_Pour
...
Initialize_Next; -- Do_Drink
...
Finally;
exception
when E : others =>
...
Finally;
raise;
end Proc;
On a final note, this strategy also makes it easier to deal with finalization exceptions. I'd suggest creating a procedure in Finally that is called when an exception is raised and recursively call Finally by manipulating the Init_Stage as well as interject any additional fail-over logic there.
Object reference not set to an instance of an object
I am getting this error sometimes.
I have a page where I load questions and for some question system generate this error. Why do I get this error? Please help me.
It means that somewhere something dereferences a null reference. Crude example:
Foobar foo = null;
foo.DoSomething(); // this line will trigger a NullReferenceException
Here's what you can do:
compile in debug mode
run it until it crashes
examine the stack trace; it will tell you where the exception originates (unless you're doing stupid things like catching and rethrowing improperly). You may have to follow the chain of inner exceptions
if that doesn't provide sufficient information, step through your code with a debugger until right before the call that makes it crash. Examine any variables that might be null.
Once you have found the culprit, you need to find out why it is null and what to do about it. The solution can be one of:
fix incorrect program logic that causes something to be null that shouldn't be
check if something is null before dereferencing it
handle the exception at a point that makes sense, and translate it into something more meaningful (probably some sort of custom exception)
You got null reference and if it can't handle it will show that message.
It can caused by some case
The return value of a method is null
A local variable or member field is declared but not initialized
An object in a collection or array is null
An object is not created because of a condition
An object passed by reference to a method is set to null
I suggest you to track the null value using a debug mode
You get this error, whenever you are trying to make use of variable with Null value in it.