I get the following error in XCode...
I've added a "Swift Error" breakpoint and left the "Type" box empty. I thought this might show some further info, but it doesn't appear to have any effect.
How can I find out where this error is arising from?
EXC_BAD_INSTRUCTION means that you have an invalid assertion (usually a force-unwrapped nil, though a bad cast could also be the culprit, here). Make sure that tourDto is a populated var, and that it can be cast to whatever toJSONString() returns (I'm not familiar with that method, and it could also be the culprit).
I was accessing REST calls, when I passed wrong parameter to GET request it does not throw any http error. Should the design be changed to throw a http error or wrong parameter can be passed to REST call.
Example 1:(parameters are optional)
https://example.com/api/fruits?fruit=apple
Give list of all apple elements
Example 2:
https://example.com/api/fruits?abc=asb
Give list of all fruits
My question is related to example 2, should example 2 throw an error or is it behaving properly?
It's pretty common to ignore parameters that you aren't necessarily expecting. I think example 2 is behaving as it should.
I know that depending on the browser I would sometimes append an extra variable with a timestamp to make sure that the rest call wouldn't be cached. Something like:
https://example.com/api/fruits?ihateie=2342342342
If you're not explicitly doing anything with the extra parameter then I can't see the harm in allowing it.
For a GET request, the request-line is defined as follows
request-line = 'GET' SP request-target SP HTTP-version CRLF
where request-target "...identifies the target resource upon which to apply the request".
That means that the path /api/fruits, the question-mark ? and the query abc=asb are all part of the identifier.
The fact that your implementation happens to use the path to route the request to a handler, and the query to provide arguments, is an accident of your current implementation.
That leaves you with the freedom to decide that
/api/fruits?abc=asb does exist, and its current state is a list of all fruits
/api/fruits?abc=asb does exist, and its current state is an empty list
/api/fruits?abc=asb does exist, and its current state is something else
/api/fruits?abc=asb does not exist, and attempting to access its current state is an error.
My question is related to example 2, should example 2 throw an error or is it behaving properly?
If abc=asb indicates that there is some sort of error in the client, then you should return a 4xx status to indicate that.
Another way of thinking about the parameter handling is in terms of Must Ignore vs Must Understand.
As a practical matter, if I'm a consumer expecting that my filter is going to result in a small result set, and instead I end up drinking a billion unfiltered records out of a fire hose, I'm not going to be happy.
I'd recommend that in the case of a bad input you find a way to fail safely. On the web, that would probably mean a 404, with an HTML representation explaining the problem, enumerating recognized filters, maybe including a web form that helps resend the query, etc. Translate that into your API in whatever way makes sense.
But choosing to treat that as a successful request and return some representation also works, it's still REST, the web is going to web. If doing it that way gives you consumers a better experience, thereby increasing adoption and making your api more successful, then the answer is easy.
I have a script that performs a bunch of experiments on different datasets, below is part of the logic to determine whether a dataset needs to be loaded. Basically we load the dataset if there is none already loaded, or if the currently loaded one doesn't match the one we need by name.
This example crashes with the error The class dataset has no property or method named 'name' at the if statement (the class does in fact have this property):
if(~exist('dataset','var')||~strcmp(dataset.name,datasets{datai}.id))
loaded=load(datasets{datai}.filename);
dataset=loaded.dataset;
end
If I debug and stop at the line, I can access dataset.name in the debugger without performing any further actions. I don't think the reason is the dataset object not existing. In the loop I ran, the first dataset was correctly loaded, but the second one (where the name check comes into play) wasn't.
This rewriting works:
if(~exist('dataset','var')||~strcmp(nom,datasets{datai}.id))
loaded=load(datasets{datai}.filename);
dataset=loaded.dataset;
nom=dataset.name;
end
Why was I able to access dataset.name in the debugger, and why does the rewriting fix the issue?
I have 2 pieces of code :
It works normal
with ADOTemp do
begin
SQL.Clear;
SQL.Add('INSERT INTO documents');
SQL.Add('(document_date,fk_id_status,money_direction,');
SQL.Add('paid,addition,saving,fk_id_base,fk_id_user)');
SQL.Add('VALUES ');
SQL.Add('(CONVERT(DATE,GETDATE(),103),:pfk_id_status,:pmoney_direction,');
SQL.Add('0,0,0,'+IntToStr(p_id_base)+',:pfk_id_user)');
Parameters.ParamByName('pfk_id_status').Value := p_id_status;
Parameters.ParamByName('pmoney_direction').Value := p_money_direction;
// Parameters.ParamByName('p').Value := p_id_base;
Parameters.ParamByName('pfk_id_user').Value := fMain.ApplicationVariablers.user_id;
ExecSQL;
end;
It does not work and returns a terrible error
with ADOTemp do
begin
SQL.Clear;
SQL.Add('INSERT INTO documents');
SQL.Add('(document_date,fk_id_status,money_direction,');
SQL.Add('paid,addition,saving,fk_id_base,fk_id_user)');
SQL.Add('VALUES ');
SQL.Add('(CONVERT(DATE,GETDATE(),103),:pfk_id_status,:pmoney_direction,');
SQL.Add('0,0,0,:p,:pfk_id_user)');
Parameters.ParamByName('pfk_id_status').Value := p_id_status;
Parameters.ParamByName('pmoney_direction').Value := p_money_direction;
Parameters.ParamByName('p').Value := p_id_base;
Parameters.ParamByName('pfk_id_user').Value := fMain.ApplicationVariablers.user_id;
ExecSQL;
end;
Error on image, line 1917
It seems that this question is in reference to your earlier question: Delphi, error :"Access violation at address xxxxxxxx. Read of address yyyyyyyy", at AdoQuery.SQL.Text:='''
You had added and accepted an answer there along the lines seen in this question. The error message, an AV in msvcrt.dll is indicative of a serious programming error. Perhaps a heap corruption. Perhaps something else. The change you made that stops the error occurring does not really fix the problem. The problem will still be there, lying dormant. You just got (un)lucky that the change you made appeared to fix the problem.
Access violations are not always reproducible. That's just their nature. When you encounter one you need to understand why it happens. Simply using trial and error to re-organise code from a completely different location will never lead to the real solution.
What you should be doing is tracking down the real cause of the problem. Do that, fix it, and either version of the code above will work. This is probably not the answer you are wanting to get. You may very well not want to accept what I say. But, speaking from experience, until you recognise that you have a more serious problem, you will make no headway here.
Looking at your code in the bitmap...I'm pretty sure you access violation is because of what David just said...you have serious Memory problems in your code...Your not freeing your objects...that you are creating...Anytime you pass a nil reference to Owner in a TComponent Constructor...your telling the compiler...that you know what your doing and will free it when your finished with it...Matter of fact good practice is to always free objects that you declare and use exclusively in the scope of your method.
procedure TForm1.MyMethod;
var
a_MyComp: TMyComp;
begin
a_MyComp := TMyComp.Create(nil);
Try
//use my a_MyComp...
Finally
a_MyComp.Free;
End;
end;
Check out your code in CreateNewDocument...you'll notice that your not freeing your AdoTemp.
-Rick
Access violation exceptions (AVs) tell you that you have made a mistake in the memory access of your program. However, the way access violations work: they are unable to guarantee always detecting the error every time you make a mistake. (Sometimes you get un-lucky and no access violations are raised, but the mistake is still there causing other things to quietly go wrong inside your application.)
You can think of the memory available to your program being represented as below (where "." means the memory is not allocated to anything, and "A" means the memory is allocated to something within your program: e.g. object, local variable, parameter, machine code).
[.......AAA..AAAA.....AA......A...A...........AAA....A.......AA.........AAAAA]
Suppose you create some object; this will require memory to be allocated for the object itself. If the object in turn creates child objects, this will also be allocated in memory. (I'll use "O" and "C")
/-ref--\
[.......AAA..AAAAO....AAC.....A...A.....C.....AAA....A.......AA.........AAAAA]
\-ref------------------/
Note that within the memory allocated to O, it might hold references to its child objects.
Conversely to the above, whenever an object is destroyed, its memory is deallocated. Let us suppose you have made a mistake in your memory access, and something has destroyed one of O's child objects before O has finished using it.
/-ref--\
[.......AAA..AAAAO....AAC.....A...A...........AAA....A.......AA.........AAAAA]
\-ref------------------/
If O now tries to use its second child object, you will get an access violation. However, you might be un-lucky and not get an access violation showing your earlier mistake IF:
You destroy O without it trying to do anything to the second child object.
Or you first create a new object that happens to be allocated in the exact same place the child object was.
The second situation tends to be worse than the first, because every time C2 is used on the assumption that it is the correct child of O: unexpected results are produced and an incorrect values are written in memory. These incorrect values may be important data, or references to other objects (making the problem get worse over time).
So: Whenever yo do get an access violation, thank your lucky stars and hunt down the root cause of the problem.
NB! NB! I cannot stress the importance of the above enough.
Investigating your particular problem
First note, when an exception pauses in the debugger, the code usually points to the next line that would have been executed if not for the exception. (This doesn't affect anything here, because the previous line is also ADOTemp.SQL.Add; - the point is be prepared to consider the previous line as the cause of an exception - and test to confirm it!)
Useful tip: When an exception is thrown within Delphi/Third Party code, it can be useful to build with Debug DCU's or recompile Third Party source with debug information to get closer to the actual line raising the exception. (This can even be useful in situations like this where the error appears to be inside a Microsoft DLL.)
Looking at your screen-shot, the exception is thrown from a brand new instance of TADOQuery. Now there really isn't any sensible explanation for a brand new query to be throwing access violations when simply adding text to its SQL query. This strongly implies a corruption problem as described earlier.
One possibility would be another thread interfering with this one. (NOTE: even if your application isn't multi-threaded, the ADO objects do have built-in support for asynchronous operations.) However, I'm going to ignore that possibility for now because threading issues tend to be less consistent than you've implied this one is.
So, assuming you have a memory access problem somewhere else that is only manifesting here with a brand new TADOQuery (used correctly at least up to the point of the AV) - what other objects are interacting with this query to possibly cause corruption?
ADOTemp.Connection := fMain.ADOConnection;
There is a very strong possibility that commenting out the above line would also eliminate your access violation. So what are the possible problems:
fMain might have been destroyed prematurely and is now a dangling pointer meaning the code to return ADOConnection could do any of a number of unexpected things.
Any of the code backing fMain.ADOConnection may be referencing corrupted memory or dangling pointers; this includes the returned connection itself.
NOTE: One very common cause of invalid memory access is to have a function such as GetADOConnection that does not correctly initialise its Result. So in some cases it returns a 'random' address in memory, resulting in all sorts of unexpected behaviour when something tries to use the connection.
Is your ADOConnection created on a different thread, and being used by multiple threads?
PS: Don't forget to follow Rick's advice, and make sure you destroy the query when you've finished using it. Who knows, there might be an internal bug in ADO that when it runs out of a particular internal resource (due to queries not being destroyed), that causes ADO to start throwing AVs.
NOTE: You should be able to test my theory without altering functionality by simply changing the ADOTemp.Connection := ... line. Simply assign a connection string to the query instead of a connection object. However, if that solves (or more correctly stated: hides) the problem, please follow my advice, and hunt down the root cause of the AV.
I am not able to understand the places where an error handling or where an exception handling should be used. I assume this, if it is an existing framework class there are delegate methods which will facilitate the programmer to send an error object reference and handle the error after that. Exception handling is for cases where an operation of a programmer using some framework classes throws an error and i cannot get an fix on the error object's reference.
Is this assumption valid ? or how should i understand them ?
You should use exceptions for errors that would never appear if the programmer would have checked the parameters to the method that throws the exception. E.g. divide by 0 or the well known "out of bounds"-exception you get from NSArrays.
NSErrors are for errors that the programmer could do nothing about. E.g. parsing a plist file. It would be a waste of resources if the program would check if the file is a valid plist before it tries to read its content. For the validity check the program must parse the whole file. And parsing a file to report that it is valid so you can parse it again would be a total waste. So the method returns a NSError (or just nil, which tells you that something went wrong) if the file can't be parsed.
The parsing for validity is the "programmer should have checked the parameters" part. It's not applicable for this type of errors, so you don't throw a exception.
In theory you could replace the out of bounds exception with a return nil. But this would lead to very bad programming.
Apple says:
Important: In many environments, use of exceptions is fairly commonplace. For example, you might throw an exception to signal that a routine could not execute normally—such as when a file is missing or data could not be parsed correctly. Exceptions are resource-intensive in Objective-C. You should not use exceptions for general flow-control, or simply to signify errors. Instead you should use the return value of a method or function to indicate that an error has occurred, and provide information about the problem in an error object.
I think you are absolutely right with your assumption for Errors and for it framework provide a set of methods (UIWebView error handling ), But your assumption for Exception partially right because the exception only occurred if we do something wrong which is not allowed by the framework and can be fixed. (for example accessing a member from an array beyond its limit).
and will result in application crash.