I was told in a comment in another thread that I should be using exceptions whenever anything out of the ordinary happens, even if it isn't fatal to the script. This came up as I was using constructs akin to the following:
return err("File could not be loaded");
which would print the error to the screen, and return false, terminating the instruction processing. It was suggested that this would be better handled with exceptions.
The trouble is that the program is, for all intents and purposes, a language interpreter, controlled through a console, which means that any time a command is input incorrectly, or there is a bug in the interpreted code, an error needs to be displayed.
Besides the fact that these issues seem to minor to be processed as exceptions, how should it be implemented? How can a try block be used to control processing paths? For example, currently my code looks as follows:
if(!validate(code))
return false; //the validate function already having output the error
else
process(code);
How should I ensure that process(code) only executes if validate(code) succeeds? Should I just return false; from the function in the catch block? This would seem to return to the original issue of using return values to handle exceptional events. It seems to me that the fundamental problem is that the issues aren't exceptions at all, but I defer to those with more experience than I.
If an operation may - by design - either succeed or fail, and both of those are common, it might be most clear to structure your command flow in such way that errors are checked "explicitly", like in boolean value of "validate" function.
Exceptions come in handy when you don't want to disturb your regular control flow with error checking and want to move the checking somewhere else, maybe some function call levels above.
Your situation sounds like you don't really need exceptions. If your code looks clean without them, stay with it.
The idea of exceptions is you wouldn't need something like a separate "validate" step. Whenever "process" gets to a point where something's broken, throw an exception.
If you did need a separate validation step for some reason, then it'd look like
validate(code);
process(code);
Validate would throw an exception on failure, in which case process would never be reached.
try
{
validate(code);
process(code);
}
catch(...)
{
return false;
}
return true;
Assuming that validate throws, process won't happen.
Maybe you want your top-level loop to look something like this:
while (!quit) {
try {
process_command_line_input();
}
catch (const std::exception& ex) {
std::cerr << "Error: " << ex.what() << std::endl;
}
}
So, process_command_line_input() gets the next line of input and does whatever it does. If any error is detected, an exception is thrown, the top level loop displays it, and then goes on to the next input line.
The issue is that you have your function set up to use a return code. You can't just drag and drop exceptions in when your code is already set up with return values.
The correct way to do it is like
std::string code;
// input into code
try {
validate(code);
process(code); // Throws an exception.
}
catch(std::runtime_error& except) {
std::cout << except.what();
// recover from here.
}
There is no one correct answer.
It is highly dependent on the code and the situation:
In your situation the simple code above (if it is withing my own class) then error codes are fine:
class MyClass
{
public:
void processesValidCodesOrIgnore(int code)
{
if (validate(code))
{
processesCode(code);
}
}
private:
bool validate(int);
void processesCode(int);
};
As the validate method is private I know its result will always be checked and not ignored so its very easy to just use error codes in this situation.
On the other hand if validate was public I would definately consider using exceptions (depending on usage). As this would force the caller to actively check for problems rather than silently ignoring them.
Related
I am currently trying to write a function for a specific type of hardware exception handling.
The program should test, whether a certain condition is true and start a short emergency procedure and finally terminate.
This test will have to be done quite often and should be rather quick.
Further, it is implemented in a project without any further exception handling.
Therefore, putting try statements everywhere is not really an option.
Ideally, such a function would look like this:
void CheckForException(){
if (Condition == true){
cout << "The chosen configuration is very dangerous, do something different!" << endl;
someHardwareFunction();
someStatementEndingTheProgram;
}
}
In principle, I am looking for someStatementEndingTheProgram.
I figured, end(), terminate() and the like are bad style.
Could a throw statement without a try work in this case?
Not answering the title, but the actual problem description: even if putting try's everywhere is not an option, you can wrap the complete code of the main() function in a try block and catch std::exception, so when CheckForException will throw, it will be caught there, so displaying a message and exiting the application can be done easily.
The downside to this approach is that there can be try blocks in your program later on that could catch this exception, so it should be rethrown.
This reeks of bad design in your app.
If you want to prevent the overhead of try-catch but still handle exceptions/error-conditions in a robust way, the code must cater for it: in effect replacing the exception-handling with a custom optimized handler.
In your example you have two basic choices:
if the someStatementEndingTheProgram needs to do cleanup you can call exit() at the exit-point of that function, otherwise just replace it with exit().
redesign your app to handle CheckForException() returning a status, and check that status higher-up the call-chain, e.g.:
int main()
{
bool okay=true;
while (okay)
{
okay=CheckForException();
//other code
}
if (okay) return 0; else return ERROR_CODE;
}
which will let your app terminate normally.
Does anyone know a opensource c++ application with a well designed/robust exception mechanism so I can get some inspiration? Most code/examples I see do questionable things like:
Throw objects with a message string as argument. Seems wrong because it marks the exception as fatal, a error message wich can be displayed to the user higher up leaves little room for client code trying to handle the exception. Even if the exception is fatal, things like diffirent locales (languages) makes formatting a message at the point of throw seem like a bad idea to me.
Use tons of different exception classes derived from a base exception class. Just feels wrong introducing a new class/type for every single thing that can go wrong (open file, read file, write file, create thread, etc...). Catching all unhandled exceptions at the highest level using the base type loses type information required to display meaningfull error messages.
Use one exception class derived from a base exception class for every component/library and give it a error code as argument to indicate the exact error. Catching with base type results in ambiguity. (Whose error code "3" did we catch? )...
Some pointers in the right direction would be welcome.
To address all three of these, I found that the best for me is to throw my own custom exception that's derived from std::runtime_error. Like this:
#include <exception>
class ChrisAException : public std::runtime_error
{
/// constructor only which passes message to base class
ChrisAException(std::string msg)
: std::runtime_error(msg)
{
}
}
It allows accepting a string, which I always put in something like the following format (assuming x negative was not a valid input and meant something calling it was in error):
#include "ChrisAException.h"
void myfunction(int x)
{
if(x < 0)
{
throw ChrisAException("myfunction(): input was negative!");
}
// rest of your function
}
For this one, keep in mind that the strings in these exceptions are more for the programmer than the end user. It's the programmer of the interface's job to display something meaningful in the locale when there's a failure. The strings in the exception can either be logged or seen at debug time (preferable!)
This way you can ultimately catch it with:
try
{
// high level code ultimately calling myfunction
}
catch(ChrisAException &cae)
{
// then log cae.what()
}
catch(std::runtime_error &e)
{
// this will also catch ChrisAException's if the above block wasn't there
}
catch(...)
{
// caught something unknown
}
I personally don't like deriving too many types of exceptions, or giving an error code. I let the string message do the reporting.
In general, I use C++ exceptions to mean "something went wrong with the program" and not to handle normal use cases. Thus, for me, a thrown exception during the algorithm execution either means "flag the user that something went wrong" or "don't tell the user" (depending on how critical that code was to what they were doing) but certainly log it and let the programmers know somehow.
I don't use C++ exceptions to handle cases that aren't essentially programming errors, e.g., some kind of incorrect logic or something being called wrong. For example, I wouldn't use C++ exceptions to handle normal program situations like an empty DVD not being in the drive for a DVD writing program. For that, I'd have explicit return code that allows the user to know whether an empty DVD was there (maybe with a dialog etc.)
Keep in mind that part of C++ exception handling is to unwind the stack up to a try-catch block. To me that means, abort what's going on in the program and clean up the stack. In the case of something like my DVD example, you shouldn't really want to unwind much of the stack. It wasn't catastrophic. You should simply let the user know and then let them try again.
But again, this is my preferred way of using C++ exceptions, based on experience and my reading. I'm open to opinions otherwise.
edit: Changed std::exception to std::runtime_error based on commenter advice.
I have this program written in C++ Builder 6. I didn't write all the code, just some of it. The language, however, is not C++ (as far as I'm aware) - it looks more like Delphi or Pascal. So that's why I included them all in the tags.
I have an int called Oversteering.
try
{
Oversteering=HoursCounter.ToInt();
}
catch(EConvertError &convertError)
{
Oversteering=0;
}
HoursCounter is an AnsiString, and it is in the form of an int.
Since this is the only try/catch statement in the whole code (that's not too good, I know), and I couldn't find any good example of such in Delphi/Pascal/???, I don't know if it's correctly written.
Well, I try to convert the string to an int. Sometimes I get this error:
That is, an exception called EConvertError has occurred.
So my question is: why is this exception NOT caught by the catch statement?
This error is shown by the debugger when running through the code,
if you run the exe and have the same situation the error message will not be shown to you
The exception is caught but the debugger is notifiying you regarding the error in the code
that is here
try
{
Oversteering=HoursCounter.ToInt();
}
since running in the debugger the ,your trying to convert (blankspace) '' to integer, the debugger will show the exception...but when running the exe, the debugger will set
Oversteering=0
check this from about.com
Break On Exceptions
When building a program with exception handling, you may not want Delphi to break on Exceptions. This is a great feature if you want Delphi to show where an exception has occurred; however, it can be annoying when you test your own exception handling.
As #PresleyDias explained, it is the debugger that is displaying the exception, not your app. The exception is being caught (you should be catching it by a const reference, though), but the debugger sees it before your app does, that's all. You can configure the debugger to ignore EConvertError, if you like.
A better solution is to avoid the exception in the first place. If you use AnsiString::ToIntDef() instead, you can remove the try/catch block completely:
Oversteering = HoursCounter.ToIntDef(0);
Alternatively, you can use TryStrToInt() instead:
if (!TryStrToInt(HoursCounter, Oversteering))
{
...;
}
If 0 is a valid value for your counter, use TryStrToInt():
if (TryStrToInt(HoursCounter, Oversteering))
{
// use Oversteering as needed, even zeros...
}
else
ShowMessage("Cannot convert HoursCounter to a valid integer!");
If 0 always represents an error, then use ToIntDef():
Oversteering = HoursCounter.ToIntDef(0);
if (Oversteering != 0)
{
// use Oversteering as needed, except zeros...
}
else
ShowMessage("Cannot convert HoursCounter to an acceptable integer!");
In C++, RAII is often advocated as a superior approach to exception handling: if an exception is thrown, the stack is unwound, all the destructors are called and resources are cleaned up.
However, this presents a problem with error reporting. Say a very generic function fails, the stack is unwound to the top level and all I see in the logs would be:
Couldn't read from socket: connection reset by peer.
...or any equally generic message. This doesn't say much about the context from which the exception is thrown. Especially if I'm running something like an event queue processing loop.
Of course I could wrap every call to socket reads with a try/catch block, catch the exception, construct a new one with more detailed context information and re-throw it, but it defeats the purpose of having RAII, and is slowly but surely becoming worse than handling return error codes.
What's a better way for detailed for error reporting in standard C++? I'm also open to suggestions involving Boost.
As James McNellis suggested here, there is a really neat trick involving a guard object and the std::uncaught_exception facility.
The idea is to write code like this:
void function(int a, int b)
{
STACK_TRACE("function") << "a: " << a << ", b: " << b;
// do anything
}
And have the message logged only in case an exception is actually thrown.
The class is very simple:
class StackTrace: boost::noncopyable // doesn't make sense to copy it
{
public:
StackTrace(): mStream() {}
~StackTrace()
{
if (std::uncaught_exception())
{
std::cout << mStream.str() << '\n';
}
}
std::ostream& set(char const* function, char const* file, unsigned int line)
{
return mStream << file << "#" << line << " - " << function << " - ";
}
private:
std::ostringstream mStream;
};
#define STACK_TRACE(func) \
StackTrace ReallyUnwieldyName; \
ReallyUnwieldyName.set(func, __FILE__, __LINE__)
One can use __PRETTY_FUNC__ or equivalent to avoid naming the function, but I have found in practice that it was too mangled / verbose for my own taste.
Note that you need a named object if you wish it to live till the end of the scope, which is the purpose here. We could think of tricky ways to generate a unique identifier, but I have never needed it, even when protecting narrower scope within a function, the rules of name hiding play in our favor.
If you combine this with an ExceptionManager (something where exceptions thrown register themselves), then you can get a reference to the latest exception and in case of logging you can rather decide to setup your stack within the exception itself. So that it's printed by what and ignored if the exception is discarded.
This is a matter of taste.
Note that in the presence of ExceptionManager you must remain aware that not all exceptions can be retrieved with it --> only the ones you have crafted yourself. As such you still need a measure of protection against std::out_of_range and 3rd party exceptions.
Say a very generic function fails, the stack is unwound to the top level and all I see in the logs would be [...]
What's a better way for detailed for error reporting in standard C++?
Error handling isn't local to a class or library - it is a application level concern.
Best I can answer you question is that the error reporting should be always implemented by looking first and foremost at the error handling. (And the error handling also including the handling of the error by the user.) Error handling is the decision making about what has to be done about the error.
That is one of the reasons why error reporting is an application level concern and strongly depends on the application workflow. In one application the "connection reset by peer" is a fatal error - in another it is a norm of life, error should be silently handled, connection should be reestablished and pending operation retried.
Thus the approach you mention - catch the exception, construct a new one with more detailed context information and re-throw it - is a valid one too: it is up to the top level application logic (or even user configuration) to decide whether the error is really an error or some special (re)action has to taken upon the condition.
What you have encountered is one of the weaknesses of so called out-of-line error handling (aka exceptions). And I'm not aware of any way to do it better. Exceptions create a secondary code path in the application and if error reporting is vital the design of the secondary code path has to treated just like the main code path.
Obvious alternative to the out-of-line error handling is the in-line error handling - good ol' return codes and log messages on the error conditions. That allows for a trick where application saves all low-severity log messages into internal (circular) buffer (of fixed or configurable size) and dumps them into the log only if high-severity error happens. This way more context information is available and different layers of application do not have to be actively aware of each other. That is also standard (and sometimes literally "standard" - mandated by law) way of error reporting in application fields like safety and mission critical software, were one is not allowed to miss errors.
I've never actually done this, but you could roll your own "stacktrace":
struct ErrorMessage {
const char *s;
ErrorMessage(const char *s) : msg(s) {}
~ErrorMessage() { if (s) std::cout << s << "\n"; }
void done() { s = 0; }
};
void someOperation() {
ErrorMessage msg("Doing the first bit");
// do various stuff that could throw
msg = "Doing the second bit";
// do more stuff that could throw
msg.done();
}
You can have several levels of this (although not necessarily use it at every level):
void handleFoo() {
ErrorMessage msg("Handling foo event");
someOperation();
msg.done();
}
And add more constructors and members:
void handleBar(const BarEvent &b) {
ErrorMessage msg(std::stringstream("Handling bar event ") << b.id);
someOperation();
msg.done();
}
And you needn't write the messages to std::cout. It could be to some logging object, and the object could queue them, and not actually output them to the log unless the catch site tells it to. That way, if you catch an exception that doesn't warrant logging, nothing is written.
It's not pretty, but it's prettier than try/catch/throw or checking return values. And if you forget to call done on success (for example if your function has multiple returns and you miss one), then you will at least see your mistake in the logs, unlike a resource leak.
[Edit: oh, and with a suitable macro you can store __FILE__ and __LINE__ in the ErrorMessage.]
You could add a call stack to your exception. I'm not sure how good this works for release builds but works like a charm with debug. You could do this in the constructor of your exception (to encapsulate it). See here for a starting point. This is possible as well on Linux - eventhough I dont remember how exactly.
I use RAII and exceptions and just have various unit-test-like assert statements throughout the code - if they fail, the the stack unwinds to where I can catch and handle them.
#define APP_ASSERT_MSG(Class,Assertion,szDescription) \
if ( !(Assertion) ) \
{ \
throw Class(__LINE__,__FILE__,szDescription); \
}
For most of my particular use case, all I care about is logging debug information, so my exception has the file and line number in it along with an error message (message is optional as I have an assert without it as well). You could easily add FUNCTION or an error code of some type for better handling.
I can then use it like this:
int nRet = download_file(...);
APP_ASSERT_MSG(DownloadException == ERR_OK, "Download failed");
This makes error handling and reporting much easier.
For really nasty debugging, I used GCC's function instrumentation to keep a trace list of what's going on. It works well, but slows down the app quite a bit.
What I do regularly, FWIW, is not use exceptions, but rather explicit error handling in a standard format (ie: using a macro). For example:
result = DoSomething();
CHECK_RESULT_AND_RETURN_ON_ERROR( result );
Now, obviously, this has many limitations design-wise:
Your return codes may need to be somewhat uniform
Code is cluttered with macros
You may need many macros for various check conditions
The upside, though, is as Dummy00001 said: you can effectively generate a stack trace on demand in the event of a serious error, by simply caching the results. I also use this paradigm to log all unexpected error conditions, so I can adjust the code later to handle unexpected conditions which occur "in the wild".
That's my 2c.
Here's how I handle error reporting in my libraries (this may or may not suit your situation).
First, as part of your design you want a "core" or "system" library that all this common logic will sit in. All you other libraries will then link to the core and use its error reporting APIs, so your whole system has a single compact chunk of logic for handling this mess.
Inside the core, provide a set of logging MACROS such as "LogWarning" and "LogFatal" with documented behavior and expected usage- for example, LogFatal should trigger a hard abort of the process but anything lower than "LogError" is simply advisory (does nothing). The macros can provide a "printf" interface that automatically appends the "LINE", "FILE", and "FUNC" macros as arguments to the underlying singleton object that handles your error reporting.
For the object itself I'll defer to you. However, you want public APIs that configure your "drains"- e.g. log to stderr, log to logfile, log to MS Services log, etc. You also want the underlying singleton to be thread safe, re-entrant where possible, and very robust.
With this system, you can make "exception reporting" ONE MORE DRAIN TYPE. Just add an internal API to that error manager object that packages your logged message as an exception, then throws it. Users can then enable AND DISABLE exceptions-on-error behavior in your code with ONE LINE in their apps. You can put try-catches around 3rd party or system code in you libraries that then calls a "Log..." macro in the catch clauses to enable clean re-throw behavior (with certain platforms and compiler options you can even catch things like segfaults using this).
When writing a function, my implementation very frequently looks like this:
Call a subfunction
If this subfunction fails to execute (because of an exceptional situation): log this failure and abort the current function
Otherwise continue calling other subfunctions, which in turn can fail
A crucial part is the logging. Every function that fails should add a short description to the log. This way, at the level where the exception is handled, the user can be shown a detailed error message.
For example, consider an application where a new user account can be created, and there is a problem with the database connection. The following inverse stack trace results:
SQLDriverConnect() -> "SQLDriverConnect error: Data source not found and no default driver specified"
OpenDatabaseConnection() -> "Failed to open database connection"
CreateUser() -> "Failed to create the new user"
ValidateAndSaveNewUserAccount() -> "Failed to save the user profile"
Catch the exception and show the logged messages to the user
Using the exceptions feature, I would implement this as follows:
void CreateUser()
{
try {
OpenDatabaseConnection();
}
catch(std::exception& e) {
e.AddLog("Failed to create the new user");
throw;
}
//...
}
Using a simple return value, I'd write the following:
bool CreateUser(Log& log)
{
if (!OpenDatabaseConnection(log))
{
log.Add("Failed to create the new user");
return false;
}
//...
return true;
}
I find both implementations equally good. Therefore, I don't see much advantage in using exceptions. I am well aware that exception handling is generally considered a useful feature, but I don't really understand why. A long time ago, I used exception handling extensively, but I didn't see the big advantage, so now I never use them anymore. Hence my questions:
Using exceptions, how can this be implemented in C++ more concisely?
If not, what is then the advantage of throwing exceptions over returning an "is successful" boolean?
Note: I use the term logging as "collecting an explanation of what went wrong, so it can be presented to the user later on". I'd rather not store that explanation in a global collection of log messages (in memory, in a file or in a database), as it directly describes the specific exception.
Update: Thanks for you responses so far. I understand that exceptions are only useful if the user doesn't need detailed feedback on what went wrong. (Please correct me if I misinterpreted this.)
Your strategy seems to avoid the most useful aspect of exceptions, you can throw an exception class which already has the text of the log information in it - that is generate the text for the log at time the exception is thrown not at the time the exception is caught. Then you don't have to catch at every level going up the stack, but only at the top level.
Thus only one try block, and one log.add - much less code in general.
Something like this seems to remove all your replication.
void OpenDatabaseConnection()
{
if (Error) throw MyException("Failed opening database");
}
void CreateUser()
{
try {
OpenDatabaseConnection();
//...... do everything here
}
catch(MyException& E) { //only one copy of this code
E.AddLog(E.getMessage());
throw;
}
}
If you always want to handle your exceptional conditions immediately after the call, then there is no real advantage.
The advantage comes when you want to handle the condition several layers up the call chain. To do that with your success flag, you'd have to bubble the flag up several layers of subroutine calls. Every layer would have to be written with the knowldege that it has to keep track of the special flag from way down in the bowels of the code. This is just a major primo PITA.
For instance, for realtime work we typically build our applications around an iteration loop. Any error during the loop generally just aborts that iteration of the loop (excepting "fatal" errors, which abort the entire app). The easiest way to handle this is to just throw exceptions from wherever they occur, and handle them all in their own catch blocks at the very outermost of the application.
I think a big case for using exceptions here is that you've now made logging part of your method signatures. In general, I don't think that should be the case, because it's a cross-cutting concern. Imagine trying to do an analogous thing with user permissions, for example. Are you going to write this everywhere?
bool CreateUser(Log& log, UserSecurityContext& u) {
if (!HasPermissionsFor(u, SecurityRoles::AddUser)) {
log.Add("Insufficient permissions");
return false;
}
//...
return true;
}
There are other reasons to want to use exceptions (see Elemental's answer), but anytime the non-use of a language feature impacts the design of your software, it's worth thinking about whether that was the right way to do it.
Exception handling removes error handling from the normal control flow. This way, the code structured more clean. Exception handling also unwinds the stack automatically. This way, you need not to include error handling code in each method called on the way to the error. If you need one of those features, go with exceptions. If you don't, use error-codes or any other method because exceptions have costs (computing time) even if they are not thrown.
Additional answers to your comment. Imagine a code, that calls several functions that may fail.
procedure exceptionTest(...)
{
try
{
call1();
call2();
call3();
call4();
}
catch (...)
{
//errorhandling outside the normal control flow
}
}
without exception:
procedure normalTest(...)
{
if (!call1())
{
//errorHandling1
}
else if (!call2())
{
//errorHandling2
}
else if ....
...
}
As you can easily see, the normal control flow is disrupted with error handling. Compared to this code, the code using exceptions is easier to read.
If you need to add error handling in each method you call, exceptions may not provide benefits. But if you have nested calls that each may generate errors, it may be easier to catch the exception at top level. That's what I meant. It is not the case in your example, still it's good to know where to benefit from exceptions.
Exceptions are using in only extreme situations. Execution of exception is too slow. For log not great errors try use return value.
Example:
int someMethod{
if(erorr_file_not_found){
logger.add("File not found");
return 1;
}
if(error_permission){
logger.add("You have not permissons to write this file");
return 2;
}
return 0;
}
In this case you can print error and process this error on higher level.
Or (more complex):
int someMethod{
int retval=0;
if(someshit){
retval=1;
goto _return;
}
//...
_return:
switch(retval){
case 1:logger.add("Situation 1");break;
case 2:logger.add("Situation 2");break;
//...
}
/*
close files, sockets, etc.
*/
return retval;
}
This way is more hard but most fast.
Depending on your circumstances, you may be able to log from a constructor of your exception (maybe asynchronously) That way your code would look like:
void CreateUser()
{
OpenDatabaseConnection();
}
Of course, you would need to throw your custom exception from OpenDatabaseConnection().
I worked on two projects when this strategy was used with success.
I would propose to separate error handling from logging and from the user interaction.
Every method can write to log files for itself. With a small log message framework, methods can output debug, informational and error message. Depending on the context your applications runs in defined by a config file, e.g., only critical error messages are actually written.
Especially in networking applications, connection failures can always occur and are not exceptional. Use exceptions for unexpected errors that should not occur or that occur only rarely. It can also make sense to use exceptions internally if you need e.g. the stack unrolling feature:
void CreateUser() {
try {
CDatabaseConnection db = ConnectToDatabase();
InsertIntoDB(db, "INSERT INTO ... ");
SetPermission(...);
} catch(...) {}
}
If InsertIntoDB throws an exception because the network connection is lost again, object CDatabaseConnection will be destroyed and SetPermission is never run. Using this can lead to better code.
The third thing you want to do is give the user feedback in an interactive application. That's a whole different thing. Let your internal methods return enumerations of possible error codes eerrorCONNECTIONLOST, eerrorUSERNAMEINVALID, etc Don't return the error strings from core methods. The user interface layer should bother which strings to display (possibly internationalizing them). Internally, the error codes will be much more useful. You could e.g. retry five times if your login method returned eerrorCONNECTIONLOST.