Stop program flow in the middle without using an exception - c++

I need to stop the program flow in the middle, and I am currently using an exception for this. This flow is the legal flow and I want to know if I can do it without using an exception.
This is an example of my code, and I cannot change func_2 and func_1:
#include "stdio.h"
void func_3()
{
printf("i am func_3\n");
throw 20;
printf("i am not supposed to be here\n");
}
void func_2()
{
printf("i am func_2\n");
func_3();
printf("i am not supposed to be here\n");
}
void func_1()
{
printf("i am func_1\n");
func_2();
printf("i am not supposed to be here\n");
}
int main()
{
try
{
func_1();
}
catch (int e)
{
printf("i am supposed to be here\n");
}
catch (...)
{
printf("i am not supposed to be here\n");
}
}

I assume that you want to handle an exceptional case and are looking for an alternative to exceptions. I.e. I hope you don't want to continue with the program "normally" after handling your exceptional case, which is possible but not recommended to implement with exceptions.
Possible but not recommended alternatives to exceptions are:
When you want to stop your whole application, then you can use std::exit(0);. You can implement your "catch"-code in a function which you call instead of your "throw"-statement, and call std::exit(0); at the end of that function (or use another exit code to indicate an "unsuccessful" exit). Or you implement an exit handler and register it using std::atexit(&handle_exit);.
Alternative to std::exit(<something>); is abort(); which throws the POSIX signal "SIGABRT" to indicate abnormal termination (which is the default behavior if your program throws and doesn't catch an exception). Your "catch"-code would then go in a signal handler which you register using the POSIX functions. Note that this requires a POSIX system and is thus not as portable as other solutions.
Another (similar) option is to use the "terminate" mechanism: Call std::terminate(); when you would normally throw your exception. Put your "catch"-code in a "terminate handler" function with signature void(*)(), i.e. no parameters and no return value, let's call the function void handle_terminate(). Install a terminate handler using std::set_terminate(&handle_terminate);. I didn't try that one, however, and it sounds damn ugly.
You could implement an exception-like behavior using assembly instructions, but please do not try this at home, as the behavior of such code is highly implementation defined (if not undefined), and way too ugly to implement.

In short, you can't (well ... you could, by using jumps instead, but then you would have two problems to solve).
The exception solution is the one to use, but do not throw a number (a number - especially a magical number in this case doesn't tell you anything).
Instead, define a struct func_3_interrupted {}; minimalistic structure, whose type name tells you it is an "interruption" of func_3, and catch that instead; The structure should be empty (or close to empty) and it should probably not inherit from the std::exception hierarchy.

Return can be used to return to the caller and stop the function being executed
int GLOBAL_FLAG = 1;
function called_function(){
printf("Inside Function")
if(/*some condition*/)
{
GLOBAL_FLAG = 0;
return;
}
/*Normal function code*/
}
int main(){
{
called_function();
if(GLOBAL_FLAG == 1)/*continue program execution*/
printf("Function had been executed.Back to normal flow")
}
So once the return statement is encountered it goes back to the caller that is main here and continues executing rest of the statements in main function.

Related

Properly terminating program. Using exceptions

Question:
Is using exceptions the proper way to terminate my program if all I want is to display an error message and close (accounting that I may be deep in the program)? Or can I just explicitly call something like exit(EXIT_FAILURE) instead?
What I'm Currently Doing:
I'm working on a game project and am trying to figure out the best way to terminate the program in the case of an error that calls for such an action. For example, in the case the textures can't be loaded I display an error message and terminate the program.
I'm currently doing this with exceptions like so:
int main()
{
Game game;
try
{
game.run();
}
catch (BadResolutionException & e)
{
Notification::showErrorMessage(e.what(), "ERROR: Resolution");
return 1;
}
catch (BadAssetException & e)
{
Notification::showErrorMessage(e.what(), "ERROR: Assets");
return 1;
}
catch (std::bad_alloc & e)
{
Notification::showErrorMessage(e.what(), "ERROR: Memory");
return 1;
}
return 0;
}
All but bad_alloc are my own defined exceptions derived from runtime_error.
I don't need any manual resource cleanup and I'm using std::unique_ptr for any dynamic allocation. I just need to display the error message and close the program.
Research/Alternatives to Exceptions:
I've looked up a lot of posts on SO and other places and have seen others say anything from don't use exceptions, to use exceptions but your using them wrong. I've also looked up explicitly calling something like exit().
Using exit() sounds nice but I read it won't go back through the call stack up to main cleaning everything up (if I can find this again I'll post the link). Additionally, according to http://www.cplusplus.com/reference/cstdlib/exit/ this should not be used if multiple threads are active. I do expect to be creating a second thread for a short time at least once, and an error could occur in that thread.
Not using exceptions was mentioned in some replies here in relation to games https://gamedev.stackexchange.com/questions/103285/how-industy-games-handle-their-code-errors-and-exceptions
Use exceptions was discussed here: http://www.quora.com/Why-do-some-people-recommend-not-using-exception-handling-in-C++
There are a number of other sources I've read but those were the most recent I looked at.
Personal Conclusion:
Due to my limited experience of working with error handling and using exceptions, I'm not sure if I'm on the right track. I've chosen the route of using exceptions based on the code I posted above. If you agree that I should tackle those cases with exceptions, am I using it correctly?
It's generally considered good practice to let all exceptions propagate through to main. This is primarily because you can be sure the stack is properly unwound and all destructors are called (see this answer). I also think it's more organised to do things this way; you always known where your program will terminate (unless the program crashes). It's also facilitates more consistent error reporting (a point often neglected in exception handling; if you can't handle the exception, you should make sure your user knows exactly why). If you always start with this basic layout
int main(int argc, const char **argv)
{
try {
// do stuff
return EXIT_SUCCESS;
} catch (...) {
std::cerr << "Error: unknown exception" << std::endl;
return EXIT_FAILURE;
}
}
then you won't go far wrong. You can (and should) add specific catch statements for better error reporting.
Exceptions when multithreading
There are two basic ways of executing code asynchronously in C++11 using standard library features: std::async and std::thread.
First the simple one. std::async will return a std::future which will capture and store any uncaught exceptions thrown in the given function. Calling std::future::get on the future will cause any exceptions to propagate into the calling thread.
auto fut = std::async(std::launch::async, [] () { throw std::runtime_error {"oh dear"}; });
fut.get(); // fine, throws exception
On the other hand, if an exception in a std::thread object is uncaught then std::terminate will be called:
try {
std::thread t {[] () { throw std::runtime_error {"oh dear"};}};
t.join();
} catch(...) {
// only get here if std::thread constructor throws
}
One solution to this could be to pass a std::exception_ptr into the std::thread object which it can pass the exception to:
void foo(std::exception_ptr& eptr)
{
try {
throw std::runtime_error {"oh dear"};
} catch (...) {
eptr = std::current_exception();
}
}
void bar()
{
std::exception_ptr eptr {};
std::thread t {foo, std::ref(eptr)};
try {
// do stuff
} catch(...) {
t.join(); // t may also have thrown
throw;
}
t.join();
if (eptr) {
std::rethrow_exception(eptr);
}
}
Although a better way is to use std::package_task:
void foo()
{
throw std::runtime_error {"oh dear"};
}
void bar()
{
std::packaged_task<void()> task {foo};
auto fut = task.get_future();
std::thread t {std::move(task)};
t.join();
auto result = fut.get(); // throws here
}
But unless you have good reason to use std::thread, prefer std::async.
There's nothing wrong with catching unrecoverable errors and shutting down your program this way. In fact, it's how exceptions should be used. However, be careful not to cross the line of using exceptions to control the flow of your program in ordinary circumstances. They should always represent an error which cannot be gracefully handled at the level the error occurred.
Calling exit() would not unwind the stack from wherever you called it. If you want to exit cleanly, what you're already doing is ideal.
You have already accepted an answer, but I wanted to add something about this:
Can I just explicitly call something like exit() instead?
You can call exit, but (probably) shouldn't.
std::exit should be reserved for situations where you want to express "exit right now!", not simply "application has nothing left to do".
As an example, if you were to write a controller for a laser used in cancer treatments, your first priority in case something went wrong would be to shut down the laser and call std::exit - or possibly std::terminate (to ensure any side effects of a hanging, slow or crashing application do not kill a patient).
Similar to how exceptions should not be used for controlling application flow, exit should not be used to stop the application in normal conditions.
Is using exceptions the proper way to terminate my program if all I want is to display an error message and close (accounting that I may be deep in the program)?
Yes. This is the reason for using exceptions. An error occurred deep down in the code and something at a higher level will handle it. In your case, at the highest level.
There are arguments for/against exceptions vs. error codes, and this is a good read:
Exceptions or error codes
Can I just explicitly call something like exit() instead?
You can, but you may end up duplicating your logging code. Also, if in the future you decide that you want to handle an exception differently you will have to change all your exit calls. Imagine you wanted a different message, or to fall back on an alternative process.
Another similar question:
Correct usage of exit() in c++?
You also have a flaw in your approach as you don't handle all (C++) exceptions. You want something like this:
int main()
{
Game game;
try
{
game.run();
}
catch (BadResolutionException & e)
{
Notification::showErrorMessage(e.what(), "ERROR: Resolution");
return 1;
}
catch (BadAssetException & e)
{
Notification::showErrorMessage(e.what(), "ERROR: Assets");
return 1;
}
catch (std::bad_alloc & e)
{
Notification::showErrorMessage(e.what(), "ERROR: Memory");
return 1;
}
catch (...)
{
// overload?
Notification::showErrorMessage("ERROR: Unhandled");
return 1;
}
return 0;
}
If you don't handle all* exceptions you could have the game terminate without telling you anything useful.
You can't handle ALL exceptions. See this link:
C++ catching all exceptions
From the documentation:
[[noreturn]] void exit (int status);
Terminate calling process
Terminates the process normally, performing the regular cleanup for terminating programs.
Normal program termination performs the following (in the same order):
Objects associated with the current thread with thread storage duration are destroyed (C++11 only).
Objects with static storage duration are destroyed (C++) and functions registered with atexit are called.
All C streams (open with functions in ) are closed (and flushed, if buffered), and all files created with tmpfile are removed.
Control is returned to the host environment.
Note that objects with automatic storage are not destroyed by calling exit (C++).
If status is zero or EXIT_SUCCESS, a successful termination status is returned to the host environment.
If status is EXIT_FAILURE, an unsuccessful termination status is returned to the host environment.
Otherwise, the status returned depends on the system and library implementation.
For a similar function that does not perform the cleanup described above, see quick_exit.

Catching and handling all exceptions in my own class

Hello to all and sorry for my english!
How can I do the title above?
For example, I have a class contains a some functions that can throw exceptions:
class cl {
public:
void f1();
void f2();
};
void cl::f1()
{
// throw exception
}
void cl::f2()
{
// throw exception
}
I need to handle them.
Are there any other method to handle exceptions (that throws in my class) in one place of the code except code like this:
void cl::f1()
{
try
{
// throw exception
}
catch (...)
{
// handling
}
}
void cl::f1()
{
try
{
// throw exception
}
catch (...)
{
// handling
}
}
or this:
int main()
{
cl c;
try
{
f1();
f2();
}
catch(...)
{
// handling
}
}
?
Thanks in advance!
Are there any other method to handle exceptions (that throws in my
class) in one place of the code except code like this:
In my opinion you would typically only handle an exception when:
You can remedy it i.e do something about it e.g allow the user to select a different file.
You can add additional information.
For the latter case, it would mean throwing a new exception (possible of different type) from the handler.
If neither of the above hold, let it propagate to a level where it can be handled. In your case, I would not have a try/catch within f1 and f2, but only at the callsite (in main).
You might ask whether one cannot(should not) do certain cleanup work in the catch handler. I personally have never found this to be necessary if/when one uses the stack/scope to clean up see RAII. I/we usually have one catch handler per thread at the highest level, and this simply performs logging. We catch (and use exceptions) for runtime errors mostly. For logic errors we use assert (even in release mode), but this can be (and have been) debated often.

How do I make a C++ console program exit?

Is there a line of code that will terminate the program?
Something like python's sys.exit()?
While you can call exit() (and may need to do so if your application encounters some fatal error), the cleanest way to exit a program is to return from main():
int main()
{
// do whatever your program does
} // function returns and exits program
When you call exit(), objects with automatic storage duration (local variables) are not destroyed before the program terminates, so you don't get proper cleanup. Those objects might need to clean up any resources they own, persist any pending state changes, terminate any running threads, or perform other actions in order for the program to terminate cleanly.
#include <cstdlib>
...
exit( exit_code );
There are several ways to cause your program to terminate. Which one is appropriate depends on why you want your program to terminate. The vast majority of the time it should be by executing a return statement in your main function. As in the following.
int main()
{
f();
return 0;
}
As others have identified this allows all your stack variables to be properly destructed so as to clean up properly. This is very important.
If you have detected an error somewhere deep in your code and you need to exit out you should throw an exception to return to the main function. As in the following.
struct stop_now_t { };
void f()
{
// ...
if (some_condition())
throw stop_now_t();
// ...
}
int main()
{
try {
f();
} catch (stop_now_t& stop) {
return 1;
}
return 0;
}
This causes the stack to be unwound an all your stack variables to be destructed. Still very important. Note that it is appropriate to indicate failure with a non-zero return value.
If in the unlikely case that your program detects a condition that indicates it is no longer safe to execute any more statements then you should use std::abort(). This will bring your program to a sudden stop with no further processing. std::exit() is similar but may call atexit handlers which could be bad if your program is sufficiently borked.
Yes! exit(). It's in <cstdlib>.
Allowing the execution flow to leave main by returning a value or allowing execution to reach the end of the function is the way a program should terminate except under unrecoverable circumstances. Returning a value is optional in C++, but I typically prefer to return EXIT_SUCCESS found in cstdlib (a platform-specific value that indicates the program executed successfully).
#include <cstdlib>
int main(int argc, char *argv[]) {
...
return EXIT_SUCCESS;
}
If, however, your program reaches an unrecoverable state, it should throw an exception. It's important to realise the implications of doing so, however. There are no widely-accepted best practices for deciding what should or should not be an exception, but there are some general rules you need to be aware of.
For example, throwing an exception from a destructor is nearly always a terrible idea because the object being destroyed might have been destroyed because an exception had already been thrown. If a second exception is thrown, terminate is called and your program will halt without any further clean-up having been performed. You can use uncaught_exception to determine if it's safe, but it's generally better practice to never allow exceptions to leave a destructor.
While it's generally always possible for functions you call but didn't write to throw exceptions (for example, new will throw std::bad_alloc if it can't allocate enough memory), it's often difficult for beginner programmers to keep track of or even know about all of the special rules surrounding exceptions in C++. For this reason, I recommend only using them in situations where there's no sensible way for your program to continue execution.
#include <stdexcept>
#include <cstdlib>
#include <iostream>
int foo(int i) {
if (i != 5) {
throw std::runtime_error("foo: i is not 5!");
}
return i * 2;
}
int main(int argc, char *argv[]) {
try {
foo(3);
}
catch (const std::exception &e) {
std::cout << e.what() << std::endl;
return EXIT_FAILURE;
}
return EXIT_SUCCESS;
}
exit is a hold-over from C and may result in objects with automatic storage to not be cleaned up properly. abort and terminate effectively causes the program to commit suicide and definitely won't clean up resources.
Whatever you do, don't use exceptions, exit, or abort/terminate as a crutch to get around writing a properly structured program. Save them for exceptional situations.
if you are in the main you can do:
return 0;
or
exit(exit_code);
The exit code depends of the semantic of your code. 1 is error 0 e a normal exit.
In some other function of your program:
exit(exit_code)
will exit the program.
This SO post provides an answer as well as explanation why not to use exit(). Worth a read.
In short, you should return 0 in main(), as it will run all of the destructors and do object cleanup. Throwing would also work if you are exiting from an error.
In main(), there is also:
return 0;
#include <cstdlib>
...
/*wherever you want it to end, e.g. in an if-statement:*/
if (T == 0)
{
exit(0);
}
throw back to main which should return EXIT_FAILURE,
or std::terminate() if corrupted.
(from Martin York's comment)
else if(Decision >= 3)
{
exit(0);
}
exit(0); // at the end of main function before closing curly braces
simple enough..
exit ( 0 );
}//end of function
Make sure there is a space on both sides of the 0. Without spaces, the program will not stop.

Is it possible to enter the terminate_handler twice?

I have some clean up in a terminate_handler and it is possible to throw an exception. Do I need to worry about catching it to prevent recursive calls to the terminate_handler? With gcc, it seems this can't happen and we just go into abort. Is that true of the standard or is the behavior undefined?
A terminate handler is not allowed to return (§18.6.​3.1/2); it must end the program (the default handler calls abort()). If it consisted of:
void my_terminate()
{
throw 5;
}
You'd get undefined behavior, because you would leave the function (because the exception propagates) without having terminated the program. So if you have code that could throw, make sure you catch all exceptions, like this:
void my_terminate()
{
try
{
// stuff
}
catch(...)
{
// too bad
}
abort();
}
However (to answer the title question), I don't see anything that restricts it from being entered again, so this should be technically be fine:
void my_terminate()
{
static int counter = 0;
if (counter++ < 5)
terminate();
abort();
}
No, you cannot resume normal program flow from std::terminate. You can, however, throw a different exception from the unexpected function. Terminate does just that -- program execution terminates after it completes.
EDIT:
That said, you shouldn't be doing anything complicated in std::terminate -- if you're in terminate then things have blown up sufficiently that you should not be trying to continue -- and you shouldn't try to do things like allocate memory in std::terminate for the same reason -- what if the program is in there as a result of a low memory condition? There's nothing you can do about it there.

Dispatching exceptions in C++

How should exceptions be dispatched so that error handling and diagnostics can be handled in a centralized, user-friendly manner?
For example:
A DataHW class handles communication with some data acquisition hardware.
The DataHW class may throw exceptions based on a number of possible errors: intermittent signal, no signal, CRC failure, driver error. Each type of error gets its own exception class.
The DataHW class is called by a number of different pieces of code that do different types of acquisition and analysis.
The proper error handling strategy depends on the type of exception and the operation being attempted. (On intermittent signal, retry X times then tell the user; on a driver error, log an error and restart the driver; etc.) How should this error handling strategy be invoked?
Coding error recovery into each exception class: This would result in exception classes that are rather large and contain high-level UI and system management code. This seems bad.
Providing a separate catch block for each type of exception: Since the DataHW class is called from many different places, each catch block would have to be duplicated at each call site. This seems bad.
Using a single catch block that calls some ExceptionDispatch function with a giant RTTI-based switch statement: RTTI and switch usually indicates a failure to apply OO design, but this seems the least bad alternative.
Avoid duplicating the catch blocks at each call site by catching (...) and calling a shared handler function which rethrows and dispatches:
f()
{
try
{
// something
}
catch (...)
{
handle();
}
}
void handle()
{
try
{
throw;
}
catch (const Foo& e)
{
// handle Foo
}
catch (const Bar& e)
{
// handle Bar
}
// etc
}
An idea I keep running into is that exceptions should be caught by levels which can handle them. For example, a CRC error might be caught by the function that transmits the data, and upon catching this exception, it might try to retransmit, whereas a "no signal" exception might be caught in a higher level and drop or delay the whole operation.
But my guess is that most of these exceptions will be caught around the same function. It is a good idea to catch and handle them seperately (as in soln #2), but you say this causes a lot of duplicate code (leading to soln #3.)
My question is, if there is a lot of code to duplicate, why not make it into a function?
I'm thinking along the lines of...
void SendData(DataHW* data, Destination *dest)
{
try {
data->send(dest);
} catch (CRCError) {
//log error
//retransmit:
data->send(dest);
} catch (UnrecoverableError) {
throw GivingUp;
}
}
I guess it would be like your ExceptionDispatch() function, only instead of switching on the exception type, it would wrap the exception-generating call itself and catch the exceptions.
Of course, this function is overly simplified - you might need a whole wrapper class around DataHW; but my point is, it would be a good idea to have a centralized point around which all DataHW exceptions are handled - if the way different users of the class would handle them are similar.
Perhaps you could write a wrapper class for the DataHW class?
The wrapper would offer the same functionality as the DataHW class, but also contained the needed error handling code. Benefit is that you have the error handling code in a single place (DRY principle), and all errors would be handled uniformly. For example you can translate all low level I/O exceptions to higher level exceptions in the wrapper.
Basically preventing low level exceptions being showed to user.
As Butler Lampson said: All problems in computer science can be solved by another level of indirection
There are three ways i see to solve this.
Writing wrapper functions
Write a wrapper function for each function that can throw exceptions which would handle exceptions. That wrapper is then called by all the callers, instead of the original throwing function.
Using function objects
Another solution is to take a more generic approach and write one function that takes a function object and handles all exceptions. Here is some example:
class DataHW {
public:
template<typename Function>
bool executeAndHandle(Function f) {
for(int tries = 0; ; tries++) {
try {
f(this);
return true;
}
catch(CrcError & e) {
// handle crc error
}
catch(IntermittentSignalError & e) {
// handle intermittent signal
if(tries < 3) {
continue;
} else {
logError("Signal interruption after 3 tries.");
}
}
catch(DriverError & e) {
// restart
}
return false;
}
}
void sendData(char const *data, std::size_t len);
void readData(char *data, std::size_t len);
};
Now if you want to do something, you can just do it:
void doit() {
char buf[] = "hello world";
hw.executeAndHandle(boost::bind(&DataHW::sendData, _1, buf, sizeof buf));
}
Since you provide function objects, you can manage state too. Let's say sendData updates len so that it knows how much bytes were read. Then you can write function objects that read and write and maintain a count for how many characters are read so far.
The downside of this second approach is that you can't access result values of the throwing functions, since they are called from the function object wrappers. There is no easy way to get the result type of a function object binder. One workaround is to write a result function object that is called by executeAndHandle after the execution of the function object succeeded. But if we put too much work into this second approach just to make all the housekeeping work, it's not worth the results anymore.
Combining the two
There is a third option too. We can combine the two solutions (wrapper and function objects).
class DataHW {
public:
template<typename R, typename Function>
R executeAndHandle(Function f) {
for(int tries = 0; ; tries++) {
try {
return f(this);
}
catch(CrcError & e) {
// handle crc error
}
catch(IntermittentSignalError & e) {
// handle intermittent signal
if(tries < 3) {
continue;
} else {
logError("Signal interruption after 3 tries.");
}
}
catch(DriverError & e) {
// restart
}
// return a sensible default. for bool, that's false. for other integer
// types, it's zero.
return R();
}
}
T sendData(char const *data, std::size_t len) {
return executeAndHandle<T>(
boost::bind(&DataHW::doSendData, _1, data, len));
}
// say it returns something for this example
T doSendData(char const *data, std::size_t len);
T doReadData(char *data, std::size_t len);
};
The trick is the return f(); pattern. We can return even when f returns void. This eventually would be my favorite, since it allows both to keep handle code central at one place, but also allows special handling in the wrapper functions. You can decide whether it's better to split this up and make an own class that has that error handler function and the wrappers. Probably that would be a cleaner solution (i think of Separation of Concerns here. One is the basic DataHW functionality and one is the error handling).