What is a good way in C++ to detect in a destructor that it is being run during unwind of the stack due to an exception being thrown as opposed to a normal exit of scope triggering the destructor? I'd like to know so that I can create a class that has some cleanup code that is always run on normal exit but skipped when an exception occurs.
std::uncaught_exception() (defined in <exception>) will tell you in your destructor if it was called because of an exception:
class A
{
public:
~A()
{
if (std::uncaught_exception()) {
// Called because of an exception
} else {
// No exception
}
}
};
Probably this article will help you. The article will show you the problems with std::uncaught_exception() and contains an advice how to deal with exceptions in destructors.
Don't do that unless you have a good reason. The stack unwinding is such a language feature that all automatic objects inside try block will be enforced to deallocate, so that the resources inside them have a chance to release.
You want skip cleanup in dtor during stack unwinding, which bypasses the original intention of it. And you'll run the risk of leaking resources.
Example
class CDBConnection
{
public:
CDBConnection()
{
m_db.open();
}
~CDBConnection()
{
if (!std::uncaught_exception())
m_db.close();
// if this is called during a stack unwinding,
// your DB connection will not be closed for sure.
// That's a resource leakage.
}
//..
private:
DB m_db;
};
void main()
{
//..
try
{
// code that may throw
CDBConnection db;
//..
}
catch(const CDBException& exp)
{
// properly handle the exception
}
}
Here is one way I can think of, but it seems clumsy:
{
myCleanupClass unwindAction;
try {
// do some work which may throw exception.
} catch (...) {
unwindAction.disableDestructorWork();
throw;
}
}
Related
Does this code
try
{
opaque_function_that_might_throw_arbitrary_exception ();
}
catch (...)
{
throw;
}
differ in any way semantically from just calling
opaque_function_that_might_throw_arbitrary_exception ();
in C++? Are there differences between the standards?
I though both snippets would be semantically identical, but I checked with the compiler-explorer and it generates different code, even on the highest optimization level. This made me wonder if there is something I'm missing.
Yes, there is a subtle difference in one particular place. Consider this main function:
int main() {
RAIIHandle important_resource{ /*...*/ };
opaque_function_that_might_throw_arbitrary_exception ();
}
Is important_resource freed upon an exception being thrwon? Well, it's implementation defined. std::terminate is called, but the stack may not be unwound if an exception escapes from main. For some resources, this could spell real trouble.
Now in this case:
int main() {
try {
RAIIHandle important_resource{ /*...*/ };
opaque_function_that_might_throw_arbitrary_exception ();
}
catch(...) {
throw;
}
}
std::terminate is still called. But not before a suitable handler is found for the exception. The stack must be unwound. So our important_resource is definitely freed.
I would like my C++ code to stop running with proper object cleanup if a certain condition is met; in a constructor of a class.
class A {
public:
int somevar;
void fun() {
// something
}
};
class B {
public:
B() {
int possibility;
// some work
if (possibility == 1) {
// I want to end the program here
kill code;
}
}
};
int main() {
A a;
B b;
return 0;
}
How can I terminate my code at that point doing proper cleanup. It's known that, std::exit does not perform any sort of stack unwinding, and no alive object on the stack will call its respective destructor to perform cleanup. So std::exit is not a good idea.
You should throw an exception, when the constructor fails, like this:
B() {
if(somethingBadHappened)
{
throw myException();
}
}
Be sure to catch exceptions in main() and all thread entry functions.
Read more in Throwing exceptions from constructors. Read about Stack unwinding in How can I handle a destructor that fails.
It is not possible to perform just from a constructor. If you throw an exception then applications need to set up a proper exception handling code at entry points, because if you just thrown an exception that won't be handled then compiler is allowed to skip stack unwinding and cleanup.
If you don't want to use use exceptions, you can have an init method in class B that returns a return code:
class B {
public:
B(/*parameters that will be used by init*/) : ...
int init(); // actually initialize instance and return non 0 upon failure
}
How can I protect myself from using object which isn't fully created when using exceptions?
Should I catch in constructor ? Or maybe it's bad practice ? If I'll catch in constructor object will be created.
#include <stdio.h>
class A
{
public:
A()
{
try {
throw "Something bad happened...";
}
catch(const char* e) {
printf("Handled exception: %s\n", s);
}
// code continues here so our bad/broken object is created then?
}
~A()
{
printf("A:~A()");
}
void Method()
{ // do something
}
};
void main()
{
A object; // constructor will throw... and catch, code continues after catch so basically we've got
// broken object.
//And the question here:
//
//* is it possible to check if this object exists without catching it from main?
// &object still gives me an address of this broken object so it's created but how can I protect myself
// from using this broken object without writing try/catch and using error codes?
object.Method(); // something really bad. (aborting the program)
};
The language itself has no concept of an object being "invalid" in any detectable way.
If the exception indicates that a valid object can't be created, then it shouldn't be handled within the constructor; either rethrow it, or don't catch it in the first place. Then the program will leave the scope of the object being created, and it won't be possible to incorrectly access it.
If that isn't an option for some reason, then you'll need your own way to mark the object as "invalid"; perhaps set a boolean member variable at the end of the constructor to indicate success. This is flaky and error-prone, so don't do it unless you've got a very good reason.
If the object is in an invalid state when a certain exception is thrown, then I would let the exception unwind the call stack so the caller can be notified (and therefore react) to such things.
However, if the exception is one you can recover from, it may be worth trying to do so depend on your application. Make sure you use something like a logger or even simply stderr to indicate this is happening though.
I am going to suggest a first iteration of doing something more like this:
try {
throw "Something bad happened...";
}
catch(const std::exception e) {
cerr << e.what () << endl ; // Better off in the main
throw ;
}
Two things here:
Unless your exception handler handles the exception, it should throw.
Always use exception classes based upon std::exception to that you can always find out what the problem was as shown above.
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.
I have an object on the stack for which I wish its destructor to skip some work when the destructor is being called because the stack is being unwound due to a specific exception being thrown through the scope of the object on the stack.
Now I could add a try catch block inside the scope of the stack item and catch the exception in question and notify the stack object to not run the work to be skipped an then rethrow the exception as follows:
RAII_Class pending;
try {
doSomeWorkThatMayThrowException();
} catch (exceptionToSkipPendingDtor &err) {
pending.notifySkipResourceRelease();
throw;
}
However, I'm hoping there is a more elegant way to do this. For example imagine:
RAII_Class::~RAII_Class {
if (detectExceptionToSkipPendingDtorBeingThrown()) {
return;
}
releaseResource();
}
You can almost do this with std::uncaught_exception(), but not quite.
Herb Sutter explains the "almost" better than I do: http://www.gotw.ca/gotw/047.htm
There are corner cases where std::uncaught_exception() returns true when called from a destructor but the object in question isn't actually being destroyed by the stack unwinding process.
You're probably better off without RAII because it doesn't match your use case. RAII means always clean up; exception or not.
What you want is much simpler: only release resource if an exception is not throw which is a simple sequence of functions.
explicitAllocateResource();
doSomeWorkThatMayThrowException();
explicitReleaseResource(); // skipped if an exception is thrown
// by the previous function.
I would do it the other way around - explicitly tell it to do its work if no exception was thrown:
RAII_Class pending;
doSomeWorkThatMayThrowException();
pending.commit(); // do or prepare actual work
This seems to circumvent the main reason to use RAII. The point of RAII is that if an exception happens in the middle of your code you can still release resources/be destructed properly.
If this isn;t the semantic you want, then don't use RAII.
So instead of:
void myFunction() {
WrapperClass wc(acquireResource());
// code that may throw
}
Just do:
void myFunction() {
Resource r = acquireResource();
// code that may throw
freeResource(r);
}
If the code in the middle throws, the resource won't be freed. This is what you want, rather than keeping RAII (and keeping the name) but not implementing RAII semantics.
Looks like bool std::uncaught_exception(); does the trick if you want to have this behavior for every exception, not just special ones!
You can do without a try-catch:
RAII_Class pending;
doSomeWorkThatMayThrowException(); // intentional: don't release if throw
pending.releaseResource();
Alternatively, you can try a little harder with RAII:
struct RAII_Class {
template<class Op>
void execute(Op op) {
op();
releaseResources();
}
private:
void releaseResources() { /* ... */ }
};
int main(int argc, char* argv[])
{
RAII_Class().execute(doSomeWorkThatMayThrowException);
return 0;
}
Although it would be a kludge at best, if you own the code for the exception class you're interested in, you could add a static data member to that class (bool) that would be set to "true" in the constructor for objects of that class, and false in the destructor (might need to be an int that you increment/decrement instead). Then in the destructor of your RAII class, you can check std::uncaught_exception(), and if true, query the static data member in your exception class. If you get true (or > 0) back, you've got one of those exceptions--otherwise you ignore it.
Not very elegant, but it would probably do the trick (as long as you don't have multiple threads).
I found this website with an interesting discussion about std::uncaught_exception() and an alternative solution to your question that seems much more elegant and correct to me:
http://www.gotw.ca/gotw/047.htm
// Alternative right solution
//
T::Close() {
// ... code that could throw ...
}
T::~T() /* throw() */ {
try {
Close();
} catch( ... ) {
}
}
In this way you're destructor does only one thing and you're protected against throwing an exception during an exception (which I assume is the problem you're trying to solve).