Thread: life cycle of exception object

Hello everyone,


1.

Suppose I have code like this. According to the standard, the storage of exception object is undefined (could be either on stack or on heap?).

2.

I am wondering what is the life cycle of the exception object which reference variable e binded to? For example, could we bind a global reference to the exception object referred by e? Or doing something on the exception object beyond the bracket?

3.

The life cycle of variable e itself should not beyond the bracket, right?

Code:

catch (exception& e)
{
    // ...
}

thanks in advance,
George

1) The storage of the exception object can be anywhere. Technically, doing "throw x;" where x is some object or value, then x ceases to exist when control passes out of the scope in which it was created. For example;

Code:

void f()
{
      {
           SomeType x;
           throw x;
      }
      //   x does not exist here
}

While x ceases to exist as soon as control is returned to an enclosing scope, a temporary copy of x exists until the exception is caught (or the program terminates due to an uncaught exception) -- and will be copied repeatedly as control passes to enclosing scopes. Then compiler implementation details kick in, as the standard explicitly allows (as opposed to "requires") the compiler to avoid creating a temporary if the only way of detecting that temporary is by tracking constructor and destructor calls.

2) and 3) The reference e exists until control leaves the scope of the catch block, as does the object e refers to. If the catch block rethrows the exception then, logically, a copy of the exception object will be created, and copied during stack unwinding. Practically, again, the compiler is allowed to be smarter than that, so it can avoid creating copies.

Thanks grumpy,


Suppose in catch block we pass the address of the exception object to a global pointer variable or make a global reference variable binded to the exception object (let me know if I have not made myself understood). And after catch block or even after function returns, is it safe to access the global reference variable or the global pointer variable?

Originally Posted by grumpy

1) The storage of the exception object can be anywhere. Technically, doing "throw x;" where x is some object or value, then x ceases to exist when control passes out of the scope in which it was created. For example;

Code:

void f()
{
      {
           SomeType x;
           throw x;
      }
      //   x does not exist here
}

While x ceases to exist as soon as control is returned to an enclosing scope, a temporary copy of x exists until the exception is caught (or the program terminates due to an uncaught exception) -- and will be copied repeatedly as control passes to enclosing scopes. Then compiler implementation details kick in, as the standard explicitly allows (as opposed to "requires") the compiler to avoid creating a temporary if the only way of detecting that temporary is by tracking constructor and destructor calls.

2) and 3) The reference e exists until control leaves the scope of the catch block, as does the object e refers to. If the catch block rethrows the exception then, logically, a copy of the exception object will be created, and copied during stack unwinding. Practically, again, the compiler is allowed to be smarter than that, so it can avoid creating copies.

regards,
George

Thanks Mats,


I have written some code which verified that the caught object instance is not the original one we thrown. Looks like we only get a temporary exception object in catch block?

Here is my code. You can see two object instance is created.

Code:

#include <iostream>

using namespace std;

class Foo {
public:
	Foo(int input)
	{
		this->a = input;
		cout << this << endl;
	}

	Foo (const Foo& input)
	{
		this->a = input.a;
		cout << this << endl;
	}
	int a;
};

void foo()
{
	try {
		Foo* f = new Foo (100);
		throw *f;
	} catch (const Foo& e)
	{
		cout << e.a << endl;
	}
}

int main()
{
	foo();
}

Originally Posted by matsp

That would equivalent of taking a reference/address of a local variable in any other context, so no, this is not valid and leads to undefined behaviour.

--
Mats

regards,
George

Thanks Mats,


1. In my sample, since I am using new, I thnk it is safe to address the original exception object later, right?

2. All we are talking about not safe to address exception object is the temporary copied object, right? Not the original one.

Originally Posted by matsp

Yes, like Grumpy said, it's a temporary copy of the original object - the original object may even be destroyed by the time the catch is entered.

--
Mats

regards,
George

Thanks Mats,


My question is answered. As a conclusion:

1. There are a temporary copy and the original object instance;
2. For the original object instance, it follows normal C++ object instance life cycle;
3. For the temporary copy object instance, it is not safe to address it outside catch block.

My conclusions are correct? :-)

Originally Posted by matsp

Sure, if you use new to create some object that you then use for throw, the object will not be destroyed as part of the trhow operation - it will be copied, but not destroyed.

--
Mats

regards,
George

Hi CornedBee,


Can you show me some pseudo code for (1) and (2) please?

Originally Posted by CornedBee

Your conclusions are wholly unnecessary.

1) They might be the same.
2) No, it might be merged with the exception object.
3) Of course not.


One addition to what grumpy said: a re-throw does not create a new exception object.

regards,
George

Thanks grumpy!


Great!! Your reply gives me more insights.

1.

Originally Posted by grumpy

A compiler is allowed to avoid creating a temporary if the only way of detecting the existence of that temporary is by tracking constructor and destructor calls (such as you have done).

Seems confusing. My code shows a temporary object is needed. Seems you mentioned a case when temporary object is not needed -- which is not the same as I have mentioned. You mean compiler optimization to avoid creating a temporary object?

What do you mean "the only way of detecting the existence of that temporary is by tracking constructor and destructor calls"?

2.

Originally Posted by grumpy

Not quite. Practically, if a compiler eliminates temporaries, the object may exist longer than you would expect.

You mean if compiler optimize the code and does not generate temporary, and let the exception object in the catch block the same as the one in try block?

If yes, I still do not know even in this way, why compiler needs to prolong the life cycle of the exception object instance in the try block?


regards,
George

Hi CornedBee,


I can understand the exception object in try and in catch may share the same object instance for compiler optimization purpose.

But what do you mean (2) -- "No, it might be merged with the exception object.". What do you mean merged?

Originally Posted by CornedBee

No, it's a compiler optimization.

regards,
George