Object destructed twice?

**DL1** · 01-31-2009

If I compile this

Code:

#include <iostream>
#include <exception>
using namespace std;

class X
{
    public:
    static int i;
    int count;
    X(){count = ++i; cout << "constructing X object " << count << endl;}
    ~X(){cout << "destructing X object " << count << endl;}
};

int X::i = 0;

void f() throw (X)
{
    throw 1;
}

void g()
{
    cout << "unexpected handler g() called\n";
    throw X();
}

int main()
{
    set_unexpected(&g);
    try
    {
        f();
    }
    catch (X)
    {
        cout << "caught exception X\n";
    }
    catch (int)
    {

        cout << "caught exception int\n";
    }

    return 0;
}

The X object thrown by g() appears to get destructed twice. Is this right ? Or is there something very wrong with the program? Or is my compiler sick?

Any ideas?

**laserlight** · 01-31-2009

Try implementing the copy constructor to match what your default constructor does

EDIT:
By the way, you should have caught the X object by const reference instead of by value, though if you did you probably would not have observed the "problem" that you described.

**phantomotap** · 01-31-2009

You should always catch by reference.

Soma

Edit: Eh...

**DL1** · 01-31-2009

So obvious when it's pointed out.

Thanks a million.

**cpjust** · 02-01-2009

Also, see here why you should avoid using Exception Specifications.

**iMalc** · 02-01-2009

Originally Posted by phantomotap

You should always catch by reference.

Yes, and const-reference when possible.

**phantomotap** · 02-01-2009

Yes, and const-reference when possible.

True. A catch for a non-constant reference is only useful when you need to patch a polymorphic exception, and if you need to do that you've probably failed anyway.

Soma

**CornedBee** · 02-01-2009

Catch by non-const is also useful if you want to supply the exception with additional information and then rethrow. Boost.Exception provides an exception class that you can attach arbitrary information to.

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