Re: Exceptions in destructors

From:

Greg Herlihy <greghe@pacbell.net>

Newsgroups:

comp.lang.c++

Date:

Mon, 30 Apr 2007 04:38:27 GMT

Message-ID:

<C25AC0CA.1839%greghe@pacbell.net>

On 4/29/07 7:50 PM, in article f13lhq$f39$1@ensb.cpsc.ucalgary.ca, "Rennie
deGraaf" <degraaf@cpsc.no-processed-pork.ucalgary.ca> wrote:

I know that if an exception is thrown from a destructor while unwinding
the stack because of another exception, terminate() is called (FAQ
17.3). How exactly does this rule work? Is it acceptable to both throw
/and/ catch an exception inside a destructor, as in the following code?

struct Foo
{
    void finalize()
    {
        throw 1.0;
    }
    ~Foo()
    {
        try
        {
            finalize();
        }
        catch (double d)
        {}
    }
};

The runtime calls terminate() if - during exception processing - another
thrown exception "escapes" from the destructor in which it was thrown.

So as long as the thrown exception in the code above is caught before Foo's
destructor exits, then terminate() will not be called. Therefore the above
code is technically safe - but it is also fragile. Specifically, Foo's
destructor assumes that finalize() will only ever throw a double - but
through there is nothing in finalize()'s implementation that enforces such a
restriction - nor is there anything in Foo's destructor to check whether its
assumption about the type of finalize()'s thrown exceptions - remains valid
since the last time it checked.

Greg