Exception Safe Programming

[Saaa]

On the website there is the following example:

Transaction abc()
{
    Foo f;
    Bar b;

    f = dofoo();
    try
    {
    	b = dobar();
    	return Transaction(f, b);
    }
    catch (Object o)
    {
    	dofoo_undo(f);
    	throw o;
    }
}

When f=dofoo() is run and doesn't succeed I suspect that f hasn't changed 
and dofoo has thrown an exception.
Because of the exception the try part isn't run, but the catch part is.
Did I understand this correctly?
The explanation of the try statement on the website is not the most 
comprehensible one (^.^)