Question

What happens when we throw from a destructor? I know that it causes terminate() to be called, and memory is indeed freed and the destructor is called, but, is this before or after throw is called from foo? Perhaps the issue here is that throw is used while the stack is unwinding that is the problem.

Answer 1

Is this before or after throw is called from foo?

This is what is happening:

• foo() is called
• An object a of type A is created on the stack
• The next statement throws
• Now, the dtor for a is called, which throws another exception
• std::terminate is called — which is nothing but abandoning the exception handling mechanism:

From C++0x draft:

15.5.1 The std::terminate() function

1 In the following situations exception
handling must be abandoned for less
subtle error handling techniques:

[…]
— when the destruction of
an object during stack unwinding
(15.2) exits using an exception, or

2 In such cases, std::terminate() is
called (18.7.3). In the situation
where no matching handler is found, it
is implementation-defined whether or
not the stack is unwound before
std::terminate() is called. In all
other situations, the stack shall not
be unwound before std::terminate() is
called. An implementation is not
permitted to finish stack unwinding
prematurely based on a determination
that the unwind process will
eventually cause a call to
std::terminate().

Note: Emphasis mine