stock unwinding from GotW #47

[puzzlecracker]

It is per Sutter's GotW #47 article. I don't understand why if a U
object is destroyed due to stack unwinding during to exception
propagation, T::~T will fail to use the "code that could throw" path
even though it safely could.

Code from the article:

// The wrong solution
//
T::~T() {
if( !std::uncaught_exception() ) {
// ... code that could throw ...
} else {
// ... code that won't throw ...
}
}


// Why the wrong solution is wrong
//
U::~U() {
try {
T t;
// do work
} catch( ... ) {
// clean up
}
}