Can I use std::current_exception during stack unwinding?

Question

Should it be possible to use std::current_exception inside destructors of objects that are destroyed during stack unwinding?

Documentation on cppreference says:

If called during exception handling (typically, in a catch clause), captures the current exception object (...)

But it's not clear for me whether stack unwinding is a part of exception handling.

In some highest-ranked answer on stackoverflow author assumes that it's possible.

I did some test on my compiler (g++ (Ubuntu 4.8.2-19ubuntu1) 4.8.2) and it seems, that std::current_exception returns empty pointer in this case.

#include <exception> #include <stdexcept> #include <iostream> struct A { ~A() { std::clog << "in destructor"<<std::endl; std::clog << "uncaught_exception: " << std::uncaught_exception() << std::endl; std::clog << "current_exception: " << (bool)std::current_exception() << std::endl; } }; int main(int argc, char **) { try { A aa; std::clog << "before throw"<<std::endl; if(argc>1) throw std::runtime_error("oh no"); } catch(...) { std::clog << "in catch block"<<std::endl; std::clog << "uncaught_exception: " << std::uncaught_exception() << std::endl; std::clog << "current_exception: " << (bool)std::current_exception() << std::endl; } return 0; }

The output is:

before throw in destructor uncaught_exception: 1 current_exception: 0 in catch block uncaught_exception: 0 current_exception: 1

Does anybody know what the standard says?

quantdev · Accepted Answer · 2015-02-02 13:31:26Z

C++ Standard defines current_exception() in section § 18.8.5 [propagation] :

(emphasis mine)

exception_ptr current_exception() noexcept;

Returns: An exception_ptr object that refers to the currently handled exception (15.3) or a copy of the currently handled exception, or a null exception_ptr object if no exception is being handled. The referenced object shall remain valid at least as long as there is an exception_ptr object that refers to it.

And § 15.3 [except.handle], notes 7 and 8 :

A handler is considered active when initialization is complete for the parameter (if any) of the catch clause. [ Note: The stack will have been unwound at that point. — end note ]

The exception with the most recently activated handler that is still active is called the currently handled exception.

The exception returned by current_exception() is defined as the "currently handled exception", which is the exception of the most recent active handler, and a handler is active only when stack unwinding completed.

As your tests have shown, there is no "active handler" during stack unwinding, so there is no "currently handled exception" either : in that case, current_exception() will return a null exception_ptr.

" there is no "active exception" during stack unwinding" unless an exception is being handled by a catch in or invoked from a destructor.
@Cheersandhth.-Alf True, I corrected the wording, please read no "active handler" (wording of the standard)
Well, it's so difficult to get right that Some™ suspect that might be the reason for std::uncaught_exception being unusable for any purpose. ;-)
That appears to be correct. I was surprised as well; I can see good uses for peaking at exceptions in d'tors; particularly RAII-based logging.

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