std::uncaught_exception,std::uncaught_exceptions(3)

NAME
std::uncaught_exception,std::uncaught_exceptions - std::uncaught_exception,std::uncaught_exceptions

Synopsis
   Defined in header <exception>
   bool uncaught_exception() throw(); (until C++11)
                        (since C++11)
   bool uncaught_exception() noexcept; (1) (deprecated in C++17)
                        (removed in C++20)
   int uncaught_exceptions() noexcept; (2) (since C++17)
   1) Detects if the current thread has a live exception object, that is, an exception
   has been thrown or rethrown and not yet entered a matching catch clause,
   std::terminate or std::unexpected. In other words, std::uncaught_exception detects
   if stack unwinding is currently in progress.
   2) Detects how many exceptions in the current thread have been thrown or rethrown
   and not yet entered their matching catch clauses.
   Sometimes it's safe to throw an exception even while std::uncaught_exception() ==
   true. For example, if stack unwinding causes an object to be destructed, the
   destructor for that object could run code that throws an exception as long as the
   exception is caught by some catch block before escaping the destructor.

Parameters
   (none)

Return value
   1) true if stack unwinding is currently in progress in this thread, false otherwise.
   2) The number of uncaught exception objects in the current thread.

Notes
   An example where int-returning uncaught_exceptions is used is the boost.log library:
   the expression BOOST_LOG(logger) << foo(); first creates a guard object and records
   the number of uncaught exceptions in its constructor. The output is performed by the
   guard object's destructor unless foo() throws (in which case the number of uncaught
   exceptions in the destructor is greater than what the constructor observed).
   std::experimental::scope_fail and std::experimental::scope_success in LFTS v3 rely
   on the functionality of uncaught_exceptions, because their destructors need to do
   different things that depend on whether is called during stack unwinding.
        Feature-test macro Value Std Feature
   __cpp_lib_uncaught_exceptions 201411L (C++17) std::uncaught_exceptions

Example
// Run this code
 #include <exception>
 #include <iostream>
 #include <stdexcept>
 struct Foo
 {
     char id{'?'};
     int count = std::uncaught_exceptions();
     ~Foo()
     {
         count == std::uncaught_exceptions()
             ? std::cout << id << ".~Foo() called normally\n"
             : std::cout << id << ".~Foo() called during stack unwinding\n";
     }
 };
 int main()
 {
     Foo f{'f'};
     try
     {
         Foo g{'g'};
         std::cout << "Exception thrown\n";
         throw std::runtime_error("test exception");
     }
     catch (const std::exception& e)
     {
         std::cout << "Exception caught: " << e.what() << '\n';
     }
 }

Possible output:
 Exception thrown
 g.~Foo() called during stack unwinding
 Exception caught: test exception
 f.~Foo() called normally
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
     DR Applied to Behavior as published Correct behavior
   LWG 70 C++98 the exception specification of specified as throw()
                     uncaught_exception() was missing

See also
   terminate function called when exception handling fails
                     (function)
   exception_ptr shared pointer type for handling exception objects
   (C++11) (typedef)
   current_exception captures the current exception in a std::exception_ptr
   (C++11) (function)

External links
   1. GOTW issue 47: Uncaught Exceptions
   2. Rationale for std::uncaught_exceptions (N4125)