Extracted uncaught_exceptions function from Boost.Log.

The uncaught_exceptions function is functionally equivalent to
unhandled_exceptions_count in Boost.Log and implements functionality
similar to the same named C++17 standard function. Tests and docs are
also included.

One notable difference from std::uncaught_exceptions is that the return
type is unsigned rather than signed. This is deliberate as uncaught_exceptions
must never return a negative value and unsigned int better documents that.
Theoretically, as a counter, it may also overflow.

test/uncaught_exceptions_np.cpp

@@ -0,0 +1,90 @@
+/*
+ *             Copyright Andrey Semashev 2018.
+ * Distributed under the Boost Software License, Version 1.0.
+ *    (See accompanying file LICENSE_1_0.txt or copy at
+ *          https://www.boost.org/LICENSE_1_0.txt)
+ */
+/*!
+ * \file   uncaught_exceptions_np.cpp
+ * \author Andrey Semashev
+ * \date   2018-11-10
+ *
+ * \brief  This file contains tests for the uncaught_exceptions function.
+ *
+ * This file contains checks that are compiler specific and not quite portable or require C++17.
+ */
+
+#include <boost/core/uncaught_exceptions.hpp>
+
+#if !defined(BOOST_CORE_UNCAUGHT_EXCEPTIONS_EMULATED)
+
+#include <boost/core/lightweight_test.hpp>
+
+struct my_exception1 {};
+struct my_exception2 {};
+
+class exception_watcher2
+{
+    unsigned int& m_count;
+
+public:
+    explicit exception_watcher2(unsigned int& count) : m_count(count) {}
+    ~exception_watcher2() { m_count = boost::core::uncaught_exceptions(); }
+};
+
+class exception_watcher1
+{
+    unsigned int& m_count1;
+    unsigned int& m_count2;
+
+public:
+    exception_watcher1(unsigned int& count1, unsigned int& count2) : m_count1(count1), m_count2(count2) {}
+    ~exception_watcher1()
+    {
+        m_count1 = boost::core::uncaught_exceptions();
+        try
+        {
+            exception_watcher2 watcher2(m_count2);
+            throw my_exception2();
+        }
+        catch (...)
+        {
+        }
+    }
+};
+
+// Tests for uncaught_exceptions when used in nested destructors while an exception propagates
+void test_in_nested_destructors()
+{
+    const unsigned int root_count = boost::core::uncaught_exceptions();
+
+    unsigned int level1_count = root_count, level2_count = root_count;
+    try
+    {
+        exception_watcher1 watcher1(level1_count, level2_count);
+        throw my_exception1();
+    }
+    catch (...)
+    {
+    }
+
+    BOOST_TEST_NE(root_count, level1_count);
+    BOOST_TEST_NE(root_count, level2_count);
+    BOOST_TEST_NE(level1_count, level2_count);
+}
+
+int main()
+{
+    test_in_nested_destructors();
+
+    return boost::report_errors();
+}
+
+#else
+
+int main()
+{
+    return 0;
+}
+
+#endif