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.

doc/core.qbk

@@ -57,3 +57,4 @@ criteria for inclusion is that the utility component be:
 [include scoped_enum.qbk]
 [include swap.qbk]
 [include typeinfo.qbk]
+[include uncaught_exceptions.qbk]

doc/uncaught_exceptions.qbk

@@ -0,0 +1,52 @@
+[/
+ / Copyright (c) 2018 Andrey Semashev
+ /
+ / Distributed under the Boost Software License, Version 1.0. (See accompanying
+ / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+ /]
+
+[section:uncaught_exceptions uncaught_exceptions]
+
+[simplesect Authors]
+
+* Andrey Semashev
+
+[endsimplesect]
+
+[section Header <boost/core/uncaught_exceptions.hpp>]
+
+The header `<boost/core/uncaught_exceptions.hpp>` defines the `boost::core::uncaught_exceptions` function,
+which is a more portable implementation of the same named function introduced in C++17. The function
+returns the number of the currently pending exceptions. When that function returns a value greater than 0,
+throwing an exception from a destructor can terminate the program.
+
+Unfortunately, the function cannot be implemented on every pre-C++17 compiler, although the most commonly
+used compilers are supported. When the compiler does not provide the necessary functionality,
+`boost::core::uncaught_exceptions` returns a non-zero value if at least one exception is pending (i.e. not
+necessarily the number of pending exceptions), and `BOOST_CORE_UNCAUGHT_EXCEPTIONS_EMULATED` macro
+is defined.
+
+[section Example]
+``
+class my_class
+{
+private:
+    const unsigned int m_exception_count;
+
+public:
+    my_class() : m_exception_count(boost::core::uncaught_exceptions())
+    {
+    }
+
+    ~my_class() noexcept(false)
+    {
+        if (m_exception_count == boost::core::uncaught_exceptions())
+            do_something_potentially_throwing();
+    }
+};
+``
+[endsect]
+
+[endsect]
+
+[endsect]

include/boost/core/uncaught_exceptions.hpp

@@ -0,0 +1,93 @@
+/*
+ *             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.hpp
+ * \author Andrey Semashev
+ * \date   2018-11-10
+ *
+ * \brief  This header provides an `uncaught_exception` function implementation, which was introduced in C++17.
+ *
+ * The code in this file is based on the implementation by Evgeny Panasyuk:
+ *
+ * https://github.com/panaseleus/stack_unwinding/blob/master/boost/exception/uncaught_exception_count.hpp
+ */
+
+#include <exception>
+#include <boost/config.hpp>
+
+#if defined(BOOST_HAS_PRAGMA_ONCE)
+#pragma once
+#endif
+
+namespace boost {
+
+namespace core {
+
+namespace detail {
+
+// cxxabi.h availability macro
+#if defined(__has_include) && (!defined(BOOST_GCC) || (__GNUC__ + 0) >= 5)
+#   if __has_include(<cxxabi.h>)
+#       define BOOST_CORE_HAS_CXXABI_H
+#   endif
+#elif defined(__GLIBCXX__) || defined(__GLIBCPP__)
+#   define BOOST_CORE_HAS_CXXABI_H
+#endif
+
+#if defined(__cpp_lib_uncaught_exceptions) && __cpp_lib_uncaught_exceptions >= 201411
+#define BOOST_CORE_HAS_UNCAUGHT_EXCEPTIONS
+#elif defined(BOOST_CORE_HAS_CXXABI_H)
+// MinGW GCC 4.4 seem to not work the same way the newer GCC versions do. As a result, __cxa_get_globals based implementation will always return 0.
+// Just disable it for now and fall back to std::uncaught_exception().
+#if !defined(__MINGW32__) || (defined(__GNUC__) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 5)))
+// Only GCC 4.7 declares __cxa_get_globals() in cxxabi.h, older compilers do not expose this function but it's there
+#define BOOST_CORE_HAS_CXA_GET_GLOBALS
+extern "C" void* __cxa_get_globals();
+#endif
+#elif defined(_MSC_VER)
+#if _MSC_VER >= 1900
+// Visual Studio 14 supports N4152 std::uncaught_exceptions()
+#define BOOST_CORE_HAS_UNCAUGHT_EXCEPTIONS
+#elif _MSC_VER >= 1400
+#define BOOST_CORE_HAS_GETPTD
+extern "C" void* _getptd();
+#endif
+#endif
+
+} // namespace detail
+
+#if !defined(BOOST_CORE_HAS_UNCAUGHT_EXCEPTIONS) && !defined(BOOST_CORE_HAS_CXA_GET_GLOBALS) && !defined(BOOST_CORE_HAS_GETPTD)
+//! This macro is defined when `uncaught_exceptions` is not guaranteed to return values greater than 1 if multiple exceptions are pending
+#define BOOST_CORE_UNCAUGHT_EXCEPTIONS_EMULATED
+#endif
+
+//! Returns the number of currently pending exceptions
+inline unsigned int uncaught_exceptions() BOOST_NOEXCEPT
+{
+#if defined(BOOST_CORE_HAS_UNCAUGHT_EXCEPTIONS)
+    // C++17 implementation
+    return static_cast< unsigned int >(std::uncaught_exceptions());
+#elif defined(BOOST_CORE_HAS_CXA_GET_GLOBALS)
+    // Tested on {clang 3.2,GCC 3.5.6,GCC 4.1.2,GCC 4.4.6,GCC 4.4.7}x{x32,x64}
+    return *(reinterpret_cast< const unsigned int* >(static_cast< const char* >(boost::core::detail::__cxa_get_globals()) + sizeof(void*))); // __cxa_eh_globals::uncaughtExceptions, x32 offset - 0x4, x64 - 0x8
+#elif defined(BOOST_CORE_HAS_GETPTD)
+    // MSVC specific. Tested on {MSVC2005SP1,MSVC2008SP1,MSVC2010SP1,MSVC2012}x{x32,x64}.
+    return *(reinterpret_cast< const unsigned int* >(static_cast< const char* >(boost::core::detail::_getptd()) + (sizeof(void*) == 8 ? 0x100 : 0x90))); // _tiddata::_ProcessingThrow, x32 offset - 0x90, x64 - 0x100
+#else
+    // Portable C++03 implementation. Does not allow to detect multiple nested exceptions.
+    return static_cast< unsigned int >(std::uncaught_exception());
+#endif
+}
+
+#undef BOOST_CORE_HAS_CXXABI_H
+#undef BOOST_CORE_HAS_CXA_GET_GLOBALS
+#undef BOOST_CORE_HAS_UNCAUGHT_EXCEPTIONS
+#undef BOOST_CORE_HAS_GETPTD
+
+} // namespace core
+
+} // namespace boost

test/Jamfile.v2

@@ -138,5 +138,8 @@ run test_lib_typeid.cpp lib_typeid : : : <link>static : test_lib_typeid_static ;
 run test_lib_typeid.cpp lib_typeid : : : <link>shared <rtti>off : test_lib_typeid_shared_no_rtti ;
 run test_lib_typeid.cpp lib_typeid : : : <link>static <rtti>off  : test_lib_typeid_static_no_rtti ;
 
+run uncaught_exceptions.cpp : : : <exception-handling>on ;
+run uncaught_exceptions_np.cpp : : : <exception-handling>on ;
+
 use-project /boost/core/swap : ./swap ;
 build-project ./swap ;

test/uncaught_exceptions.cpp

@@ -0,0 +1,55 @@
+/*
+ *             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.cpp
+ * \author Andrey Semashev
+ * \date   2018-11-10
+ *
+ * \brief  This file contains tests for the uncaught_exceptions function.
+ *
+ * This file only contains the very basic checks of functionality that can be portably achieved
+ * through C++03 std::uncaught_exception.
+ */
+
+#include <boost/core/uncaught_exceptions.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+struct my_exception {};
+
+class exception_watcher
+{
+    unsigned int& m_count;
+
+public:
+    explicit exception_watcher(unsigned int& count) : m_count(count) {}
+    ~exception_watcher() { m_count = boost::core::uncaught_exceptions(); }
+};
+
+// Tests for uncaught_exceptions when used in a destructor while an exception propagates
+void test_in_destructor()
+{
+    const unsigned int root_count = boost::core::uncaught_exceptions();
+
+    unsigned int level1_count = root_count;
+    try
+    {
+        exception_watcher watcher(level1_count);
+        throw my_exception();
+    }
+    catch (...)
+    {
+    }
+
+    BOOST_TEST_NE(root_count, level1_count);
+}
+
+int main()
+{
+    test_in_destructor();
+
+    return boost::report_errors();
+}

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