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1132. JP-30: nested exceptions

Section: 17.9.8 [except.nested] Status: NAD Submitter: Seiji Hayashida Opened: 2009-06-01 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [except.nested].

View all issues with NAD status.

Discussion:

Addresses JP 30

C++0x nested_exception cannot handle a structured exception well. The following codes show two types of tree structured exception handling.

The first one is based on nested_exception in C++0x, while the second one is based on my library trickerr.h (in Japanese). http://tricklib.com/cxx/dagger/trickerr.h

Assume that Function A() calls two sub functions A_a() and A_b(), both might throw tree structured exceptions, and A_b() must be called even if A_a() throws an exception.

List A (code of tree structured exception handling based on nested_exception in C++0x)

void A()
{
    try
    {
        std::vector<exception_ptr> exception_list;
        try
        {
            // A_a() does a similar processing as A().
            A_a();
        }
        catch(...)
        {
            exception_list.push_back(current_exception());
        }

        // ***The processing A() has to do even when A_a() fails. ***
        try
        {
            // A_b() does a similar processing as A().
            A_b();
        }
        catch(...)
        {
            exception_list.push_back(current_exception());
        }
        if (!exception_list.empty())
        {
            throw exception_list;
        }
    }
    catch(...)
    {
        throw_with_nested(A_exception("someone error"));
    }
}
void print_tree_exception(exception_ptr e, const std::string & indent ="")
{
    const char * indent_unit = " ";
    const char * mark = "- ";
    try
    {
        rethow_exception(e);
    }
    catch(const std::vector<exception_ptr> e)
    {
        for(std::vector<exception_ptr>::const_iterator i = e.begin(); i!=e.end(); ++i)
        {
            print_tree_exception(i, indent);
        }
    }
    catch(const std::nested_exception  e)
    {
        print_tree_exception(evil_i(e), indent +indent_unit);
    }
    catch(const std::exception e)
    {
        std::cout << indent << mark << e.what() << std::endl;
    }
    catch(...)
    {
        std::cout << indent << mark << "unknown exception" << std::endl;
    }
}
int main(int, char * [])
{
    try
    {
        A();
    }
    catch()
    {
        print_tree_exception(current_exception());
    }
    return EXIT_SUCCESS;
}

List B ( code of tree structured exception handling based on trickerr.h. ) "trickerr.h" (in Japanese), refer to: http://tricklib.com/cxx/dagger/trickerr.h.

void A()
{
    tricklib::error_listener_type error_listener;
    // A_a() is like A(). A_a() can throw tree structured exception.
    A_a();

    // *** It must do process so that A_a() throws exception in A(). ***
    // A_b() is like A(). A_b() can throw tree structured exception.
    A_b();

    if (error_listener.has_error()) // You can write this "if block" in destructor
                                    //  of class derived from error_listener_type.
    {
        throw_error(new A_error("someone error",error_listener.listener_off().extract_pending_error()));
    }
}
void print_tree_error(const tricklib::error_type &a_error, const std::string & indent = "")
{
    const char * indent_unit = " ";
    const char * mark = "- ";

    tricklib::error_type error = a_error;
    while(error)
    {
        std::cout << indent << mark << error->message << std::endl;
        if (error->children)
        {
            print_tree_error(error->children, indent +indent_unit);
        }
        error = error->next;
    }
}
int main(int, char * [])
{
    tricklib::error_thread_power error_thread_power_on; // This object is necessary per thread.

    try
    {
        A();
    }
    catch(error_type error)
    {
        print_tree_error(error);
    }
    catch(...)
    {
        std::cout << "- unknown exception" << std::endl;
    }
    return EXIT_SUCCESS;
}

Prospect

We will focus on the method A() since the other methods, also main(), occur only once respectively.

• In the List A above (of the nested exception handling), it is hard to find out an active reason to use the nested exception handling at this scene. Rather, we can take a simpler description by throwing the entire exception_list directly to the top level.
• The code in the same example gives us a kind of redundant impression, which might have come from the fact that the try-throw-catch framework does not assume a tree structured exception handling.

According to the above observation, we cannot help concluding that it is not so easy to use the nested_exception handling as a tree structured exception handling mechanism in a practical sense.

This text is based on the web page below (in Japanese). http://d.hatena.ne.jp/wraith13/20081231/1230715424

[ 2009-10 Santa Cruz: ]

Mark as NAD. The committee agrees that nested_exception is not a good match for this usage model. The committee did not see a way of improving this within the timeframe allowed.

Proposed resolution: