Re: Advice on using templates and vector to improve exception handling

From:
Goran <goran.pusic@gmail.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Sat, 30 Jan 2010 15:34:40 CST
Message-ID:
<58f3bc17-5939-4b7e-b63f-64a969a08ea7@m25g2000yqc.googlegroups.com>
On Jan 30, 1:37 am, jdm <james.d.mclaugh...@googlemail.com> wrote:

Hi,

I've been trying to improve on my program's exception handling, due to
the amount of code duplication resulting from the following (not to
mention the need to recode when the relevant data required five
variables):

struct three_long_int_misc_exception
{
        std::string ident_string;
        long int err_number1, err_number2, err_number3;
        three_long_int_misc_exception(std::string i="Unknown exception
(three_long_int_misc_exception).", long int x=0, long int y=0, long
int z=0)
        {
                ident_string = i;
                err_number1 = x;
                err_number2 = y;
                err_number3 = z;
        }

};

struct three_unsigned_long_int_misc_exception
{
        std::string ident_string;
        unsigned long int err_number1, err_number2, err_number3;
        three_unsigned_long_int_misc_exception(std::string i="Unknown
exception (three_unsigned_long_int_misc_exception).", unsigned long
int x=0, unsigned long int y=0, unsigned long int z=0)
        {
                ident_string = i;
                err_number1 = x;
                err_number2 = y;
                err_number3 = z;
        }

};

struct four_long_int_misc_exception
{
        std::string ident_string;
        long int err_number1, err_number2, err_number3, err_number4;
        four_long_int_misc_exception(std::string i="Unknown exception
(four_long_int_misc_exception).", long int w=0, long int x=0, long int
y=0, long int z=0)
        {
                ident_string = i;
                err_number1 = w;
                err_number2 = x;
                err_number3 = y;
                err_number4 = z;
        }

};

(There were also structs for when the exception data consisted only of
one or two ints.)

Although the following was a bit more complicated to throw, it seemed
a better idea:

template <typename T> struct multiple_integer_misc_exception
{
        std::string ident_string;
        std::vector<T> error_numbers;

        multiple_integer_misc_exception()
        {
                ident_string = "";
                error_numbers = std::vector<T>(1);
                error_numbers[0] = 0;
        }

        multiple_integer_misc_exception(std::vector<T>& v, std::string
s="Unknown exception (multiple_integer_misc_exception).")
        {
                ident_string = s;
                error_numbers = std::vector<T>(v.size());
                for (int i=0; i < v.size(); i++)
                {
                        error_numbers[i] = v[i];
                }
        }

};

However, this doesn't seem to eliminate much of the code duplication
involved in catching the exceptions, as the main() function now needs
to include:

        catch (multiple_integer_misc_exception<long int> lmime) //Is there a
better way than these two almost identical catches?
        {
                std::cout<<'\n'<<lmime.ident_string<<'\t';
                for (int i=0; i<lmime.error_numbers.size(); i++)
                {
                        std::cout<<lmime.error_numbers[i]<<' ';
                }
                std::cout<<std::endl;
                return 1;
        }
        catch (multiple_integer_misc_exception<unsigned long int> ulmime)
        {
                std::cout<<'\n'<<ulmime.ident_string<<'\t';
                for (int i=0; i<ulmime.error_numbers.size(); i++)
                {
                        std::cout<<ulmime.error_numbers[i]<<' ';
                }
                std::cout<<std::endl;
                return 1;
        }

Can anyone suggest a better way to do this?


First, obvious complaint: you better have a god damn good reason not
to catch a const exc_type& there (especially if you plan to copy a
vector!).

Second, what exactly are you doing with the exceptions once they're
caught? If only read their data for display, then how about mandating
base class with a method that provides text for "display" (IOW, it's
no wonder that one of rare virtual methods in STL is exception::what()
- such usage is really that common). Note that this simple abstraction
removes any need to care about quantity and types of your exception
members at catch site. If you have more operations on your exception
types, you have two options: either add that as virual methods in the
base class, either make "processing" functions that operate on
specific exception types e.g. using dynamic_cast.

HTH,
Goran.

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