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

From:
"Martin B." <0xCDCDCDCD@gmx.at>
Newsgroups:
comp.lang.c++.moderated
Date:
Sat, 30 Jan 2010 15:34:48 CST
Message-ID:
<hk1gv0$a3f$1@news.eternal-september.org>
jdm 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;
    }
};

(....)
(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(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)
    {

(.... snip identical code - better use a function)

     }

Can anyone suggest a better way to do this?


1) always catch by-ref or by-const-ref
2) Ask yourself why you have n different exceptions, when the handling
code apparently always is the same? If the handling code always is the
same, ONE exception type seems more appropriate.
3) If you really need differnt exc-types and multiple catches, but the
handling code is (nearly) the same, then encapsulate that in a (tmpl)
function.
4) You could use a vector of boost::variant for the different types
5) Have a look at boost::exception - it allows for adding arbitrary info
to it.

br,
Martin

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