Re: Why leave the error handling to the caller?


"Richard Heathfield" <rjh@xxxxxxxxxxxxxxx> wrote in message news:crOdnbKFN_t84-bbnZ2dnUVZ8vmdnZ2d@xxxxxxxxx
Malcolm McLean said:

<snip>

No, the specification was to produce a function that filled out the
structure giving a list of strings, from an input consisting of a line
of strings separated by commas.

Separate the concept of a list of strings from the concept of input
parsing.

Now separate the concept of a list from the concept of a string.

Now the error handling is becoming manageable (you will recall that you
originally wrote that code to demonstrate that the error handling was a
PITN to write), and the program's complexity is being reduced as more
and more work is farmed out to re-usable library modules.

What you end up with is rather elegant (if done properly), and eminently
re-usable.

The problem is you start introducing dependencies.
The example is a little bit artificial because we haven't said what we are using our list of strings for.
However a programming interface

STRINGS *list = makestrings("One,two,three,four");
if(!list)
/* that old problem again - a 2GB computer is more likely to break
than to execute this */
exit(EXIT_FAILURE);
/* use strings */
killstrings(list);

is perfectly reasonable; you might want to pass a series of strings in one go and one parameter rather than mess about with add_string functions. Certainly an object constructor very often needs to take a list.

The question is the internals. High-level languages achieve their neatness by using dynamic arrays. We can do that in C, but only by seriously messing about with the language.

/*
add an item to a dynamic list.
Params: ptr - pointer allocated with malloc(), can be null
N - number of items pointer currently holds
item - item to add to list
sz - element size
Returns: pointer to new array on success, 0 on fail.
*/
void *dlist_add(void *ptr, int N, void *item, size_t sz)
{
unsigned char *temp;

temp = realloc(ptr, (N + 1) * sz);
if(!temp)
return 0;
memcpy(temp + N * sz, item, sz);
return temp;
}

Now we've got the equivalent of an STL-style vector, but it is horrid, horrid. No one is going to use that.
Then if we put dlist_add into a separate file, suddenly we got a dependency, and the old problem of you want a list of strings and that requires pulling in a whole hierarchy of files.

--
Free games and programming goodies.
http://www.personal.leeds.ac.uk/~bgy1mm

.

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