On Fri, 19 Sep 2003 11:59:52 -0400, Eric Sosman <Eric.Sosman@sun.com>
wrote in comp.lang.c:
[color=blue]
> Joona I Palaste wrote:[color=green]
> >
> > dreamcatcher <member39454@dbforums.com> scribbled the following:[color=darkred]
> > > I always have this idea that typedef a data type especially a structure
> > > is very convenient in coding, but my teacher insisted that I should use
> > > the full struct declaration and no further explanations, so I wonder is
> > > there any good using typedef ? and I also know that when a data type
> > > being typedefed become an abstract data type, so what exactly is an
> > > abstract data type, is it any good ?[/color]
> >
> > Typedeffing structs is a bad idea, but there *are* good uses for
> > typedef. Particularly with function pointers. [...][/color]
>
> Using or avoiding typedef with struct and union types
> seems to be a matter of taste. I haven't seen any solid
> technical reason to favor one practice over the other, so
> it comes down to "de gustibus." (Personally, I'm in the
> use-the-typedef camp -- but I have no particular quarrel
> with people who feel otherwise.)
>
> In addition to Joona's example of using typedef to
> clarify gnarly function declarations, another place I find
> typedef useful is in accommodating the uncertainties about
> the sizes of C's types. For example, if I need an integer
> type capable of representing values up to one million, it
> is unsafe to use `int' because it might only go as high as
> 32767. `long' will go to at least 2147483647 and will
> therefore suffice, but on some machines `long' will be
> serious overkill and a waste of space. If I need to store a
> large quantity of these numbers, I'd like to use the smallest
> possible type.
>
> Preprocessor tests coupled with typedef provide a way to
> handle this portably and with minimal ugliness:
>
> #include <limits.h>
> #if CHAR_MIN <= -1000000 && 1000000 <= CHAR_MAX
> typedef char Million;
> #elif SHRT_MIN <= -1000000 && 1000000 <= SHRT_MAX
> typedef short Million;
> #elif INT_MIN <= -1000000 && 1000000 <= INT_MAX
> typedef int Million;
> #else
> typedef long Million; /* known to suffice */
> #endif
>
> Thereafter, I can just write `Million' whenever I mean "a
> sufficiently but not unnecessarily wide integer:"
>
> Million *ptr = malloc(1234567 * sizeof *ptr);
> Million smallest, largest;
> Million func(void);
>
> ... and so on; the result of all the grungy decision-making
> has been conveniently packaged into the single word `Million'.
>
> One shortcoming, though, is illustrated by this code:
>
> Million value = 42;
> printf ("value = %d\n", value); /* WRONG */
>
> The problem is that if `Million' actually turns out to be an
> alias for `long', the "%d" format specifier ought to be "%ld"
> instead. One way to deal with this is to define a FMT_MILLION
> macro as either "d" or "ld" in the testing above; you could
> then write
>
> printf ("value = %" FMT_MILLION "\n", value);
>
> This works, but is clumsy to write and makes a mess for some
> kinds of tools that assist with translating message strings
> to multiple languages. I prefer the simpler
>
> printf ("value = %ld\n", (long)value);
>
> ... even though it may cost a little more at run time.
>
> In short: typedef is convenient when you'd like to hide
> the details of some type decisions from the eventual user
> (possibly yourself).[/color]
You realize, I hope, that the current C standard has done all of this
work for you?
You can include <inttypes.h> and use:
int_least32_t value;
....to define your type, and:
printf ("value = %" PRIdLEAST16 "\n", value);
....to print it, without creating your own type definitions and macros?
Note that it is quite possible and relatively easy to create your own
<stdint.h> and <inttypes.h> headers, at least up to 32 bit types, for
any conforming C89/90 compiler? If you do it properly all code that
uses them will still work just fine on an actual C99 compiler that
provides them.
--
Jack Klein
Home:
http://JK-Technology.Com
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