Re: Program to compute elapsed time, learning structures and functions

From	Keith Thompson <kst-u@mib.org>
Newsgroups	comp.lang.c
Subject	Re: Program to compute elapsed time, learning structures and functions
Date	2015-10-09 14:06 -0700
Organization	None to speak of
Message-ID	<ln37xjg3b7.fsf@kst-u.example.com> (permalink)
References	<b151ca82-94b3-40ee-8444-2c0e61f5ed79@googlegroups.com> <mv8bku$22a$1@dont-email.me> <d861f166-2cc0-4879-80c3-68bb2759139d@googlegroups.com> <mv8lrv$chl$1@dont-email.me>

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Bartc <bc@freeuk.com> writes:
> On 09/10/2015 14:49, Alla _ wrote:
[...]
>> I don't see why the compiler is complaining about hour? Is it
>> because I don't have a space between first left curly brace and the dot?
>
> It doesn't like .hour because I believe that's only allowed, in this 
> context, within a compound literal. And those need a cast in front.

The existing code is (reformatted to avoid long lines):

    struct time result;
    // ...
    result = {.hour = time_2.hour - time_1.hour,
              .minutes = time_2.minutes - time_1.minutes,
              .seconds = time_2.seconds - time_1.seconds};

The correct version of that code would be:

    result = (struct time){.hour = time_2.hour - time_1.hour,
                           .minutes = time_2.minutes - time_1.minutes,
                           .seconds = time_2.seconds - time_1.seconds};

The parenthesized type name looks like a cast operator (and
deliberately so, since it does something fairly similar), but
that's not actually what it is.  It's part of the syntax of a
compound literal.

You can tell because a cast operator is always followed by an
expression, whereas the `(struct time)` construct here is followed
by something that *isn't* an expression.

Alla, you mentioned that you thought you could omit the type
because it's being assigned to an object of type struct time.
The thing to keep in mind is that, in almost all cases, the type
of an expression is determined by the expression itself, not by
the context in which it appears.  In principle a clever compiler
*could* infer the required type for your `{ ... }` based on what it's
assigned to -- but it wouldn't be a C compiler.  The parenthesized
type name is not optional.

Also, consider that compound literals were not introduced into the
language until the C99 standard.  Not all compilers implement them,
even now in 2015.  Whether you use them or not depends in part on
whether you care about compiling your code with pre-C99 compilers.
("No" is a perfectly good answer to that question.)

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

Thread

Program to compute elapsed time, learning structures and functions Alla _ <modelling.data@gmail.com> - 2015-10-09 02:36 -0700
  Re: Program to compute elapsed time, learning structures and functions Bartc <bc@freeuk.com> - 2015-10-09 10:48 +0100
    Re: Program to compute elapsed time, learning structures and functions Alla _ <modelling.data@gmail.com> - 2015-10-09 03:06 -0700
  Re: Program to compute elapsed time, learning structures and functions Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-09 11:01 +0100
    Re: Program to compute elapsed time, learning structures and functions Alla _ <modelling.data@gmail.com> - 2015-10-09 03:07 -0700
  Re: Program to compute elapsed time, learning structures and functions Richard Heathfield <rjh@cpax.org.uk> - 2015-10-09 13:26 +0100
    Re: Program to compute elapsed time, learning structures and functions Alla _ <modelling.data@gmail.com> - 2015-10-09 06:49 -0700
      Re: Program to compute elapsed time, learning structures and functions Bartc <bc@freeuk.com> - 2015-10-09 16:20 +0100
        Re: Program to compute elapsed time, learning structures and functions Alla _ <modelling.data@gmail.com> - 2015-10-09 09:27 -0700
        Re: Program to compute elapsed time, learning structures and functions Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-09 19:29 +0100
        Re: Program to compute elapsed time, learning structures and functions Keith Thompson <kst-u@mib.org> - 2015-10-09 14:06 -0700
          Re: Program to compute elapsed time, learning structures and functions Bartc <bc@freeuk.com> - 2015-10-09 22:24 +0100
            Re: Program to compute elapsed time, learning structures and functions Keith Thompson <kst-u@mib.org> - 2015-10-09 15:51 -0700
      Re: Program to compute elapsed time, learning structures and functions Richard Heathfield <rjh@cpax.org.uk> - 2015-10-09 18:26 +0100
        Re: Program to compute elapsed time, learning structures and functions Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-09 19:24 +0100
          Re: Program to compute elapsed time, learning structures and functions Richard Heathfield <rjh@cpax.org.uk> - 2015-10-09 19:50 +0100
            Re: Program to compute elapsed time, learning structures and functions Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-10-09 15:56 -0400
      Re: Program to compute elapsed time, learning structures and functions James Kuyper <jameskuyper@verizon.net> - 2015-10-09 15:31 -0400
      Re: Program to compute elapsed time, learning structures and functions Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-10 06:57 -0700
        Re: Program to compute elapsed time, learning structures and functions Bartc <bc@freeuk.com> - 2015-10-10 17:25 +0100
          Re: Program to compute elapsed time, learning structures and functions Bartc <bc@freeuk.com> - 2015-10-10 19:12 +0100
          Re: Program to compute elapsed time, learning structures and functions Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-10 20:56 +0100
            Re: Program to compute elapsed time, learning structures and functions Bartc <bc@freeuk.com> - 2015-10-10 21:52 +0100
          Re: Program to compute elapsed time, learning structures and functions Keith Thompson <kst-u@mib.org> - 2015-10-10 17:57 -0700
            Re: Program to compute elapsed time, learning structures and functions raltbos@xs4all.nl (Richard Bos) - 2015-10-11 10:09 +0000
            Re: Program to compute elapsed time, learning structures and functions Bartc <bc@freeuk.com> - 2015-10-11 11:26 +0100
              Re: Program to compute elapsed time, learning structures and functions Keith Thompson <kst-u@mib.org> - 2015-10-11 14:12 -0700
            Re: Program to compute elapsed time, learning structures and functions Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-15 10:51 -0700
              Re: Program to compute elapsed time, learning structures and functions Keith Thompson <kst-u@mib.org> - 2015-10-15 11:25 -0700
                Re: Program to compute elapsed time, learning structures and functions Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-16 05:34 -0700
                Re: Program to compute elapsed time, learning structures and functions Keith Thompson <kst-u@mib.org> - 2015-10-16 08:52 -0700
          Re: Program to compute elapsed time, learning structures and functions Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-15 10:46 -0700
            Re: Program to compute elapsed time, learning structures and functions Bartc <bc@freeuk.com> - 2015-10-15 19:06 +0100
              Re: Program to compute elapsed time, learning structures and functions Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-16 05:24 -0700
          Re: Program to compute elapsed time, learning structures and functions glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-17 00:10 +0000
      Re: Program to compute elapsed time, learning structures and functions Barry Schwarz <schwarzb@dqel.com> - 2015-10-18 09:52 -0700
  Re: Program to compute elapsed time, learning structures and functions Alla _ <modelling.data@gmail.com> - 2015-10-09 06:52 -0700
    Re: Program to compute elapsed time, learning structures and functions Barry Schwarz <schwarzb@dqel.com> - 2015-10-18 10:21 -0700
      Re: Program to compute elapsed time, learning structures and functions Alla _ <modelling.data@gmail.com> - 2015-11-05 00:56 -0800
        Re: Program to compute elapsed time, learning structures and functions Barry Schwarz <schwarzb@dqel.com> - 2015-11-05 01:55 -0800
          Re: Program to compute elapsed time, learning structures and functions Alla _ <modelling.data@gmail.com> - 2015-11-05 02:55 -0800
          Re: Program to compute elapsed time, learning structures and functions Alla _ <modelling.data@gmail.com> - 2015-11-05 03:01 -0800
            Re: Program to compute elapsed time, learning structures and functions Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-05 03:41 -0800
              Re: Program to compute elapsed time, learning structures and functions Alla _ <modelling.data@gmail.com> - 2015-11-05 06:55 -0800
          Re: Program to compute elapsed time, learning structures and functions Alla _ <modelling.data@gmail.com> - 2015-11-05 03:03 -0800
            Re: Program to compute elapsed time, learning structures and functions Richard Heathfield <rjh@cpax.org.uk> - 2015-11-05 13:27 +0000
              Re: Program to compute elapsed time, learning structures and functions Alla _ <modelling.data@gmail.com> - 2015-11-05 06:54 -0800
        Re: Program to compute elapsed time, learning structures and functions Keith Thompson <kst-u@mib.org> - 2015-11-05 12:42 -0800
          Re: Program to compute elapsed time, learning structures and functions Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-05 15:50 -0800

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