Structure initialization

[CBFalconer]

Re: Structure initialization

John Bode wrote:

jb.si...@lmco.c om wrote:
>

>Recently I was pinged in a code review about my use of the
>initializati on method
>AStruct myStruct = { 0 } ;
>>
>Which initializes all elements of the myStruct to 0.
>>

.... snip ...

>>
>so it seems that the construct ={0}, sets the fisrst element to
>0 and since there are no more initializers present, all other
>structure members are set to zero by default. Is this correct ?

>
Sort of. Here's the language from n1124:
>
"6.7.8. 21 If there are fewer initializers in a brace-enclosed list
than there are
elements or members of an aggregate, or fewer characters in a string
literal used
to initialize an array of known size than there are elements in the
array, the
remainder of the aggregate shall be initialized implicitly the same
as objects that
have static storage duration."
>
And the section on initialization of static objects:
>
"10 If an object that has automatic storage duration is not
initialized explicitly,
its value is indeterminate. If an object that has static storage
duration is not
initialized explicitly, then:
— if it has pointer type, it is initialized to a null pointer;
— if it has arithmetic type, it is initialized to (positive or
unsigned) zero;
— if it is an aggregate, every member is initialized (recursively)
according to these rules;
— if it is a union, the first named member is initialized
(recursively) according to
these rules."

A little more readable if taken from N869.txt:

[#21] If there are fewer initializers in a brace-enclosed
list than there are elements or members of an aggregate, or
fewer characters in a string literal used to initialize an
array of known size than there are elements in the array,
the remainder of the aggregate shall be initialized
implicitly the same as objects that have static storage
duration.

and

[#10] If an object that has automatic storage duration is
not initialized explicitly, its value is indeterminate. If
an object that has static storage duration is not
initialized explicitly, then:

-- if it has pointer type, it is initialized to a null
pointer;
-- if it has arithmetic type, it is initialized to
(positive or unsigned) zero;
-- if it is an aggregate, every member is initialized
(recursively) according to these rules;
-- if it is a union, the first named member is initialized
(recursively) according to these rules.

See <http://cbfalconer.home .att.net/download/n869_txt.bz2>

--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home .att.net>
Try the download section.


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[santosh]

Re: Structure initialization

pete wrote:

santosh wrote:

>pete wrote:

>

>>If the only reason that a programmer
>>can't understand a particular C code construct,
>>is because the programmer doesn't know enough C,
>>then it's time to learn more C.

>>
>Only if your boss gives you time off to do so.

>
Do I understand you to mean
that if your boss saw you reading a C reference
while you were supposed to be writing C code,
that your boss would be displeased?
>
That notion seems bizarre to me.
What do you really mean?

I mean that here, outside of transnational companies, it's very unusual
in the middle to low end jobs here for your "boss" to let you take time
off, even if it might benefit your work efficiency in the long run.

Specifically, with regards to C, clc's standards are, IME _very_ high.
For example in the code bases I have had the opportunity to examine I
have never once come across the form of structure initialisation
demonstrated in this thread.

[none]

Re: Structure initialization

Harald van Dijk wrote:

On Tue, 10 Jun 2008 10:53:14 -0700, jb.simon wrote:

>so it seems that the construct ={0}, sets the fisrst element to 0 and
>since there are no more initializers present, all other structure
>members are set to zero by default. Is this correct ?

>
That is correct. And when the first member is an array or structure (or a
union starting with one of these), it applies to the remaining members or
elements of the first member as well. That is, when X is a simple integer
constant, possibly 0, possibly something else,
>
struct A {
struct B {
int a, b, c;
} s;
int d;
} a = {X};
>
initialises a.s.a to X, while a.s.b, a.s.c, and a.d are all given their
default values of 0.

Thanks for all of the replies.

I forwarded the relevant parts of the standard to the person who
commented, indicating it *is* standard, and then I also changed my code
to add functions (4 of them) to explicitly initialize all fields to 0.
It was not a big deal, however, now if any of those structures change
the appropriate initialization function also needs to change.

In a way this is really not so bad, because this supports an setting the
variables to non zero at initialization if desired.

P.S. The reason I put these in a function is these structures can be
kinda large (50-100 members) and i didn't want to clutter the caller
with all of the initialization. To me it makes it harder to read the
function if you have lines and lines of initializaiton code.

Thanks All !
Joe