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Initialize struct fields

P: n/a
Hi,

how is it possible, to only initialize parts of a structure.

Example:

typedef struct{
int a, b;
... (huge lot of members);
double x,y;
}_s;

_s s={a=10,x=23.0};
This shall be done BEFORE any code is executed! I mean no initialize
functions!

Anyone an idea?
Andreas
Nov 13 '05 #1
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7 Replies


P: n/a
On Wed, 9 Jul 2003, Andi.Martin wrote:
how is it possible, to only initialize parts of a structure.


In C99, you can use designated initializers:

typedef struct { int a, b, c, d, e, f, g, h; } thing_t;
thing_t os = {
.b = 0,
.e = 5
};

Tak-Shing

Nov 13 '05 #2

P: n/a
"Andi.Martin" <An*********@freenet.de> writes:
how is it possible, to only initialize parts of a structure.

Example:

typedef struct{
int a, b;
... (huge lot of members);
double x,y;
}_s;

_s s={a=10,x=23.0};


If you mean, by "initialize parts of a structure", to specify
values for some members, and let the others receive the value 0
or a null pointer, then you can do it in C99 using the syntax
{.a = 10, .x = 23.0}
If you don't have a C99 compiler, you're out of luck. I suggest
putting the members you want to initialize at the beginning of
the structure.

If you mean, by "initialize parts of a structure", to specify
values for some members, and leave the other ones indeterminate,
there is no way to do that. C doesn't have partial
initialization in declarations: an object is either indeterminate
or fully initialized.

By the way, _s is a poor choice of names. Names beginning with
an underscore are generally reserved to the implementation.
--
"Your correction is 100% correct and 0% helpful. Well done!"
--Richard Heathfield
Nov 13 '05 #3

P: n/a
On Wed, 9 Jul 2003 22:17:25 +0200, "Andi.Martin"
<An*********@freenet.de> wrote:
_s s={a=10,x=23.0};


C99 allows:

_s s={.a=10, .x=23.0};

If your compiler is C90, then it is difficult. Can you move the
members around in the structure definition so that ones to be
initialized are first?

BTW, don't begin identifiers with underscores. They are reserved for
use in file scope for ordinary and tag name spaces.

Best wishes,

Bob
Nov 13 '05 #4

P: n/a
Andi.Martin wrote:
Hi,

how is it possible, to only initialize parts of a structure.

Example:

typedef struct{
int a, b;
... (huge lot of members);
double x,y;
}_s;

_s s={a=10,x=23.0};
This shall be done BEFORE any code is executed! I mean no initialize
functions!


Another common way to do this is to create a special instance of the structure
which is initialized to the values you want:

/* file scope */
struct _s init = {0};

/* in main */
init.a = 10;
init.x = 23.0;

/* in other functions */
struct _s s = init;

/david

--
FORTRAN was the language of choice
for the same reason that three-legged races are popular.
-- Ken Thompson, "Reflections on Trusting Trust"

Nov 13 '05 #5

P: n/a
In <be*************@news.t-online.com> "Andi.Martin" <An*********@freenet.de> writes:
how is it possible, to only initialize parts of a structure.

Example:

typedef struct{
int a, b;
... (huge lot of members);
double x,y;
}_s;

_s s={a=10,x=23.0};
This shall be done BEFORE any code is executed! I mean no initialize
functions!

Anyone an idea?


If you need a portable solution, your only chance is to put the members
that need initialisation at the beginning on the structure:

struct {
int a;
double x;
... (huge lot of members);
} s = {10, 23.0};

The members without an explicit initialiser will be initialised to the
right type of zero.

Dan
--
Dan Pop
DESY Zeuthen, RZ group
Email: Da*****@ifh.de
Nov 13 '05 #6

P: n/a
Da*****@cern.ch (Dan Pop) wrote in news:be**********@sunnews.cern.ch:
If you need a portable solution, your only chance is to put the
members that need initialisation at the beginning on the structure:

struct {
int a;
double x;
... (huge lot of members);
} s = {10, 23.0};

The members without an explicit initialiser will be initialised to the
right type of zero.

Dan


IIRC in C99 you can do struct { int a; double x; /* ... */ } s = {.a = 10,
..x = 23.0 };

Nov 13 '05 #7

P: n/a
In article <Xn***************************@167.206.3.3>,
gr*****@t2n.org says...
Da*****@cern.ch (Dan Pop) wrote in news:be**********@sunnews.cern.ch:
If you need a portable solution, your only chance is to put the
members that need initialisation at the beginning on the structure:

struct {
int a;
double x;
... (huge lot of members);
} s = {10, 23.0};

The members without an explicit initialiser will be initialised to the
right type of zero.

Dan


IIRC in C99 you can do struct { int a; double x; /* ... */ } s = {.a = 10,
.x = 23.0 };


Requiring C99 for your implementation (which is by no means portable
today) is the likely reason for Dan's alternate suggestion.
--
Randy Howard
remove the obvious bits from my address to reply.
Nov 13 '05 #8

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