Re: Passing an array of structuresfrom a pointer?
- From: Barry Schwarz <schwarzb@xxxxxxxx>
- Date: Sun, 28 Sep 2008 16:59:07 -0700
On Sun, 28 Sep 2008 15:51:00 -0700, Robby
<Robby@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
Hello,
I have a two part question. I do understand the principle of an array like so:
int xxx[10];
where xxx holds the address of the first item. But, when using this analogy
First problem: xxx does not hold the address of the first item. In
many (but not all) contexts, the expression xxx is ***converted*** to
the address of the first item, equivalent to the expression &xxx[0].
with arrays of structures, I feel a little unsure! Anyways, here is my
problem!
-[Q#1]-
I am trying to pass the address of an array of structures, that have already
been declared in one of my headers. I don't know... but I feel like I am
It is extremely undesirable that an array of struct be defined in a
header. Headers contain typedef definitions, external declarations,
function prototypes, and macro definitions but hardly ever object
definitions.
drawing a blank ! I know I did similar stuff like this but not from the same
exact perspective and the fact that I need to pass the address of an array of
strucutes somehow confuses me! Basically I need to pass the address of the
array of structures to a fiunction that will then store this address in
another structure for later use.
There is no difference in passing an array of struct to a function
than in passing any other array (do you use strcpy or sprintf or
sscanf). Arrays are passed by value and the actual value used is the
address of the first element of the array.
If you have a declaration of the form
struct x{...};
and an array definition of the form
struct x y[N];
and a function that expects such an array (that is the function
prototype specifies either
void func(struct x []...);
or
void func(struct x *...);
which are equivalent as noted in my first comment and where the return
type and other parameters are not part of this discussion), then you
pass the array of struct to the function with
func(y ...);
Here's what I am trying to accomplish! In a header file I will be declaring
several strucutures that have several members. Furthermore, an array for
every one of these structures will be declared. For example, as shown below,
Don't define arrays in headers. How will you avoid the duplicate
definition problem if your code has more than once source file? Arrays
should be defined in the primary function that uses them and passed to
any secondary functions as arguments. If you absolutely must have
global arrays, declare them as external in the header and define them
in the same source file as main.
the "ddlbColl1" structure will have an array of itself declared as
"DDLB_COLL1". In my example below I only declared one (1) such strucure
called "ddlbColl1", but someday I could end up having 2, 3 or even 5 of them!
I'm sorry but this makes no sense. A structure may not contain
itself. The significance of the case change is not apparent.
The thing is, that in my main calling function I want to configure all the
necessary components for my drop down list box by calling the
ULC_DDLB_config_ddlb ( ) function which will setup informations in the
"ddListBox" structure . One of the parameters of this function will be the
desired array of strucutres collections such as the DDLB_ARY[] !
Then once back in main I would call the:
ULC_DDLB_ddlb (obj_DDLB);
function and pass the pointer to a ddListBox structure and manipulate its
data, such as the data from the DDLB_COLL1 array within the function's
implementation code.
-[Q#2]-
When I am in the void ULC_DDLB_ddlb () function, will I be able to reference
data from the DDLB_COLL1 array? I believe that to retreive or modify the data
in the DDLB_COLL1 array I can do it like this: right?
DDLB_COLL1[1]. LIST_NUMBER;
I could be wrong, this is the first time I am passing arrays around this
way!!!!
Here's the code sample, I really am unsure of the lines flagged with "[***]"
!!!
All help is appreciated!
==============================================
typedef struct ddListBox{
int x;
long y;
struct ddlbColl1 DDLB_ARY[]; //Store address of DDLB_COLL1 here ! [***]
If you want to store an address here, use a pointer, not an array.
} DDLB;
DDLB *obj_DDLB;
Do not define objects in your header. However, do declare obj_DDLB as
an external pointer with
extern DDLB *obj_DDLB;
so that when you define it in a source file other source files will
have access to it. (Ignoring the issue of whether a global pointer is
a good idea or not.)
struct ddlbColl1{
This struct must be declared prior to struct ddListBox.
long LINK_ID;
int LIST_NUMBER;
} DDLB_COLL1[] =
Do not define any objects of type struct ddbColl1 in this header. Save
the definitions for a source (.c or .cpp) file. Since you only use
this object in main, you should define it there.
{
200, 4,
201, 8};
DDLB* ULC_DDLB_config_ddlb
Surely this function definition is not in a header but in a separate
source file.
( struct ddlbColl1 DDLB_ARY[], int x, int y)
{
obj_DDLB = NULL;
If you put the previous definition of obj_DDLB at file scope in this
source file, it will be global.
obj_DDLB = (DDLB*) malloc (sizeof (struct ddListBox));
It would be a good idea to loose the cast since it is unnecessary in C
(in C++ you would use new) and has undesirable side effects.
obj_DDLB->x = x;
obj_DDLB->y = y;
obj_DDLB->DDLB_ARRAY = DDLB_ARY; //Store address of DDLB_COLL1 ! [***]
obj_DDLB is a pointer to struct ddListBox. There is no member
DDLB_ARRAY in that struct. Did you mean DDLB_ARY? Assuming yes, this
is a syntax error. An array cannot be the left operand of the
assignment operator. If you change the member to a pointer as
suggested, then this will work.
It works because the right hand operand (the parameter DDLB_ARY - it
would have been nice if you had used a different name) is actually a
pointer (even though it looks like an array). You can obviously
assign a pointer value to a pointer.
return obj_DDLB;
Since you return this value, it seems to me that you really don't need
obj_DDLB to be global. In that case, you should delete its
declaration from your header and define it local to this function.
}
void ULC_DDLB_ddlb ( DDLB *obj_DDLB)
{
// This is the implementation file where I would like to use the
informations from
// the DDLB_COLL1 array!
Go right ahead.
1 - The declaration of obj_DDLB as a parameter to this function will
completely hide the global object with the same name. Further proof
that you don't need the global object.
2 - You can refer to the elements of the array using normal subscript
notation, obj_DDLB[0] is the first element. The fact that each
element is a struct doesn't matter until you start to access the
members of the struct. Then you have operands of the form
obj_DDLB[3].x
}
int main()
{
Since you don't need obj_DDLB to be global, you should define it here.
You will end up with three objects with that name, one each in main,
ULC_DDLB_config_ddlb, and ULC_DDLB_ddlb. Since they all have
different scope, there is no problem, However, to avoid confusion, you
might want to give the object in each function a slightly different
name.
obj_DDLB = ULC_DDLB_config_ddlb ( DDLB_COLL1, 10, 5);
ULC_DDLB_ddlb (obj_DDLB);
return 0;
}
====================================================
If this is too complicated I can simply find an easier more ineffecient way,
It isn't complicated at all. Your only problem was trying to declare
the struct member as an array instead of a pointer possibly compounded
by defining objects in your header.
but I really find this would be a good fit for my project since all I have to
do, is set my drop down list box once with the ULC_DDLB_config_ddlb()
function and then just fetch the ULC_DDLB_ddlb(obj_DDLB) function and access
all the info's via the obj_DDLB pointer using the points to operator "->".
I really would appreciate any help on how I can store the address of an
array of structures in the respective places in the above example flagged by
[***] !
Sincerely, thanking all in advance!
PS. PLease note, I haven't shown the code for any testing of pointers
returned by malloc or any free()'s in order to keep the code short and simple!
--
Remove del for email
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