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passing copy of a pointer to a variable vs passing the copy

which one do you think is better ? I need to make my program efficient
and in some places I have passed the copy of a variable which makes
life some what easy while writing huge expressions but they do requrie
much more memory than a simple pointer.
Jun 27 '08 #1
7 2123
On May 30, 9:20 am, pereges <Brol...@gmail.comwrote:
which one do you think is better ? I need to make my program efficient
and in some places I have passed the copy of a variable which makes
life some what easy while writing huge expressions but they do requrie
much more memory than a simple pointer.
Can you give an example? What sort of variable?

-David
Jun 27 '08 #2
pereges <Br*****@gmail.comwrote:
which one do you think is better ? I need to make my program efficient
and in some places I have passed the copy of a variable
In C you can only pass copies of variables to functions.
which makes life some what easy while writing huge expressions
I what respect?
but they do requrie much more memory than a simple pointer.
Which variables take much more memory than a simple pointer?
Most variables you can pass to afunction are at least of
comparable size to the one of a pointer. The only exception
is structures, which can definitely be quite a lot larger than
pointers, and that's why they often get passed via a pointer
instead of by value, despite C allowing it.

But I guess I am misunderstanding your intents. Could you
post some code that illustrates what yiu are doing?

And, of course, the usual warning: don't optimize prematurely
but instead measure where the bottle-necks are in your program.

Regards, Jens
--
\ Jens Thoms Toerring ___ jt@toerring.de
\__________________________ http://toerring.de
Jun 27 '08 #3
On May 30, 7:14 pm, j...@toerring.de (Jens Thoms Toerring) wrote:
Which variables take much more memory than a simple pointer?
Most variables you can pass to afunction are at least of
comparable size to the one of a pointer. The only exception
is structures, which can definitely be quite a lot larger than
pointers, and that's why they often get passed via a pointer
instead of by value, despite C allowing it.

But I guess I am misunderstanding your intents. Could you
post some code that illustrates what yiu are doing?

And, of course, the usual warning: don't optimize prematurely
but instead measure where the bottle-necks are in your program.
Yes, I'm talking about a structure. For example I want to store a mesh
containing close to 200,000 vertices and 400,000 triangles. Here's my
data structure:

typedef struct vector_struct
{
double x, y, z;
}vector;

typedef vector vertex;

typedef struct triangle_struct
{
int v[3];
vector normal;
}triangle;

/* ======== Data structure for the mesh ========== */

typedef struct object_struct
{
vector *vertexlist;
triangle *trianglelist;
unsigned long int nvert;
unsigned long int ntri;
}object;
Jun 27 '08 #4
On May 30, 7:23 pm, pereges <Brol...@gmail.comwrote:
Yes, I'm talking about a structure. For example I want to store a mesh
containing close to 200,000 vertices and 400,000 triangles. Here's my
data structure:

typedef struct vector_struct
{
double x, y, z;

}vector;

typedef vector vertex;

typedef struct triangle_struct
{
int v[3];
vector normal;

}triangle;

/* ======== Data structure for the mesh ========== */

typedef struct object_struct
{
vector *vertexlist;
triangle *trianglelist;
unsigned long int nvert;
unsigned long int ntri;

}object;
In this mesh structure, vertexlist and triangle list are pointers to
the array of vertices and triangles respectively.
Jun 27 '08 #5
pereges <Br*****@gmail.comwrote:
On May 30, 7:23 pm, pereges <Brol...@gmail.comwrote:
Yes, I'm talking about a structure. For example I want to store a mesh
containing close to 200,000 vertices and 400,000 triangles. Here's my
data structure:

typedef struct vector_struct
{
double x, y, z;

}vector;

typedef vector vertex;

typedef struct triangle_struct
{
int v[3];
vector normal;

}triangle;

/* ======== Data structure for the mesh ========== */

typedef struct object_struct
{
vector *vertexlist;
triangle *trianglelist;
unsigned long int nvert;
unsigned long int ntri;

}object;
In this mesh structure, vertexlist and triangle list are pointers to
the array of vertices and triangles respectively.
If you pass a pointer to the structure to a function you safe
a bit of work in copying it and a bit of memory since no copy
needs to be made (but not that much for such a rather small
structure). The main difference is that you can easily change
all the elements in the original structure without passing it
also back to the caller since you are working on the original
itself and not a copy (although you can prevent that by quali-
fying the pointer as 'const' inthe functions argument list).

I don't see why dealing with a pointer instead of a copy of
the structure would be more difficult, you just have to use
a '->' between the pointer and the elements name you want
to access instead of a '.' between the structures name and
the element.

In the overwhelming majority of programs I have seen pointers
to structures get passed to fucntions instead of the structures
by value, so it's a very common idiom and most people will un-
derstand it quite fine. And, yes it might be a bit more effi-
cient speed- and memory-wise, but I wouldn't expect a doubling
of the speed of your application (or anything near that) just
from using pointers;-)
Regards, Jens
--
\ Jens Thoms Toerring ___ jt@toerring.de
\__________________________ http://toerring.de
Jun 27 '08 #6
pereges <Br*****@gmail.comwrites:
On May 30, 7:14 pm, j...@toerring.de (Jens Thoms Toerring) wrote:
>Which variables take much more memory than a simple pointer?
Most variables you can pass to afunction are at least of
comparable size to the one of a pointer. The only exception
is structures, which can definitely be quite a lot larger than
pointers, and that's why they often get passed via a pointer
instead of by value, despite C allowing it.

But I guess I am misunderstanding your intents. Could you
post some code that illustrates what yiu are doing?

And, of course, the usual warning: don't optimize prematurely
but instead measure where the bottle-necks are in your program.

Yes, I'm talking about a structure. For example I want to store a mesh
containing close to 200,000 vertices and 400,000 triangles. Here's my
data structure:

typedef struct vector_struct
{
double x, y, z;
}vector;

typedef vector vertex;

typedef struct triangle_struct
{
int v[3];
vector normal;
}triangle;

/* ======== Data structure for the mesh ========== */

typedef struct object_struct
{
vector *vertexlist;
triangle *trianglelist;
unsigned long int nvert;
unsigned long int ntri;
}object;
On one implementation I tried, your type "object" has a size of 16
bytes; that's probably typical. On most systems, it's probably going
to be about 4 times the size of a pointer.

Passing the structure itself will obviously impose some overhead
because you're passing more information.

On the other hand, it might save some code space because, within the
function, you save a pointer dereference every time you access a
member of the structure. Or, depending on how member access is
implemented by your compiler, it might not help at all.

Measure it.

--
Keith Thompson (The_Other_Keith) ks***@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
Jun 27 '08 #7
On May 30, 8:35 pm, j...@toerring.de (Jens Thoms Toerring) wrote:
If you pass a pointer to the structure to a function you safe
a bit of work in copying it and a bit of memory since no copy
needs to be made (but not that much for such a rather small
structure). The main difference is that you can easily change
all the elements in the original structure without passing it
also back to the caller since you are working on the original
itself and not a copy (although you can prevent that by quali-
fying the pointer as 'const' inthe functions argument list).

I don't see why dealing with a pointer instead of a copy of
the structure would be more difficult, you just have to use
a '->' between the pointer and the elements name you want
to access instead of a '.' between the structures name and
the element.

In the overwhelming majority of programs I have seen pointers
to structures get passed to fucntions instead of the structures
by value, so it's a very common idiom and most people will un-
derstand it quite fine. And, yes it might be a bit more effi-
cient speed- and memory-wise, but I wouldn't expect a doubling
of the speed of your application (or anything near that) just
from using pointers;-)
Well those are not the only structures that my ray tracing program is
using. There is also a structure for BSP tree(binary space
partitioning) which takes up a lot of memory. The list of rays also
consume a lot of memory. My program works very well for smaller number
of rays < 400 K or 500 K but I have seen it fails beyond that. I was
thinking fo storing the the list of rays in a file rather than as an
array in the memory. Probably that could help. I haven't used unions
either.
Jun 27 '08 #8

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