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determine array size

Started by Daivuk Sep 3, 2003 at 11:08 AM 9 replies 1.5k views

Original Post

Sep 3, 2003 at 11:08 AM

(sorry about my english, I''m from quebec) Hi, I just want to know the size of one of my array. What I mean is : I have a type : dk_Poly And I have an array of poly : dk_Poly *PolyList; Then I create a array of a specified length : PolyList = new dk_Poly[NbPoly]; But I dont want to pass NbPoly to all functions and proc. So I''m just asking if there is a way to know the size of PolyList?? I tried sizeof(), but this only give me the size of the type or pointer. thx

CloudNine

Sep 3, 2003 at 11:09 AM


int size=sizeof(*ptr)/sizeof(ptr);

should do the trick

Fruny

Sep 3, 2003 at 11:13 AM

quote:
Original post by CloudNine

int size=sizeof(*ptr)/sizeof(ptr);

should do the trick

No. No. No. No. No. No.
It will *never* work with a pointer.

Daivuk - either use a C++ vector, or pass the size explicitely. There are no other options.

Daivuk

Sep 3, 2003 at 11:15 AM

ya, ok. So i''ll will pass my NbPoly to all functions, proc and bla bla bla....

thanks

CloudNine

Sep 3, 2003 at 11:15 AM

Why not? If the pointer is valid, I don''t see why it wouldn''t work.

Daivuk

Sep 3, 2003 at 11:17 AM

I tried it.

sizeof(ptr) return 4 (size of a pointeur dah!)
sizeof(*ptr) return me 192 (size of my struct (type) dk_Poly)

Polymorphic OOP

Sep 3, 2003 at 11:21 AM

quote:
Original post by CloudNine
Why not? If the pointer is valid, I don''t see why it wouldn''t work.

Because a dereferenced pointer to a dk_Poly points to a single dk_Poly, not a whole array.

Fruny

Sep 3, 2003 at 11:28 AM

An array knows its size. A pointer doesn't know if it's pointing to a single element or an array and therefore cannot be used to find the size of the array.

You can write a function that either returns the correct result (with an array) or fails to compile (with a pointer) instead of returning an incorrect result.


template<class T, size_t n> size_t array_size( T(&)[n] )
{
  return n;
}

int array[100];
int *ptr = array;
int i = array_size(array); // Ok, returns 100

int j = array_size(ptr);   // Doesn't compile

Daivuk - learn to use C++ vectors. They do the job nicely.

GekkoCube

Sep 4, 2003 at 10:30 AM

template

i''ve seen template when defining a template class.
i''ve seen template when defining templates functions.
but what is that line aboe mean?
is this part of the stl vector???
please explain that.

BungoMan85

Sep 4, 2003 at 6:41 PM

like i just said in another post. STL is your friend. theres no excuse to not use an STL vector now days (unless you have an old old compiler).

Bungo!

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