Find the size of a c array in function

Question

I am trying to write a short function which takes a pointer to an array and simply returns it's size. So far what I have is something like this:

int main (void) {
   double MyArray[3] = {0, 1, 2};

   int Temp = ArraySize(MyArray);

   return 0;
}

int ArraySize(double * MyArray) {
   return sizeof(MyArray) / sizeof(*MyArray);
}

But this doesnt seem to be working. Any help appreciated, Jack

Please before answering the question, make an educated guess on how many times this question would've been asked :) — legends2k, Mar 24 '14 at 11:27
I had a quick look but nothing seemed to match my question. My Bad. — JMzance, Mar 24 '14 at 11:35

score 3 · Answer 1 · answered Mar 24 '14 at 11:27

3

That's impossible - the pointer simply points at the first element of the array. There's no way to extract the array size from that.

You could pass the array by reference, and infer the size as a template parameter:

template <typename T, size_t N>
size_t ArraySize(T (&)[N]) {return N;}

This will only work if you have access to the array itself (as you do in your example). If it's already decayed to a pointer, then the size information has been lost.

answered Mar 24 '14 at 11:27

Mike Seymour

249,747
28
448
644

Great! That's done the trick, can I check that I understand how this works...When you pass the array to the function the template call figures out how long the array is and that is then accessible in the function? – JMzance Mar 24 '14 at 11:34
1

@JackMedley: Exactly. The template infers both the element type and the array size from the (array) type of the function argument. – Mike Seymour Mar 24 '14 at 11:36

Pranit Kothari · Answer 2 · 2014-03-24T11:32:32.873

0

No it is not possible, you need to pass length in function with array. Like,

int ArraySize(double * MyArray, size_t length)

edited Mar 24 '14 at 11:32

answered Mar 24 '14 at 11:27

Pranit Kothari

9,721
10
61
137

You function's signature does not fit the OP's requirements, thus your answer is not an answer. – Sebastian Mach Mar 24 '14 at 11:37
The first 4 words are the answer “No it is not possible”. The rest is extra. Unless you arrange to put a sentinel at the end of the array (similar to how c strings work). That also changes the interface to the function. – ctrl-alt-delor Mar 24 '14 at 11:47
@richard Hmmmm I see. – Pranit Kothari Mar 24 '14 at 12:47

score 0 · Answer 3 · answered Mar 24 '14 at 11:27

0

You cannot do what you want. MyArray in function ArraySize is a pointer to a double, not an array of doubles. You must explicitly pass the array length along with the base address of the array.

answered Mar 24 '14 at 11:27

ajay

9,402
8
44
71

Find the size of a c array in function

3 Answers3