Calculate length of double pointer array

Question

I have a double pointer Array of a structure:

typedef struct Position{
    int x;
    int y;
} Position;

Position** array = (Position**)malloc(sizeof(Position*)*10); //10 elements
array[0] = (Position*)malloc(sizeof(Position*));
array[0]->x = 10;
array[0]->y = 5;

Can I calculate the length of set array and if so, how?

The normal way for arrays does not work :

int length = sizeof(<array>)/sizeof(<array>[0]);

it's an array of pointer, so probably you want to do `malloc(sizeof(Position*)*10)` — yar, May 21 '17 at 16:01
Define "does not work." Compile error? Result wrong? Something else entirely? — anonymoose, May 21 '17 at 16:03
@Olaf you can define an array in multiple ways! In C/C++ you can allocate space and use pointers! — PrinzJuliano, May 21 '17 at 16:03
@PrinzJuliano: No, you cannot! A pointer is not an array! One of the effects is exactly what you see here! Instead of insisting on wrong assumptions, please think about it. The details will be explained in a good C book. It might be different in the language C/C++, but then don't use the C or C++ tags, as these three are pairwise different languages (one of them does not even exist - find out which). — too honest for this site, May 21 '17 at 16:06
and `array[0] = (Position*)malloc(sizeof(Position*))` should be `array[0] = (Position*)malloc(sizeof(Position))`, right? — Stephan Lechner, May 21 '17 at 16:13
Possible duplicate of [Length of array in function argument](http://stackoverflow.com/questions/8269048/length-of-array-in-function-argument) — too honest for this site, May 21 '17 at 16:15
@Olaf I am using C++ there! It would error in gcc if I did not cast it! — PrinzJuliano, May 21 '17 at 16:18
See, that's **one** reason why you should not double-tag! (as a sidenote: don't use the language names in the title; that's exactly what the tag is for) — too honest for this site, May 21 '17 at 16:20
Pick a language. C is not C++, and C++ is not C. C/C++ is not a thing. — Donnie, May 21 '17 at 16:23
All this talk of `C/C++` reminds me of when `Java/JavaScript` was a thing :P — SiggiSv, May 21 '17 at 16:53

Mike Nakis · Answer 1 · 2017-05-21T16:08:07.227

1

Once you have dynamically allocated an array, there is no way of finding out the number of elements in it.

I once heard of some hacky way to obtain the size of a memory block, (msize) which would allegedly allow you to infer the size of the data within the block, but I would advice against any such weird tricks, because they are not covered by the standard, they represent compiler-vendor-specific extensions.

So, the only way to know the size of your array is to keep the size of the array around. Declare a struct, put the array and its length in the struct, and use that instead of the naked array.

edited May 21 '17 at 16:08

answered May 21 '17 at 16:05

Mike Nakis

56,297
11
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The question is about C, not C++! – too honest for this site May 21 '17 at 16:07
@Mike Nakis Thanks – PrinzJuliano May 21 '17 at 16:11
@PrinzJuliano: There is no such language! – too honest for this site May 21 '17 at 16:14
@PrinzJuliano If you are using C/C++ perhaps you should tag your question with the C/C++ language tag. – n. m. could be an AI May 21 '17 at 16:14

score 0 · Answer 2 · answered May 21 '17 at 16:25

As you marked the question as C++, I would suggest that you use std::vector, then, after you "allocated some memory" (or requested some memory to allocated by std::vector constructor or by using push_back, or resize), you can simply get the size back using by using std::vector::size.

typedef struct Position{
    int x;
    int y;
} Position;

std::vector<Position> array(10);
array[0].x = 10;
array[0].y = 5;
size_t size = array.size(); // will be 10

score 0 · Answer 3 · answered May 21 '17 at 16:28

Having only a pointer to some memory block, you cannot defer the size of this memory block. So you cannot defer the number of elements in it.

For arrays of pointers, however, you could infer the number of elements in it under the following conditions:

make sure that every pointer (except the last one) points to a valid object.
for the last pointer in the array, make sure that it is always NULL.

Then you can derive the length by counting until you reach NULL.

Maybe there are some other similar strategies.

Solely from the pointer itself, however, you cannot derive the number of elements in it.

score -1 · Accepted Answer · answered Jul 16 '22 at 11:07

-1

Old question, but in case someone needs it:

#include <stdio.h>
...
int main()
{
    char **double_pointer_char;
    ...
    int length_counter = 0;
    while(double_pointer_char[length_counter])
        length_counter++;
    ...
    return 0;
}

answered Jul 16 '22 at 11:07

5ama7

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7

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As it’s currently written, your answer is unclear. Please [edit] to add additional details that will help others understand how this addresses the question asked. You can find more information on how to write good answers [in the help center](/help/how-to-answer). – Community Jul 18 '22 at 17:16

Calculate length of double pointer array

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