finding size of multidimensional array

Question

How would I find the size in bytes of an array like this?

    double dArray[2][5][3][4];

I've tried this method but it returns "2". Maybe because the array is only declared and not filled with anything?

     #include <iostream>
     #define ARRAY_SIZE(array) (sizeof((array))/sizeof((array[0])))
     using namespace std;

      int main() {
        double dArray[2][5][3][4];
        cout << ARRAY_SIZE(dArray) << endl;
       }

do you want the size in terms of bytes or the size in terms of how many elements are in the entire array? — Boumbles
– Boumbles, Commented Apr 23, 2013 at 17:26
To get the total number of elements in an array (here: 2*5*3*4), you can use something like template<class T> constexpr std::size_t get_element_count(T const& a) { return sizeof(T)/sizeof(typename std::remove_all_extents<T>::type); } — dyp
– dyp, Commented Apr 23, 2013 at 17:41

Drew Dormann · Accepted Answer · 2013-04-23 17:44:42Z

5

How would I find the size in bytes

This tells you how many elements are present in an array. And it gives the expected answer of 2 for dArray.

#define ARRAY_SIZE(array) (sizeof((array))/sizeof((array[0])))

If what you want it the byte count, this will do it.

#define ARRAY_SIZE(array) (sizeof(array))

Maybe because the array is only declared and not filled with anything?

That won't affect the behavior of sizeof. An array has no concept of filled or not filled.

edited Apr 23, 2013 at 17:44

answered Apr 23, 2013 at 17:25

Drew Dormann

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5 Comments

dyp Over a year ago

Is there a reason for the double brackets in the first macro ((array))?

Drew Dormann Over a year ago

@DyP No functional reason that I'm aware of. It's the code presented in the question. :)

djechlin Over a year ago

Pretty sure OP is having more trouble getting 2*5*3*4 than converting from elem to bytes.

dyp Over a year ago

One could argue the name of the first macro would be clearer if it was ARRAY_LENGTH (or ARRAY_EXTENT). Also, there's the C++11 approach using std::extent, like template<class T> constexpr std::size_t get_extent(T const& a) { return std::extent<T>::value; }

djechlin Over a year ago

OIC. Nevermind, this is right - problem is array[0] was giant so OP should have been dividing by sizeof array[0][0][0][0] to get elem count.

Dave Rager · Accepted Answer · 2013-04-23 17:30:40Z

2

array[0] contains 5 * 3 * 4 elements so sizeof(array) / sizeof(array[0]) will give you 2 when the first dimension of your array is 2

Rewrite your macro as:

#define ARRAY_SIZE(array, type) (sizeof((array))/sizeof((type)))

to get the number of elements,

or simply sizeof(array) to get the number of bytes.

answered Apr 23, 2013 at 17:30

Dave Rager

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