How to find the size of any array in C++

Question

I want to find the size of any array type. My code is:

#include <iostream>

using namespace std;

template <typename t>
int some_function(t arr []){
    int s;
    s = sizeof(arr)/sizeof(t);
    return s;
}

int main(){
    int arr1 [] = {0,1,2,3,4,5,6,7};
    char arr2 [] = {'a','b','c','d','e'};
    int size;
    size = some_function(arr1);
    cout << "Size of arr1 : "<<size<<endl;
    size = some_function(arr2);
    cout << "Size of arr2 : "<<size<<endl;
    return 0;
}

When I run this code on cpp.sh output is:

Size of arr1 : 2
Size of arr2 : 8

and when I run it on CodeBlocks and Visual Studio output is:

Size of arr1 : 1
Size of arr2 : 4

I want it to print the exact size of an array which is:

Size of arr1 : 8
Size of arr2 : 5

SOLUTION

I found the solution with the help of rsp and Cheersandhth.-Alf. I was passing the array by value which is implicitly converted to pointer. After reading this article and answer provided by rsp I passed the array by reference. So final code is:

#include <iostream>

using namespace std;

template <typename t, int s>
int some_function(t (&arr)[s]){
    return s;
}

int main(){
    int arr1 [] = {0,1,2,3,4,5,6,7};
    char arr2 [] = {'a','b','c','d','e'};
    int size;
    size = some_function(arr1);
    cout << "Size of arr1 : "<<size<<endl;
    size = some_function(arr2);
    cout << "Size of arr2 : "<<size<<endl;
    return 0;
}

Thank you everyone for help...

Hey, who removed my https://stackoverflow.com/questions/43444235/c-does-gcc-provide-extended-macro-function-like-countof-the-size-of-an-arra link? — melpomene, Nov 04 '17 at 10:16
@melpomene: I did. It appeared to be about compiler-specific functionality, which would be entirely wrong as an answer to this question. It's a FAQ. — Cheers and hth. - Alf, Nov 04 '17 at 10:20
@Cheersandhth.-Alf Well, the [answer](https://stackoverflow.com/a/43444328/4944425) to that question is not compiler specific. — Bob__, Nov 04 '17 at 10:31
@Bob__: Right, I see. It would be nice with some other reference to C++ `std::size`, because I don't think that's in the (old) FAQ. The linked answer suffers from appearing to be about gcc functionality, and does not mention the limitation discussed in the FAQ for a Do It Yourself `constexpr` `size`, inability to find size of array by reference, which unfortunately still applies to `std::size` (it's not been fixed). I think a question purely about `std::size` could be nice along with the FAQ ref. Or better, if Someone™ updated that FAQ. :) — Cheers and hth. - Alf, Nov 04 '17 at 10:50
It isn't really to do with *passing an array by value*, because that doesn't exist in C++. What happens is that a function parameter `T t[N]` gets adjusted to `T* t` — juanchopanza, Nov 04 '17 at 11:24

msc · Accepted Answer · 2017-11-04T10:05:38.760

2

It is called arrays decaying into pointers. When you pass an array by value they decay into a pointer. So, the size of the array is nothing but the size of a pointer which is depends on the system.

edited Nov 04 '17 at 10:05

answered Nov 04 '17 at 10:00

msc

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So how can I find the size of an array, without passing the size explicitly – MAY Nov 04 '17 at 10:02
@MAY - Please read my comment above – Ed Heal Nov 04 '17 at 10:03
@MAY One way is to use a sentinel value like the `'\0'` character used to terminate c-style strings. – user0042 Nov 04 '17 at 10:03
@EdHeal Thanks for suggesting that, but what if I don't want to use vector or array container – MAY Nov 04 '17 at 10:12
@may: You can just read the FAQ linked as a duplicate. – Cheers and hth. - Alf Nov 04 '17 at 10:20
@Cheersandhth.-Alf: I am reading that, thanks – MAY Nov 04 '17 at 10:23
1

Thanks to everyone for help and especially **rsp** and @Cheersandhth.-Alf. I am passing the array by value which is converted into pointer implicitly so I passed it by reference. `template ` `int some_function(t (&arr)[N]){` `return N;` `}` – MAY Nov 04 '17 at 10:50
1

What really happens is that `t arr []` is *type adjusted* to `t* arr`. Where array decaying comes in is that it allows passing an array to the function to compile. But the function parameter is always a pointer. – juanchopanza Nov 04 '17 at 11:23

score -4 · Answer 2 · edited Nov 04 '17 at 10:30

-4

While calling your function try adding datatype of your array

size = some_function<int>(arr1);

size = some_function<char>(arr2);

edited Nov 04 '17 at 10:30

msc

33,420
29
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answered Nov 04 '17 at 10:04

Utpal Dutt

383
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18

How to find the size of any array in C++

2 Answers2