How to Find the Size of an Array Using Pointer to its First Element?

In C++, arrays are plain old data types that do not have any associated functions to find their size. In this article, we will discuss how we can determine the size of the array in C++ using the pointer to its first element.

Example

Input: 
int arr[]={1,2,3,4,5}

Output:
Size of the array is 5

Find the Size of an Array from a Pointer to its First Element

Unfortunately, there is no way we can find the size of an Array using only the pointer to its first element. Even if we try to use sizeof to that pointer, we will only get the size of the pointer itself but not the array.

We can see that in this example,

// C++ program to illustrate that we cannot find the size of
// the array only using the pointer to its first element
#include <iostream>
using namespace std;

int main()
{
    // array
    int arr[] = { 1, 2, 3, 4, 5 };
    // pointer
    int* ptr = arr;

    int size = sizeof(ptr);

    cout << "The size of the array is: " << size << endl;

    return 0;
}

The size of the array is: 8

Explanation: In 64-bit architectures, the size of pointers is uniform, typically being 8 bytes. This uniformity is dictated by the operating system, ensuring that pointers to different data types (such as int, float, and double) all occupy the same amount of memory. Consequently, regardless of the data type, a pointer will consistently be 8 bytes on a 64-bit system.

However, if you still want to work only with the pointer, you'll need to ensure that the size of the array is available through another mechanism, such as storing it in a variable, use the inbuilt containers like std::vectors, std::arrays, etc. If you still need to use arrays, keep a variable that stores the size of the array.