How to Find Size of an Array in C++ Without Using sizeof() Operator?

In C++, generally, we use the sizeof() operator to find the size of arrays. But there are also some other ways using which we can find the size of an array. In this article, we will discuss some methods to determine the array size in C++ without using sizeof() operator.

Methods to Find the Size of an Array without Using sizeof() Operator

Given an array (you don’t know the type of elements in the array), find the total number of elements in the array without using the sizeof() operator. So, we can use the methods mentioned below:

1. Using pointer hack
2. Using Macro Function
3. Using own self-made sizeof( )
4. Using Template Function
5. Using a Sentinel Value
6. Using a Class or Struct

1. Using Pointer Hack

The following solution is concise when compared to the other solution. The number of elements in an array A can be found using the expression:

// &arr returns a pointer 
int size = *(&arr + 1) - arr; 

How Does this Method Work?

Here the pointer arithmetic does its part. We don’t need to explicitly convert each of the locations to character pointers.

• &arr – Pointer to an array of 6 elements. [See this for difference between &arr and arr]
• (&arr + 1) – After adding 1 to the address of array of 6 integers, the pointer will point to the memory location immediately after the end of the array.
• *(&arr + 1) – Same address as (&arr + 1), but type of pointer is “int *”.
• *(&arr + 1) – arr – Since *(&arr + 1) points to the address 6 integers ahead of arr, the difference between two is 6.

C++ Program to Find the Size of an Array Using Pointer Hack

C++

// C++ program to find size
// of an array by using a
// pointer hack
#include <iostream>
using namespace std;

int main()
{
    int arr[] = { 1, 2, 3, 4, 5, 6 };
    int size = *(&arr + 1) - arr;
    cout << "Number of elements in arr[] is " << size;
    return 0;
}

Output

Number of elements in arr[] is 6

Complexity Analysis

• Time complexity: O(1)
• Auxiliary space: O(1)

2. Using Macro Function

We can define a macro that calculates the size of an array based on its type and the number of elements.

#define array_size(arr) (sizeof(arr) / sizeof(*(arr)))

• array_size(arr): Name of the macro
• (sizeof(arr): size of entire array in bytes
• sizeof(*(arr)): size of single element in bytes

Dividing the total size of the array by the size of a single element gives the number of elements in the array.

C++ Program to Find the Size of an Array using Macro

C++

// C++ program to find size of
// an array using Macro Function
#include <bits/stdc++.h>
using namespace std;

// Defining Macro
#define array_size(arr) (sizeof(arr) / sizeof(*(arr)))

int main()
{
    int arr[] = { 1, 2, 3, 4, 5, 6 };
    int size = array_size(arr);
    cout << "Number of elements in arr[] is " << size;
    return 0;
}

// This code is contributed by Susobhan Akhuli

Output

Number of elements in arr[] is 6

Complexity Analysis

• Time complexity: O(1)
• Auxiliary space: O(1)

3. Implement Our Own sizeof( )

Using custom user-defined sizeof function which can provide the functionality same as sizeof( ).

C++ Program to Implement Custom User-Defined sizeof Function

C++

// C++ program to find size of
// an array by writing our
// own sizeof operator
#include <iostream>
using namespace std;

// User defined sizeof macro
#define my_sizeof(type)                                    \
    ((char*)(&type + 1) - (char*)(&type))

int main()
{
    int arr[] = { 1, 2, 3, 4, 5, 6 };
    int size = my_sizeof(arr) / my_sizeof(arr[0]);

    cout << "Number of elements in arr[] is " << size;

    return 0;
}

Output

Number of elements in arr[] is 6

Complexity Analysis

• Time complexity: O(1)
• Auxiliary space: O(1)

To know more about the method refer to Implement our own sizeof.

4. Using Template Function

We can use a template function to find the size of an array.

C++ program to Find the Size of an Array Using Template Function

C++

// C++ program to find size of
// an array using Template Function
#include <bits/stdc++.h>
using namespace std;

// Calculating size of an array
template <typename T, size_t N>

int array_size(T (&arr)[N])
{
    return N;
}

int main()
{
    int arr[] = { 1, 2, 3, 4, 5, 6 };
    int size = array_size(arr);
    cout << "Number of elements in arr[] is " << size;
    return 0;
}

// This code is contributed by Susobhan Akhuli

Output

Number of elements in arr[] is 6

Explanation

The array_size() function is a template function that takes two parameters:

• T: Type of array elements
• N: Size of the array

The parameter T (&arr)[N] means the function accepts a reference to any type and any size of array. When array_size function is called with the array name as parameter, T is deduced as int and N is deduced as 6.

Complexity Analysis

• Time complexity: O(1)
• Auxiliary space: O(1)

5. Using a Sentinel Value

We can add a special value to the end of the array to indicate the end of the array, and then loop through the array until you find the sentinel value. This method is commonly used in string manipulation.

C++ program to Find the Size of an Array using a Sentinel Value

C++

// C++ program to find size of
// an array using sentinel value
#include <iostream>
using namespace std;

int main()
{
    // maximum size of the array
    const int MAX_SIZE = 100;
    // declare the array
    int arr[MAX_SIZE];
    // sentinel value to indicate the end
    // of the array
    int sentinel = -1;

    // read values into the array until the sentinel value
    // is entered
    int i = 0; // counter variable
    while (i < MAX_SIZE) {
        cout << "Enter a value for element " << i
             << " (or -1 to stop): ";
        cin >> arr[i];
        // exit the loop if the sentinel value is
        // entered
        if (arr[i] == sentinel) {
            break;
        }
        i++;
    }

    // calculate the size of the array
    // the size of the array is the number of
    // elements entered
    int size = i;

    // print the size of the array
    cout << "Size of the array: " << size << endl;

    return 0;
}

// This code is contributed by Susobhan Akhuli

Output

Enter a value for element 0 (or -1 to stop): Enter a value for element 1 (or -1 to stop): Enter a value for element 2 (or -1 to stop): Enter a value for element 3 (or -1 to stop): Enter a value for el...

Output

Enter a value for element 0 (or -1 to stop): 3
Enter a value for element 1 (or -1 to stop): 7
Enter a value for element 2 (or -1 to stop): 2
Enter a value for element 3 (or -1 to stop): 9
Enter a value for element 4 (or -1 to stop): 5
Enter a value for element 5 (or -1 to stop): -1
Size of the array: 5

Complexity Analysis

• Time complexity: O(n), where n is the number of elements in the array.
• Auxiliary space: O(1)

6. Using a Class or Struct

We can define a class or struct that contains an array as a member variable, and then define a member function that returns the size of the array. This method is useful if we need to pass the array and its size as a single parameter to a function.

C++ program to Find the Size of an Array using a Class and Member Function

C++

// C++ program to find size of
// an array usinga Class or Struct
#include <iostream>
using namespace std;

class MyArray {
private:
    // declare the array
    int arr[100];
    // size of the array
    int size;

public:
    // read values into the array
    void readArray();
    // get the size of the array
    int getSize();
};

void MyArray::readArray()
{
    // initialize size to 0
    size = 0;
    while (size < 100) {
        cout << "Enter a value for element " << size
             << " (or -1 to stop): ";
        cin >> arr[size];
        // exit the loop if -1 is entered
        if (arr[size] == -1) {
            break;
        }
        // increment size for each element entered
        size++;
    }
}

int MyArray::getSize()
{
    // return the size of the array
    return size;
}

int main()
{
    // declare an object of MyArray
    MyArray a;
    // read values into the array
    a.readArray();
    // get the size of the array
    int size = a.getSize();
    // print the size of the array
    cout << "Size of the array: " << size << endl;
    return 0;
}

// This code is contributed by Susobhan Akhuli

Output

Enter a value for element 0 (or -1 to stop): Enter a value for element 1 (or -1 to stop): Enter a value for element 2 (or -1 to stop): Enter a value for element 3 (or -1 to stop): Enter a value for el...

Output

Enter a value for element 0 (or -1 to stop): 8
Enter a value for element 1 (or -1 to stop): 4
Enter a value for element 2 (or -1 to stop): 2
Enter a value for element 3 (or -1 to stop): 7
Enter a value for element 4 (or -1 to stop): 5
Enter a value for element 5 (or -1 to stop): 6
Enter a value for element 6 (or -1 to stop): -1
Size of the array: 6

Complexity Analysis

• Time complexity: O(1), getSize() method returns size member variable that takes constant time.
• Auxiliary space: O(1)