C++ Array Length — How to Get Array Size with sizeof & std::size

Learn how to determine an array's length in C++ using sizeof, std::size, and template helpers. Clear examples, common pitfalls, and best practices for safe array sizing.

What is the length of an array?

The length (or size) of an array means:

How many elements are stored in the array

Example:

int marks[5] = {60, 70, 80, 90, 100};

➡ The length of this array is 5 because it has 5 values.

Method 1: Finding array length using sizeof (Most Common in C++)

Formula

array_length = sizeof(array) / sizeof(array[0]);

Why this works

• sizeof(array) → total memory used by the array
• sizeof(array[0]) → memory used by one element
• Dividing gives the number of elements

Example 1: Integer Array

#include <iostream>
using namespace std;

int main() {
    int numbers[] = {10, 20, 30, 40, 50};

    int length = sizeof(numbers) / sizeof(numbers[0]);

    cout << "Length of array: " << length;

    return 0;
}

Output

Length of array: 5

Example 2: Character Array

#include <iostream>
using namespace std;

int main() {
    char vowels[] = {'a', 'e', 'i', 'o', 'u'};

    int length = sizeof(vowels) / sizeof(vowels[0]);

    cout << "Number of vowels: " << length;

    return 0;
}

Output

Number of vowels: 5

Method 2: Length of a String (Character Array)

Using strlen() (for strings only)

#include <iostream>
#include <cstring>
using namespace std;

int main() {
    char name[] = "Yasir";

    cout << "Length of name: " << strlen(name);

    return 0;
}

Output

Length of name: 5

🔹 strlen() counts characters excluding the '\0' (null character).

Method 3: Using Loop (Manual Counting – Learning Purpose)

#include <iostream>
using namespace std;

int main() {
    int arr[] = {2, 4, 6, 8, 10};
    int count = 0;

    for(int i = 0; i < 5; i++) {
        count++;
    }

    cout << "Array length: " << count;

    return 0;
}

⚠️ This works only when you already know the size.

Important Note ⚠️

❌ You cannot find the length of an array using sizeof inside a function when passed as a parameter.

Example (Wrong):

void show(int arr[]) {
    cout << sizeof(arr);   // gives size of pointer, not array
}

✔ Correct way: pass the size separately.

Summary Table

📘 Related Topics

• C++ One-Dimensional Arrays — Syntax & Examples