Getting the Array Size

Getting the Array Size

An array doesn't store its length for you to read, so C++ programmers compute it - with a small trick.

The sizeof trick

Every element in an array is the same size. So the total size divided by the size of one element gives you the count:

int nums[] = {10, 20, 30, 40, 50};
int len = sizeof(nums) / sizeof(nums[0]);
cout << len;   // 5

• sizeof(nums) - the total bytes of the whole array.
• sizeof(nums[0]) - the bytes of a single element.
• Dividing them gives the element count.

Modern C++: std::size

Since C++17 there's a cleaner, less error-prone helper:

#include <iterator>
std::size(nums);   // 5

std::size says what you mean in one word and is safer to read.

One important catch

The trick works only when nums is still a real array. Pass it to a function and the name decays to a pointer - then sizeof(nums) measures the pointer, not the data. So only trust the trick in the same scope where the array was created.

TL;DR

• sizeof(nums) / sizeof(nums[0]) gives the number of elements.
• std::size(nums) (C++17) is the safer, cleaner version.
• Both work only while the name is still a real array, not a pointer.
• Hardcoding sizes invites bugs - calculate the count instead.