In this code why output is always 12, irrespective of value of n.

Question

include <bits/stdc++.h>
using namespace std;
int main() {
int n;
cin >> n;
vector<int> arr(n);
for(int arr_i = 0; arr_i < n; arr_i++){
cin >> arr[arr_i];
}
cout<<sizeof(arr)<<endl;

return 0;
}

Answer 1

These may explain the odds:

https://www.quora.com/STL-C++-Why-does-sizeof-return-the-same-value-for-all-vectors-regardless-of-the-type-and-number-of-elements-in-that-vector

https://stackoverflow.com/questions/34024805/c-sizeof-vector-is-24

Hth, cmiiw

Answer 2

sizeof() by default returns the size of the object of type vector, and that may have a fixed size of 12 bytes. So to get the number of elements or the maximum current capacity, use size() or capacity().

Eg : cout<<vec.size()<<endl;

Also, visit : https://stackoverflow.com/questions/2373189/sizeof-a-vector

Answer 3

sizeof is known at compile time. It is how many bytes an object takes on the stack. But the data of a vector is on the heap.
To get the vectors size, use arr.size().