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Arrays are often passed to functions as parameters in C++. However, since arrays decay into pointers when passed to a function, there are some subtle issues and limitations to keep in mind.

Understanding Array Parameters in Functions

When an array is passed to a function, it decays into a pointer to its first element.
The function receives only the address, not the whole array.
Using sizeof inside the function returns the pointer size, not the array size.
Therefore, the array length cannot be determined correctly inside the function.

C++

#include <iostream>
using namespace std;

void f(int a[]) {
    int n = sizeof(a) / sizeof(int); // Incorrect size calculation
    for (int i = 0; i < n; i++) {
        cout << a[i] << ' ';
    }
    cout << '\n';
}

void f(int a[], int N) {
    int n = N; // Correct size passed explicitly
    for (int i = 0; i < n; i++) {
        cout << a[i] << ' ';
    }
    cout << '\n';
}

int main() {
    int a[] = {1, 2, 3, 4, 5};
    int n = sizeof(a) / sizeof(int); // Correct size calculation in main
    for (int i = 0; i < n; i++) {
        cout << a[i] << ' ';
    }
    cout << '\n';

    f(a);    
    f(a, n); 
    return 0;
}

Output

1 2 3 4 5 
1 2 
1 2 3 4 5

Explanation:

In the "main" function, the array a has 5 elements.
The size is correctly calculated in main using sizeof(a) / sizeof(int) because a is treated as an array there.
When the array is passed to "f(int a[])", it decays into a pointer.
sizeof(a) inside this function gives the size of the pointer, not the array, so the size calculation becomes incorrect.
In "f(int a[], int N)", the array size is passed explicitly as N.
This is the correct way to handle arrays when their size is needed inside a function.

Array Parameters and Pointers

Understanding Array Parameters in Functions

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