c++伝達配列を表示する場合、なぜその長さを指定しますか?

#include <iostream>
const int SIZE = 5;
int sum_arr(int p[],int size);
int main()
{
        int sum;
        int arr[SIZE] = {1,2,3,4,5};
        std::cout << "sizeof (arr)=" << sizeof(arr) << ", address=" << arr <<std::endl;
        sum = sum_arr(arr,SIZE);
        std::cout << "sum of arr=" << sum <<std::endl;
        return 0;

}

int sum_arr(int p[], int size)
{
        int sum = 0;
        std::cout << "sizeof (p)=" << sizeof(p) << ", address=" << p <<std::endl;
        for (int i =0; i < size; i++)
                sum += p[i];
        return sum;
}

出力は次のとおりです.
 

ackerman@ubuntu:~/  /c++test$ ./a.out 
sizeof (arr)=20, address=0xbf9afa68
sizeof (p)=4, address=0xbf9afa68
sum of arr = 15