编写程序计算数组元素之和。要求编写函数三次，每次以不同的方法处理数组边界。

1.使用标准库规范

int sum_array( const int *beg, int *en )
{
  int sum = 0;
  while ( beg != en )
  {
    sum += *beg++;
  }
  return sum;
}

int main()
{
  int a[] = { 1,1,2,3,4,5,6,7,8,9 };
  cout << sum_array( a, a + sizeof(a)/sizeof(*a) );
  return 0;
}

2.显式传递表示数组大小形参
int sum_array( const int *a, int size )
{
  int sum = 0;
  for ( int i = 0; i < size; i++ )
  {
    sum += *(a + i);
  }
  return sum;
}

int main()
{
  int a[] = { 1,1,2,3,4,5,6,7,8,9 };
  cout << sum_array( a, sizeof(a)/sizeof(*a) );
  return 0;
}



3.数组大小
int sum_array( const int a[], int size )
{
  int sum = 0;
  for ( int i = 0; i < size; i++ )
  {
    sum += *(a + i);
  }
  return sum;
}


int main()
{
  int a[] = { 1,1,2,3,4,5,6,7,8,9 };
  cout << sum_array( a, sizeof(a)/sizeof(*a) );
  return 0;
}