why is this result ?

[horseriver <horserivers@xxxxxxxxx>]

hi:
  I am doing a test for c++;

  here is my code:  

#include <stdio.h>
class A
{};

class B
{
public:
  B(){};
  ~B(){};
};

int main()
{
  
  printf("size of A is %d \n",sizeof(A));
  //printf("size of B is %d \n",sizeof(B));
}

output is   "size of A is 1 " ,I can not understand this result ,
 there is no data in class A ,why here its size is 1?