Why This behaviour in C++?

/*
  Difference below  array object inside class and inside main ..
 */   

#include<iostream>
using namespace std;


class MChar
{
};


class MInt
{
 private:
 MChar mChar[]; /* empty array  doesnt cause any array*/
 int mInt[]; /* empty array again*/
};


int main(void)
{
 MInt a;
// int p[]; // error: storage size of ‘p’ isn’t known 

 return 0;
}