why does my bitset store a max 64 bit value instead of a max 32 bit value when i pass in the max 32 bit value?

0xffffffff is only 32 bit, so can someone tell me why would b be 64 1s? Shouldn't it be printing out 32 0s follow by 32 1s?

#include <iostream>
#include <bitset>


using namespace std;

int main()
{
   int a = 0xffffffff;

   string b = bitset<64>(a).to_string();
   cout << b;


    return 0;
}