C++ bitset all()

bool all() const no except; 

#include <iostream>
#include <bitset>
using namespace std;
int main()
{
bitset<4> b;
bitset<4> mask("1111");
if (!b.all())
cout<< "All bits are not set." <<endl;
b |= mask;
if (b.all())
cout<< "All bit are set." <<endl;
return 0;
} 

All bits are not set.
All bit are set.