bit field in C++

In C++, A class can define a (nonstatic) data member as a bit-field. We indicate that a member is a bit-field by following the member name with a colon and a constant expression specifying the number of bits. 
* A bit-field shall not be a static member. 
* Ordinarily it is best to make a bit-field an unsigned type. The behavior of bit-fields stored in a signed type is implementation defined.
* The address-of operator & shall not be applied to a bit-field, so there are no pointers to bit-fields.
```
#include <iostream>
using namespace std;

class File {
public:    
    unsigned int mode:2;
};

int main()
{
    enum modes { READ = 01, WRITE = 02, EXECUTE = 03 };
    File file;
    
    cout << file.mode << ",";
    file.mode |= READ;
    cout << file.mode << ",";
    file.mode |= WRITE;
    cout << file.mode << endl;
    return 0;
}

// output 0, 1, 3
```
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