Bit Fields

It is possible to specify the fields within a structure in terms of bits. This can be useful when interfacing with hardware devices, for example, or simply to store boolean values using one eighth of the memory amount taken up by the bool type. Bit fields can either be a non floating-point type (typically unsigned int), or an enumeration. For example, if you want to store the name of a person and two boolean values in two bits, corresponding to married status and European citizenship you can use: 
struct Id
{
     char name[20];
     unsigned married: 1; // uses one bit
     unsigned europeanCitizen: 1;
} myId;
To set a bit field: 
myId.married = 1;
It might also be necessary to define a structure corresponding to a byte for example, in order to save the data coming from a byte received serially from another computer. In such a case, if only the last two bits are significant, a structure can be defined in which the first six bits' meaning is not specified. Further, a bit-field might take up more than one bit: 
struct IdFromByte
{
     unsigned: 3; // three bits are used. meaning unspecified
     unsigned married: 1;
     unsigned europeanCitizen: 1;
     unsigned childrenNumber: 3;
};


Example


struct Id
{
     char name[20];
     unsigned married: 1;
     unsigned europeanCitizen: 1;
};

struct IdFromByte // structure takes up a byte
{
     unsigned: 3; // three bits are used, the meaning unspecified
     unsigned married: 1;
     unsigned europeanCitizen: 1;
     unsigned childrenNumber: 3; // maximum number stored: 7
};


#include <iostream>
#include <cstring>

using namespace std;

int main()
{
    Id myId;
    strcpy(myId.name, "Erika");
    myId.married = 1;
    myId.europeanCitizen = 0;

    IdFromByte anIdFromByte;
    anIdFromByte.married = 1;
    anIdFromByte.europeanCitizen = 1;
    anIdFromByte.childrenNumber = 3;

    cout << "married: " << anIdFromByte.married << endl;
    cout <<  "europeanCitizen: " << anIdFromByte.europeanCitizen << endl;
    cout << "childrenNumber: " << anIdFromByte.childrenNumber << endl;
}

Output

married: 1
europeanCitizen: 1
childrenNumber: 3