C++ - How to use bitfields

So I recently came across something like this

unsigned char ch : 7;

inside a struct. I read about it a little. Apparently these are called bit fields. They are used to set the width of data that a char can receive. But how do we use these things. For example, I know that we can set variable ch to be a byte

unsigned char ch = 0x61;
cout << ch << endl;

This will output

a

However, what do we do with the bitfields?

unsigned char ch : 7;
ch = 0x61;                //This doesn't work for some reason

unsigned char ch : 7;
unsigned char ch = 0x61;  //Neither does this.

Thanks for the help

Bitfields can only be used inside structured data types, i.e. struct, class, and union types. The purpose is to allow you to pack multiple members inside a single byte, without having to write lots of explicit shifts and masks. For instance, you can write:

struct halfbyte_t {
    unsigned int half1: 4;
    unsigned int half2: 4;
} halfbyte;

This declares a variable named halfbyte that contains two 4-bit members, which will be packed into a single 8-bit byte, rather than having to use 2 bytes if you just declared them unsigned char.

You access them just like any other structure member:

halfbyte.half1 = 3;
cout << halfbyte.half3;

1-bit fields are especially useful if you have lots of boolean flags in a structure, since you don't have to have a separate byte for each flag.

struct flag_t {
    unsigned int flag1: 1;
    unsigned int flag2: 1;
    unsigned int flag3: 1;
    unsigned int flag4: 1;
    ...
};