如何阅读 C++ 位域定义的语法？

Question

I need some help in understanding the syntax of a Bitfield definition. I read the Microsoft documentation page on it but the example there still leaves me with my question. Given a Bitfield and main method like this:

struct {
    unsigned short character : 8;
    unsigned short color     : 4;
} text[80];

int main() {
    text[20].character = 'a';
    text[20].color = 5;
}

For better reference of what I'm asking, here numbered:

1. What do the [80] and the [20] 's mean here?
2. Does text[80] mean an array of 80 such structs?
3. Does text[20].character = 'a' mean, that at position 20 of the array there is a character 'a' ?

Answer 1

"What do the [80] and the [20] 's mean here?"

text[80]; declares an array with 80 elements and text[20] accesses the 21st element.

"Does text[80] mean an array of 80 such structs?"

Yes.

"Does text[20].character = 'a' mean, that at position 20 of the array there is a character 'a'?"

Yes.

Answer 2

What do the [80] and the [20]'s mean here?

The [80] means defining an array of the anonymous type that you defined(the defined structure). let me clearly tell what is happening. As unsigned short is 16 bits, this struct will be a 16 bits memory that first 8 bits are named as the character, and the 4 middle bits are named as color, and 4 ending bits are not used.

You created an array of 80 elements from this structure. It means you have 80 consecutive 16 bit in the memory; each of these elements(16 bit) has a character(8 bit) and a color(4 bit) and 4 unused bit.

Bitfield is very useful in low-level software and embedded systems. For example in your scenario, it seems a consumer waiting for data that its 8 first bits will be treated as a character(ASCII), and its 4 next bits are treated as the color of that character. So your 80 element array means 80 characters that have its own color.