How to implement message class with configurable bit as either 0 or 1 in C++?

Question

I am trying to create a message class with configurable as either 0 or 1 such as this

1 Byte, unit 8
Bit 0: Port A, 1 connected, 0 disconnected
Bit 1: Port A, 1 connected, 0 disconnected
Bit 2: Port A, 1 connected, 0 disconnected
Bit 3, 4, 5, 6, 7: Reserved

This I have been able to implement using union and type punning in C++ but this is seen as undefined behavior in C++.

union Msg{
    uint8_t bitfield;
    struct {
        uint8_t PORTA : 1;
        uint8_t PORTB : 1;
        uint8_t PORTC : 1;
        uint8_t : 5;
    } msg;
};

How can I do this in C++?

And is it possible to create a function to pack data into this structure?

Answer 1

You can use bitwise operations. First, define helper functions for getting/setting a bit:

bool get_bit(uint8_t bits, uint8_t index) {
    return bool((bits >> index) & 1U);
}
uint8_t set_bit(uint8_t bits, uint8_t index, bool value) {
    return (bits & ~(1U << index)) | (uint8_t(value) << index);
}

Then, you can use uint8_t instead of Msg or define Msg as an alias for uint8_t .

// Msg is an alias for uint8_t
using Msg = uint8_t;
Msg message = 0;

message = set_bit(message, 0, true); // Setting the PORT A on
message = set_bit(message, 1, true); // Setting the PORT B on
message = set_bit(message, 2, false); // Setting the PORT C off
// You can use 1 or 0 instead of true or false
message = set_bit(message, 0, 0); // Setting the PORT A off

bool port_b = get_bit(message, 1);