C++ - Set and remove specific bits from a single

Question

I am developing a Gameboy emulator and one of the tasks is to set the flags after each instruction:

    ... 

    //Representa pares de registradores do gameboy
    union RegPair {

        std::uint16_t reg;
        struct pair
        {
            std::uint8_t lo;
            std::uint8_t hi;
        }p;

    };

    struct Registers{

        RegPair AF, BC, DE, HL;
        std::uint16_t SP, PC;

    };

    ...

Set of Gameboy flags

...

//Define as flags do processador
#define FLAG_ZERO_FLAG      7
#define FLAG_SUBSTRCAT_FLAG 6
#define FLAG_HALF_CARRY     5
#define FLAG_CARRY_FLAG     4
#define FLAG_NONE           0

...

Initialization of registers:

    ...

    reg.AF.reg = 0x01B0;
    reg.BC.reg = 0x0013;
    reg.DE.reg = 0x00D8;
    reg.HL.reg = 0x014D;
    reg.SP = 0xFFFE;
    reg.PC = 0x0000;    //Ponto de entrada também pode ser 0x100

        ...

I’ll use the opcode example 0xD Decrement Register C, opcode table can be found here:

void CPU::DEC8(std::uint8_t& target, int cyclesToUpdate)
{
    cycles += cyclesToUpdate;
    target--;

    if (target == 0) {
        wxLogDebug("ZERO FLAG OCORREU");
        reg.AF.p.lo |= FLAG_ZERO_FLAG;
    }

    reg.PC++;

}

It turns out that if there is no zero flag I have to specifically reset the zero flag, and it can also occur to add the half carry flag, and if there is no reset to its position, I found this answer in stackoverflow but I found it confusing, for example, when passing a parameter to fstream:

("filename", std::ios::in | std::ios::binary) 

It seems to be something simpler than the way it is in the answer.