That has more to do with decimal <-> binary conversion than arithmetic/comparison. Using hex literals makes it clearer

     0x1.999999999999ap-4 ("0.1")
    +0x1.999999999999ap-4 ("0.1")
    ---------------------
    =0x3.3333333333334p-4 ("0.2")
    +0x1.999999999999ap-4 ("0.1")
    ---------------------
    =0x4.cccccccccccf0p-4 ("0.30000000000000004")
    !=0x4.cccccccccccccp-4 ("0.3")

I also like how a / b can result in infinity even if both a and b are strictly non-zero[1]. So be careful rewriting floating-point expressions.

[1]: https://www.cs.uaf.edu/2011/fall/cs301/lecture/11_09_weird_f... (division result matrix)

.125 + .375 == .5

You should be using == for floats when they're actually equal. 0.1 just isn't an actual number.

I would argue that

    double m_D{}; [...]

    if (m_D == 0) somethingNeedsInstantiation();

can avoid having to carry around, set and check some extra m_HasValueBeenSet booleans.

Of course, it might not be something you want to overload beginner programmers with.