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Comment by svalorzen | Hacker News Reader

I recently actually tried to do a very similar thing, although a bit tighter in scope. What stopped me what that actually deserializing floating points cannot currently be done at compile time; the only utility available to do so is `from_chars` and it is only constexpr for ints.

I did not see any mention of this in the post; so are you actually simply extracting the string versions of the numbers, without verifying nor deserializing them?

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I was able to do the primitive

long double result = 0.0;

while (...) {

  if (json[head] == '.') ...

  result *= 10; result += json[head] - '0';

}

in a constexpr function with no problem :)

How about floats in scientific notation?

You can do something similar, no? std::pow is not constexpr (most float stuff is not, presumably due to floating point state) but you can implement 10^x anyway

The problem with this is that it will not actually parse double in IEEE 754, as you will accumulate inaccuracies at every step of the loop. In theory, when parsing a float, you are supposed to return the floating point that is closest to the original string number. Your code will not do that. Even if you accept the inaccuracy, if you for some reason load the JSON using a runtime library, you'll get different numbers and consequently and result that depend on those numbers. For my use-case this was not acceptable unfortunately..

Yes, very true. I noticed that even already at 3dp the floats start to compare unequal. The long double helped but it's not really.

I googled and found two examples of constexpr float parsing repositories, but from the sounds of things, you understand this problem better than I and will have seen them already

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