Hello! I wrote this short blog post about using pattern-matching-like template metaprogramming to deserialize JSON at build time - please let me know what you think (especially if you see improvements)
If you're looking for an interesting follow-up project, here's something I had to do once that's now become much easier: compute a compile-time hash of the compilation for the current translation unit, e.g. __BASE_FILE__ hashed together with __TIMESTAMP__ or the equivalents for each platform.
This allows you to dynamically invalidate on-disk caches and trigger new-build tripwires based on ongoing revisions. Development and release builds are handled identically: if source file X handles a cache and X was recompiled, discard the cache.
I've written 2 iterations of a reflection library where you needed to annotate structs slightly with an ugly macro but once done you could just do: Message msg; if (parse_json(str,msg)) { ..process msg struct.. }
The previous iterations were for C++11 and C++17 but it seems that with C++20 features you don't even seem to need the macro uglyness so I personally think libraries need to move in the direction of plain old structs.
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?
long double result = 0.0;
while (...) {
if (json[head] == '.') ...
result *= 10; result += json[head] - '0';
in a constexpr function with no problem :)
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
In terms of the implementation ... I feel like C++ is best when used in an "orthodox style" and minimizing the use of templates as much as possible.
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1: https://learn.microsoft.com/en-us/dotnet/fsharp/tutorials/ty...