Read/write of std::chrono data

[inter-otter]

I'm using Glaze on a C++23 project to add JSON import/export of existing data structures.

Where the data structures use "normal" types, things work well. However I'm struggling to use Glaze to serialise/deserialise classes which have std::chrono members. I'm quite new to Glaze.

For example, one data structure is a std::vector of a type which has a std::chrono::year_month_day member as well as a std::chrono::system_clock::time_point member. I have tried to use custom reader/writer code with lambdas but I can't get it to compile.

I think part of the problem is that std::chrono::year_month_day has getters but no setters; I would need to create a new instance. I could replace std::chrono::year_month_day with a custom struct which does basically the same thing, but I am reluctant to change the current data definitions.

Has anyone got any snippet of code that uses Glaze to read/write std::chrono data? I'm not able to find any examples so far.

[stephenberry]

I've been wanting to add std::chrono support: #1110. It won't be too difficult to add custom to/from specializations to make this possible, but what I haven't thought through is how the JSON should look.

If you could provide some examples of std::chrono data and the associated JSON that you think would make sense, that would be really helpful for adding support. We want to make sure we can roundtrip std::chrono data.

[inter-otter]

For std::chrono::year_month_day I think the JSON is fairly simple:

using namespace std::chrono;
std::chrono::year_month_day ymd{2025y, std::chrono::March, 4d};

translates to

{"year":2025,"month":3,"day":4}

For std::chrono::system_clock::time_point we probably should include the 'unit', e.g.

std::chrono::system_clock::time_point tp = std::chrono::system_clock::now();

might translate to

{"unit":"microseconds","count":1739234225419488}

It would be nice to have std::chrono support built in to Glaze, but until then I think I need to use custom to/from lambdas, but I'm not having any luck yet. My "plan B" is to replace the std::chrono types in this baseline with simple structs which are easier to serialise.