A weird building problem :-(

[tanpero]

I have a small problem.
I need to keep the insertion order of the JSON keys and output them. According to the existing suggestion, I used nlohmann::fifo_map instead of the original std::map, which is:

using json = nlohmann::basic_json<nlohmann::fifo_map>;

However, when I build, the compiler tells me this error:

add_key: isn't the member of std::less<void>

I guess nlohmann::fifo_map may need its own template parameters, but this depends on the type in the JSON library, so it won't work.
I checked the related issue, but others didn't seem to have encountered this problem. I want to know how this problem should be solved.
My development environment is: Visual Studio 2017
Thanks! :-D

[tanpero]

I tried change the code like this: ##1763 (comment)
and it looks like:

template<class K, class V, class dummy_compare, class A>
using my_fifo_map = nlohmann::fifo_map<K, V, nlohmann::fifo_map_compare<K>, A>;
using fifo_json = nlohmann::basic_json<my_fifo_map>;
using json = nlohmann::json;

But it still can't be compiled successfully. The error info is:

undefined identifier: `my_fifo_map`

[tete17]

@tanpero can you leave a small sample code, that produces the error you are seeing?

Maybe like a small main function?