Enable forward declaration of json by making json a class instead of a using declaration

[pgoodman]

I would like to be able to be able to declare the conversion operators in a header without having to #include the json.h file. I would then define the conversion operators in a source file, which actually includes json.h.

Here is an example of what I want do do, but can't:

namespace nlohmann {
class json;
void to_json(json &j, const MyType &blah);
void from_json(const json &j, MyType &blah);
}  // namespace nlohmann

What is stopping me is using json = basic_json<>;. A simple fix should be class json : public basic_json<> {};.

[theodelrieu]

Making this change would break everything, but since version 3.1.0 you can use the multiple header version, to include the nlohmann/json_fwd.hpp, which does what you want :)

[nlohmann]

@pgoodman Did you have the chance to test whether include/nlohmann/json_fwd.hpp helps?

[pgoodman]

I didn't test the multiple header approach because I wanted to put the single header version into my project at third_party/JSON/json.h, rather than duplicate the nlohmann directory structure. I might change this in the future, though.

[gregmarr]

I would think that you could put both the single file and the fwd file in your project.

[pgoodman]

this had not occurred to me ;-)

[pgoodman]

That didn't work for me.

[theodelrieu]

Did you include json_fwd.hpp?

[nlohmann]

@pgoodman Could you provide more information like a small example program or the concrete error message?

[pasbi]

copying both json/include/nlohmann/json_fwd.hpp and json/single_include/nlohmann/json.hpp into my src/json/ directory enables me to use #include "json/json_fwd.hpp" whenever I need the declarations and #include "json/json.hpp" whenever definitions are required.