Added forward declaration header json_forward.hpp. Minimise build times.

[zerodefect]

This pull request aims to reduce compile time by the introduction of a header dedicated to providing forward declaration of nlohmann::json.

One of the great merits of the this library is that it is header only. Unfortunately, that does also come with some disadvantages, namely the compile times are often higher with such code dependencies.

This pull request introduces a 'json_forward.hpp' header file (similar to how Boost offer forward header files) in an attempt to try reduce build times.

Things to note (and some of these come down to personal preference):

• json.hpp includes json_forward.hpp to minimise code replication
• json.hpp includes some of same std library headers as json_forward.hpp (technically these could be removed) but with #include guards, I don't feel this is much of an issue. Discuss.

Points to consider:

• Code change is small.
• It reduces build times (at least what I've seen).

So I tried integrating these code changes with my own project and ran a comparison test.

Build time without forward declaration header:

real 10m7.981s
user 36m40.768s
sys 2m1.432s

Build time making extensive use of forward declaration header (where possible):

real 9m21.282s
user 33m53.744s
sys 1m52.880s

Please note that this test was not wholly scientific. I imagine mileage will vary across environments/compilers and the level of use of this library in the said project.

Tested with cmake and gcc 6.3 running on Ubuntu's 17.04 (Zesty).

[theodelrieu]

Hi! This is already done in #700, although not merged (yet).

[zerodefect]

Thanks @theodelrieu, I see the changes in your pull request are numerous. Impressive! Any idea when your changes may be merged?

Could my changes function as a half-way house? Do your changes preclude my changes from being merged? Further clarification would be much appreciated.

[coveralls]

Coverage remained the same at 100.0% when pulling 08d5d76 on zerodefect:add_forward_declaration_hdr into 4c4f60f on nlohmann:develop.

[theodelrieu]

@zerodefect Soon I hope! Well the main issue is that we still want to provide an amalgamated version of json.hpp (you can see it in my PR) for people that still wish to use the single header. With your changes, users are forced to download two files.

All of that work is already done on #700, I need to rebase to solve minor conflicts.

Meanwhile you can use the json_fwd.hpp file on your projects, that's what we do at my work while waiting for the 3.0 release.

If you have more questions, let me know :)

[nlohmann]

I do not like breaking the header into two files at this moment. PR #700 proposes a restructuring into several files, but also provides a script to re-create a single header.

[nlohmann]

Sorry for not commenting earlier. As I wrote in #823 (review), I do not like the idea of splitting the header into several files. Furthermore, PR #700 already goes further and not only splits the code, but also provides a script to re-create a single header. Unless there I missed a strong argument for this PR, I would close it.