Simple ordered_json that works on all supported compilers

[gatopeich]

Solved the Allocator issue that prevented using a container whose value_type is not the same as std::map's.
The trick was in instantiating AllocatorType with ObjectType::value_type, instead of hard-coding to std::pair<const Key, basic_json>. This it stays the same for std::map but adapts to whatever other container we use in other specializations.

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• Changes are described in the pull request, or an existing issue is referenced.
• The test suite compiles and runs without error.
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Please don't

[X] checked too 🙂

[coveralls]

Coverage increased (+0.02%) to 99.981% when pulling 47d154d on gatopeich:issue2179 into 6ee9e5f on nlohmann:issue2179.

[gatopeich]

Almost there. Only missing default template arguments for our new ordered_map...

[gatopeich]

Looks like GCC 4.9 on Ubuntu 14 cannot handle the partial template definition required for my current solution.
Not sure it is reasonable to try to support such obsolete platform?

[nlohmann]

When I look at https://travis-ci.org/github/nlohmann/json/builds/700984944, there seem to be a lot of other compilers complaining. I restarted all failed builds, let's see.

[gatopeich]

Using std::map allocator as a placeholder (acd748e) is the most old-compiler friendly way I could find for the recursive declaration of the Allocator. And it is still not working on some compilers.

Summarizing:

• Current implementation has object_t::value_type hardcoded to that of std::map: std::pair<const Key, T>, cause AllocatorType is always specialized for that.
• Providing a non-hardcoded allocator seems impossible for certain compilers without potentially complex refactoring.

So I am trying the next alternative: Implement VecMap:erase that works with the same value_type as std:map...

[nlohmann]

Any ideas how proceed here?

[gatopeich]

I am quite frustrated about how it fails on GCC 4.9 (quite old anyway) and specially XCode 9.3 (not that old but I have no access to such build system).

For the time being we could restrict ordered_json to recent compilers?

Fact is, if we cannot make this simple container work, the claim that users can roll their own seems ambitious... However the same applies: a recent compiler is required.

[gatopeich]

Fixed for GCC 5.5, which was giving the same error as v4.9 on travis-ci.
After isolating the issue to one of constructor instantiation, I found a cryptic reference to a retroactive change to the C++11 standard related to issues around constructor inheritance...
So I declared explicit constructors instead, and GCC 5.5 seems happy now.

[gatopeich]

Now it works on all environments.
Reviewing the comments and any unintended changes to white space etc...

[gatopeich]

@nlohmann this is all yours now... 🙂

[nlohmann]

Some minor comments.

[gatopeich]

Ops! done now.

[nlohmann]

Looks good to me.

[nlohmann]

Thanks a lot!!!

[nlohmann]

I opened a new pull request #2258 to bring the feature to the main branch. Thanks so much for your patience and bringing this forward!