Allowing std::map object with serialized Enum key type to be converted to a JSON object and vice versa - #4378

[amirghaz]

Abstract

This PR allows an std::map object with serialized Enum key type to be converted to a JSON object and vice versa (instead of the original JSON array). Now, a map object defined as std::map<TaskState, std::string> t1 = {{TS_STOPPED, "aa"}, {TS_COMPLETED, "bb"}} will be translated to {"stopped":"aa","completed":"bb"} instead of the original [["stopped","aa"],["completed","bb"]]. This constructed JSON object can also be translated back to the respective map object. fixes #4378.

Changes Proposed

Create new templated to_json and from_json functions and some helper functions to allow for this conversion, and also functions that can detect whether an Enum type have been serialized or not.

Possible Issues

This changes will make changing the map object to JSON object (when the key type is a serialized Enum type) as the default behavior, though this might be the expected and desired behavior.

Validation

The respective unit test will be added if the proposed change is accepted.

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Pull request checklist

Read the Contribution Guidelines for detailed information.

• Changes are described in the pull request, or an existing issue is referenced.
• The test suite compiles and runs without error.
• Code coverage is 100%. Test cases can be added by editing the test suite.
• The source code is amalgamated; that is, after making changes to the sources in the include/nlohmann directory, run make amalgamate to create the single-header files single_include/nlohmann/json.hpp and single_include/nlohmann/json_fwd.hpp. The whole process is described here.

Please don't

• The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
• Specifically, I am aware of compilation problems with Microsoft Visual Studio (there even is an issue label for this kind of bug). I understand that even in 2016, complete C++11 support isn't there yet. But please also understand that I do not want to drop features or uglify the code just to make Microsoft's sub-standard compiler happy. The past has shown that there are ways to express the functionality such that the code compiles with the most recent MSVC - unfortunately, this is not the main objective of the project.
• Please refrain from proposing changes that would break JSON conformance. If you propose a conformant extension of JSON to be supported by the library, please motivate this extension.
• Please do not open pull requests that address multiple issues.

[coveralls]

coverage: 99.649%. remained the same
when pulling f26bec8 on amirghaz:pr1
into 9f60e85 on nlohmann:develop.

[gregmarr]

There's a .clang-format file for this repo. You should make sure that your changes are properly formatted.

[gregmarr]

Looks like it's already incosistent for template<typename vs template < typename.

[gregmarr]

These includes are sorted.