Feat/explicit conversion operator #1559

theodelrieu · 2019-04-04T12:20:14Z

Here it is!

Thanks to @gracicot, who found a good solution to #958, I'm finally attempting to solve it.

This PR introduces a CMake option: JSON_ExplicitConversions, defaulted to OFF.

To help users move away from the implicit conversion operator, a warning will be issued each time it is used. This warning can be silenced by users defining JSON_USE_IMPLICIT_CONVERSIONS.

On the other hand, defining JSON_USE_EXPLICIT_CONVERSIONS will very likely break code, but will ease the transition to the next major version, where only explicit conversion operator will be kept.

Meanwhile, I've removed every implicit conversion from the library's code, including tests.
It might be a good idea to add a short ifdefed test case, which tests implicit conversions (to avoid duplicating a lot of test code).

With explicit conversions, we can also remove the constraints that are present today, e.g. forbidding conversion from char, std::initializer_list<char>. Note that this has not been done in this PR.

I think this change is more suited to be released in a minor version, so we might release 3.6.2 first?

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 file single_include/nlohmann/json.hpp. The whole process is described here.

theodelrieu · 2019-04-04T13:11:20Z

Where is the LGTM config file?

gracicot

Yeah, sorry I didn't had the time to submit the patch. Thanks for the submission!

My proposed solution was not as perfect as I thought, see my comment. Other than that, the diff seems fine for me!

gracicot · 2019-04-04T13:26:24Z

include/nlohmann/detail/macro_scope.hpp

+#ifndef JSON_USE_EXPLICIT_CONVERSIONS
+    #define JSON_EXPLICIT
+    #ifndef JSON_USE_IMPLICIT_CONVERSIONS
+        #warning Implicit conversions are being deprecated and will be \


I realized that the #warning statement is not standard, and not supported on msvc. There is some solution like #pragma message() but it's not a real warning.

I think we could use [[deprecated("...")]] or the msvc non standard __declspec(deprecated("...")) for earlier versions in that case.

I've added JSON_DEPRECATED_MSG, and tried building the tests by removing the Wno-deprecated warnings, it works \o/

Thank you, the patch is great now. I would just wait for @nlohmann to do a review too and merge it!

I like the idea of having a switch for the conversions, but I am still not convinced that implicit conversions should be deprecated.

The main reason is because it is broken today. The pile of issues related to #958 is a good indicator of that. Not to mention the why is quite hard to grasp.

The required changes are quite minimal as well:

std::string s; s = j; // becomes std::string s; j.get_to(s); std::string s = j; // becomes std::string s(j);

After thinking about it I think @nlohmann is right on not deprecate right now (I still think it should eventually). The compile switch must be there for a while before any deprecation, and the documentation should also rely on explicit conversion and it should recommend enabling the switch.

I would like to see the implicit operator be deprecated too but I also agree that even though the change in the syntax is minimal, its use is widespread so it must be done very gradually.

Edit: moved the example into the issue

coveralls · 2019-04-04T14:12:30Z

Coverage remained the same at 100.0% when pulling 7973293 on theodelrieu:feat/explicit_conversion_operator into 2cd10a7 on nlohmann:develop.

nlohmann · 2019-04-04T15:48:13Z

Where is the LGTM config file?

There is no config file. Here is a link to the log: https://lgtm.com/projects/g/nlohmann/json/logs/rev/pr-e7a2ac9be19e57d05404374d62bf6a8ab3794bfe/lang:cpp/stage:Build%20merge_d94187b754143b1064d1f7bcd3000e3db0419555. Strange that the test fails.

theodelrieu · 2019-04-05T08:00:00Z

But where do you define how to build the library?

theodelrieu · 2019-04-09T09:00:19Z

Ok, the LGTM build uses default CMake arguments, therefore it builds with the implicit conversions.
All the other builds use explicit conversions.

@nlohmann do you wish to do like with JSON_MultipleHeaders? i.e. define a variable in the CI files.

stale · 2019-05-09T09:50:24Z

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

theodelrieu · 2019-05-09T10:05:59Z

It would be nice to merge this PR and discuss what is the long-term plan after.

stale · 2019-06-08T10:31:20Z

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

stale · 2019-07-08T10:58:35Z

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

theodelrieu · 2019-07-30T09:15:39Z

@nlohmann Shall we merge this PR now that 3.7.0 has been released?

stale · 2019-08-29T09:43:22Z

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

gracicot · 2019-10-02T16:55:52Z

Since tackling the conversion operator is being candidate for version 4.0.0, I'd vote to add the deprecation back when JSON_ExplicitConversions is off.

If we want to use explicit conversion on the next major version, deprecation and the switch is the first step to transition the community.

theodelrieu · 2020-07-21T12:07:12Z

@nlohmann Seems good finally 🙏

test/src/unit-conversions.cpp

include/nlohmann/detail/meta/type_traits.hpp

test/src/unit-conversions.cpp

nlohmann · 2020-07-21T20:20:17Z

CMakeLists.txt

@@ -24,6 +24,7 @@ endif ()
 option(JSON_BuildTests "Build the unit tests when BUILD_TESTING is enabled." ON)
 option(JSON_Install "Install CMake targets during install step." ON)
 option(JSON_MultipleHeaders "Use non-amalgamated version of the library." OFF)
+option(JSON_ExplicitConversions "Enable explicit conversion operator" OFF)


I'm still struggling with the wording. To me, the switch "enable explicit conversion" sounds like an additional feature. Instead, it breaks existing code, because the assumed implicit conversions do not work any longer. If the code already only used explicit conversions, then flipping the switch does not change anything.

theodelrieu · 2020-07-22T07:24:44Z

@nlohmann About your last comment: I can swap the CMake option definition

wrap implicit conversions tests around the JSON_USE_IMPLICIT_CONVERSIONS macro

theodelrieu · 2020-07-22T12:08:46Z

@nlohmann GitHub was unreachable again, sigh. Can you restart the job please? 🙏 (https://travis-ci.org/github/nlohmann/json/jobs/710669524#L5359)

nlohmann

Looks good to me.

theodelrieu · 2020-07-22T13:55:13Z

@nlohmann It seems appveyor is stuck as well 🤔

nlohmann · 2020-07-22T15:56:42Z

No, it's just slow...

aokellermann · 2020-07-23T03:09:45Z

CMakeLists.txt

@@ -24,6 +24,7 @@ endif ()
 option(JSON_BuildTests "Build the unit tests when BUILD_TESTING is enabled." ON)
 option(JSON_Install "Install CMake targets during install step." ON)
 option(JSON_MultipleHeaders "Use non-amalgamated version of the library." OFF)
+option(JSON_ImplicitConversions "Enable implicit conversions" ON)


Maybe put period at end of description for consistency's sake.

gracicot · 2020-07-23T04:10:59Z

I wonder if this can be removed to re-enable conversions to std::string_view once we switch to explicit conversions:

    && !std::is_same<ValueType, typename std::string_view>::value

I think it was disabled because of a problem with string views and visual studio, but I'm not sure. If someone can find the issue/pull request, that would be awesome.

nlohmann · 2020-07-23T05:07:00Z

@gracicot Was this changed in this PR?

gracicot · 2020-07-23T05:16:35Z

@gracicot Was this changed in this PR?

No. The pull request mentioned that this hasn't been done forstd::initializer_list but I was wondering if std::string_view was worth to do. Right now json_value.get<std: string_view>() works, but not the conversion and is_gettable returns false for it.

nlohmann · 2020-07-23T06:50:36Z

No. The pull request mentioned that this hasn't been done forstd::initializer_list but I was wondering if std::string_view was worth to do. Right now json_value.get<std: string_view>() works, but not the conversion and is_gettable returns false for it.

I see. Let's handle this in a follow-up issue. This PR has been open for too long...

nlohmann · 2020-07-23T06:53:05Z

Thank you all so much!

nlohmann · 2020-07-23T06:53:49Z

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Add option to switch off implicit conversions. operator T() considered harmful #958 Feat/explicit conversion operator #1559

theodelrieu · 2020-07-23T08:40:12Z

@gracicot I think all these type of explicit SFINAE disabling can be removed when using explicit conversions. IIRC there is one for char as well somewhere.

gracicot · 2020-07-23T11:34:58Z

Thank you so much! 🎉

tlemo · 2020-07-23T21:59:05Z

Thanks @theodelrieu for driving this!

theodelrieu force-pushed the feat/explicit_conversion_operator branch from edc99df to 5ec90c8 Compare April 4, 2019 12:41

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gracicot mentioned this pull request Apr 11, 2019

operator T() considered harmful #958

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theodelrieu force-pushed the feat/explicit_conversion_operator branch from 8ce2e52 to ba4ee31 Compare December 2, 2019 09:19

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theodelrieu added 2 commits July 22, 2020 10:49

add a switch to enable implicit conversions (defaults to true)

74b446f

wrap implicit conversions tests around the JSON_USE_IMPLICIT_CONVERSIONS macro

add a CI job for explicit conversions on every os

7973293

theodelrieu force-pushed the feat/explicit_conversion_operator branch from cc74efb to 7973293 Compare July 22, 2020 08:49

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operator T() considered harmful #958

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nlohmann merged commit a048b72 into nlohmann:develop Jul 23, 2020

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theodelrieu mentioned this pull request Jul 28, 2020

nlohmann_json: add 3.9.0 (and implicit_conversions option) conan-io/conan-center-index#2279

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thomasgt mentioned this pull request Jan 7, 2022

[nlohmann-json] Add port feature to control implicit conversions behaviour microsoft/vcpkg#22408

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Feat/explicit conversion operator #1559

Feat/explicit conversion operator #1559

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theodelrieu commented Apr 4, 2019 • edited

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theodelrieu commented Jul 23, 2020

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