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add new is_constructible_* traits used in from_json #1301

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merged 2 commits into from
Oct 18, 2018

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theodelrieu
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@theodelrieu theodelrieu commented Oct 16, 2018

is_compatible_* traits were used in from_json, but it made no sense
whatsoever, since they check if a type can be converted TO json.

It used to work because of non-SFINAE correctness + json_ref
unconstrained variadic template constructor.

SFINAE checks are becoming quite complex, we need a specification of
some sort describing:

  • which concepts the library uses
  • how the conversion to/from json works in detail

Having such a specification would really help simplifying the current
code (as well as having meaningful checks).

Fixes #1299


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is_compatible_* traits were used in from_json, but it made no sense
whatsoever.

It used to work because of non-SFINAE correctness + json_ref
unconstrained variadic template constructor.

SFINAE checks are becoming quite complex, we need a specification of
some sort describing:

* which concepts the library uses
* how the conversion to/from json works in detail

Having such a specification would really help simplifying the current
code (as well as having meaningful checks).

Fixes !1299
@nlohmann
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I can only trust the code - I can't provide a thorough review right now.


// This trait checks if JSONSerializer<T>::from_json(json const&) exists
// this overload is used for non-default-constructible user-defined-types
template <typename BasicJsonType, typename T, typename = void>
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Maybe add a comment on whether or not 'T' is expected to be a concrete type (string) or is allowed to be a CV-qualified type (string const&). It's not clear if these need std::decay's in there or whether the layer above it should add them?

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IIRC, the only places where CV-qualified types are specified in the current code is in is_detected related code.

I can add a comment at the top of the file.

@theodelrieu
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I'll open an issue in the next few days to explain in detail what I meant in the commit message.

I hope it will be a lot clearer then :)

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coveralls commented Oct 17, 2018

Coverage Status

Coverage remained the same at 100.0% when pulling a946dfc on theodelrieu:fix/1299 into dd67293 on nlohmann:develop.

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Looks good to me.

@nlohmann nlohmann self-assigned this Oct 18, 2018
@nlohmann nlohmann added this to the Release 3.3.1 milestone Oct 18, 2018
@nlohmann nlohmann merged commit 9294e25 into nlohmann:develop Oct 18, 2018
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Thanks!

@theodelrieu theodelrieu deleted the fix/1299 branch October 18, 2018 22:43
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4 participants