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Niels Lohmann
6a852ab96d
* Split the json_view feature page into sub-pages
The single features/json_view.md page grew to ~600 lines covering eight
topics. Split it into features/json_view/ with an overview page and
pages for ownership and lifetime, differences from basic_json, reading
values, generic code, editing, and images.
The new differences page collects what was spread over several sections
("What is the same", "What is different", "Duplicate keys", and notes in
the conversion and serialization sections) under one summary table, and
adds the parts of the basic_json interface a view does not have. The
other pages now link to it instead of restating the rules.
Internal links across the documentation point to the new pages.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
* Docs: fix stale comments and trim repeated text in json_view pages
- fig-jsonview.js/.css: point the header comments at the new pages
- reading.md: views' strings are valid as long as the document (and a
borrowed source text), as lifetime.md says
- reading.md, generic_code.md: link to differences.md instead of
restating the member-order and repeated-key rules
- differences.md: drop the third mention of ordering comparisons, name
rbegin()/rend() in the iterator row
- exceptions.md: link editable documents to editing.md
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
---------
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
57 lines
1.8 KiB
Markdown
57 lines
1.8 KiB
Markdown
# <small>nlohmann::</small>is_json_view
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<small>Defined in header `<nlohmann/json_view.hpp>`</small>
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```cpp
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template<typename T>
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struct is_json_view;
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```
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A type trait: `#!cpp is_json_view<T>::value` is `#!cpp true` if `T` is a [`basic_json_view`](basic_json_view/index.md)
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of any `BasicJsonType` and any [`view_mode`](view_mode.md) -- [`json_view`](json_view.md),
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[`json_editable_view`](json_editable_view.md), [`ordered_json_view`](ordered_json_view.md), ... -- and
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`#!cpp false` otherwise, for [`basic_json`](basic_json/index.md) as well.
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It is meant for constraining templates that accept views, in particular a `from_json()` that should read views
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directly: [`get<T>()`](basic_json_view/get.md) on a view calls a `from_json()` with the view only if that `from_json()`
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accepts views but not arbitrary types.
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## Template parameters
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`T`
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: the type to check (without cv-qualifiers or references)
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## Notes
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A view is recognized by its member type `basic_json_type` (a [`basic_json`](basic_json/index.md) specialization) and
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its [`materialize()`](basic_json_view/materialize.md) returning that type, so the trait needs no declaration of the
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view class.
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## Examples
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??? example
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The example below shows the trait, and a `from_json()` constrained to `json` values and views: `get<T>()` on a view
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reads the view directly.
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```cpp
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--8<-- "examples/is_json_view.cpp"
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```
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Output:
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```
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--8<-- "examples/is_json_view.output"
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```
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## See also
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- [basic_json_view](basic_json_view/index.md) - a handle to one value of a document
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- [get](basic_json_view/get.md) - which conversions read the view directly
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- [view_mode](view_mode.md) - read-only or editable
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- [Writing generic code](../features/json_view/generic_code.md) - functions and `from_json()` for views
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## Version history
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Added in version 3.13.0.
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