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Niels Lohmann
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* docs: writing generic code for json_view A "Writing generic code" section on the json_view page: why a function taking json_view accepts no other view, the three signatures for editable, read-only, or either kind of view (passed by value), why the modes do not convert, is_json_view, and how a from_json() reads views (constrained to json values and views; the macros and the library's conversions do it already) with what differs then (duplicate keys, JSON_DIAGNOSTICS paths). Signed-off-by: Niels Lohmann <mail@nlohmann.me> * docs: name the documents the generic view function accepts Signed-off-by: Niels Lohmann <mail@nlohmann.me> * docs: views of read-only documents convert to editable views 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.md#writing-generic-code) - 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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