Niels Lohmann
94fe31eb9b
* 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>
3.7 KiB
nlohmann::view_mode
Defined in header <nlohmann/json_view.hpp>
enum class view_mode : std::uint8_t
{
read_only,
editable
};
The second template parameter of basic_json_document and
basic_json_view: whether a document can be changed after parsing, and whether a view is
of such a document.
read_only- the default: the document is parsed once and then only read. It carries none of the bookkeeping edits need, and
calling
set,push_back,insert, oreraseon it fails to compile. editable- the document also offers
set,push_back,insert, anderase; its views see every edit, see Edits.
The shortcuts name both modes: json_document and json_view are read-only,
json_editable_document and json_editable_view are editable
(and likewise for ordered_json).
Notes
The mode is a compile-time property of the document's and the view's type, not a setting of a call. A read-only view
reads a container in place, while a view of an editable document follows the links that edits leave behind. A view of
a read-only document converts implicitly to a view of an editable document of the same BasicJsonType: a read-only
document has no edits, so the converted view reads it like an editable document before its first edit (at the cost of
one check per access). There is no conversion the other way, because a read-only view of an editable document would
read wrong values after the next edit.
A function that takes a json_editable_view therefore accepts the views of both modes. A
function template deduces the mode from its argument instead, without conversions, so it names the modes it accepts in
its signature:
// views of editable documents only
template<typename BasicJsonType>
void f(basic_json_view<BasicJsonType, view_mode::editable> v);
// views of read-only documents only
template<typename BasicJsonType>
void f(basic_json_view<BasicJsonType, view_mode::read_only> v);
// views of either document type
template<typename BasicJsonType, view_mode Mode>
void f(basic_json_view<BasicJsonType, Mode> v);
A function that takes a json_view accepts neither an ordered_json_view
nor a json_editable_view. Views of the two modes compare with
operator==.
Examples
??? example
The example below shows a function template that accepts the views of both modes, one that accepts only views of
editable documents, and a function taking a `json_editable_view` that also accepts a view of a read-only document.
```cpp
--8<-- "examples/view_mode.cpp"
```
Output:
```
--8<-- "examples/view_mode.output"
```
See also
- basic_json_document - a parsed JSON text
- basic_json_view - a handle to one value of a document
- json_editable_document - an editable document of
json - (constructor) - the conversion from a read-only to an editable view
- Writing generic code - functions that accept views
Version history
Added in version 3.13.0.