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Niels Lohmann
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* docs: cross-reference json_view and json_document for 3.13.0 Document the new zero-copy document/view API across the docs: add a "Documents and views" section to the basic_json API hub linking to all specializations, point parsing/conversion/modifying pages to the view-based alternatives, add document/view sections to feature pages, update the landing page to mention json_view.hpp/json_literals.hpp, and cross-link debugging/exceptions. Also document wide-string inputs (and ill-formed UTF-16/32 rejection), add parse/accept examples for wide strings, fix related doc gaps, and align features/parsing and FAQ with the added support. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * docs: fix article before std::istream in FAQ Signed-off-by: Niels Lohmann <mail@nlohmann.me> * docs: clarify document inputs, decoding, get<T>() and editing Address review comments: say which inputs are borrowed and that a wide string pointer is an extra input, which values are decoded during parsing, that get<T>() reads only built-in types directly, all forms of set() and that the document's insert() is array-only, and how to store wide strings. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * docs: integers are converted while parsing a document Also say "They accept every input" in basic_json_document::parse. Signed-off-by: Niels Lohmann <mail@nlohmann.me> --------- Signed-off-by: Niels Lohmann <mail@nlohmann.me>
109 lines
4.8 KiB
Markdown
109 lines
4.8 KiB
Markdown
# Modifying values
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Once a JSON value exists, its content can be changed: elements can be added, replaced, merged, and removed. This page
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gives an overview of the available operations. For read access, see [element access](element_access/index.md).
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## Adding to arrays
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New elements are appended to an array with [`push_back`](../api/basic_json/push_back.md) or constructed in place with
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[`emplace_back`](../api/basic_json/emplace_back.md). If the value is `#!json null`, it is converted to an array first, so
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these functions can also be used to build an array from scratch.
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```cpp
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json j; // null
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j.push_back(1); // [1]
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j.push_back(2); // [1,2]
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j.emplace_back(3); // [1,2,3]
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// operator+= is a shorthand for push_back
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j += 4; // [1,2,3,4]
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```
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## Adding to objects
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The most common way to add or replace a member is [`operator[]`](element_access/unchecked_access.md), which inserts the
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key if it does not exist yet:
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```cpp
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json j;
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j["name"] = "Mary"; // {"name":"Mary"}
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j["name"] = "John"; // {"name":"John"} (replaced)
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```
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[`emplace`](../api/basic_json/emplace.md) inserts a member only if the key is not already present, and reports whether
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the insertion happened — useful for "add if absent" semantics.
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## Merging objects
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To merge one object into another, [`update`](../api/basic_json/update.md) copies all members from another object
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(similar to Python's `dict.update`). This is the idiomatic way to combine two objects. It has two modes:
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- By default, the merge is shallow: existing keys are overwritten, even if both values are objects.
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- With `merge_objects = #!cpp true`, keys whose values are objects in both JSON values are merged recursively.
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Everything else is overwritten. In particular, arrays are replaced, not concatenated.
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??? example
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```cpp
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--8<-- "examples/update.cpp"
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```
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Output:
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```json
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--8<-- "examples/update.output"
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```
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A common use of the recursive mode is combining defaults with user settings. Nested defaults that the user did not set
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are kept:
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```cpp
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json defaults = {{"log", {{"level", "info"}, {"file", "app.log"}}}, {"retries", 3}};
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json user_settings = {{"log", {{"level", "debug"}}}};
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json config = defaults;
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config.update(user_settings, true);
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// {"log":{"file":"app.log","level":"debug"},"retries":3}
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```
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[JSON Merge Patch](merge_patch.md) ([RFC 7396](https://tools.ietf.org/html/rfc7396)) also merges objects recursively,
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but it is a different tool: a `#!json null` in the patch means "remove this key". It is meant for applying merge patch
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documents (e.g., received via HTTP PATCH). To merge configuration-like objects, use `#!cpp update(..., true)`. To apply
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a sequence of well-defined edit operations, see [JSON Patch](json_patch.md).
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## Removing elements
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Elements are removed with [`erase`](../api/basic_json/erase.md), which accepts an object key, an array index, or an
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iterator. [`clear`](../api/basic_json/clear.md) empties a value while keeping its type, and
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[`operator[]`](element_access/unchecked_access.md) combined with assignment can overwrite a value entirely.
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```cpp
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json j = {{"a", 1}, {"b", 2}, {"c", 3}};
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j.erase("b"); // {"a":1,"c":3}
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json a = {1, 2, 3, 4};
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a.erase(1); // [1,3,4] (erase by index)
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```
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## Editing a document
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The operations above work on a `basic_json` value. A JSON text that was parsed into a
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[document](json_view.md#editing-a-document) can also be changed in place, without building a `basic_json` tree for the
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parts that are not touched: [`set`](../api/basic_json_document/set.md) replaces a value, adds or replaces an object
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member, or replaces an array element (also through a JSON pointer),
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[`push_back`](../api/basic_json_document/push_back.md) appends to an array,
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[`insert`](../api/basic_json_document/insert.md) inserts into an array (unlike `basic_json::insert`, it has no object
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overload; members are added with `set`), and [`erase`](../api/basic_json_document/erase.md) removes a member or an
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element. These functions are available on the
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editable document types, such as [`json_editable_document`](../api/json_editable_document.md).
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## See also
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- [`push_back`](../api/basic_json/push_back.md) / [`emplace_back`](../api/basic_json/emplace_back.md) - append to an array
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- [`emplace`](../api/basic_json/emplace.md) - insert into an object if the key is absent
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- [`update`](../api/basic_json/update.md) - merge objects (shallow, or recursive with `merge_objects`)
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- [`merge_patch`](../api/basic_json/merge_patch.md) - apply an RFC 7396 merge patch
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- [`erase`](../api/basic_json/erase.md) / [`clear`](../api/basic_json/clear.md) - remove elements
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- [JSON Patch and Diff](json_patch.md) and [JSON Merge Patch](merge_patch.md) - structured modifications
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- [Editing a document](json_view.md#editing-a-document) - change a parsed document in place
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