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features/json_view/reading.md

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+# Reading values
+
+## Getting values out without copying
+
+[`get<T>()`](../../api/basic_json_view/get.md) converts most `T` directly from the flat index, without ever building a
+`basic_json` value for the conversion: `#!cpp bool`, arithmetic types, `#!cpp std::nullptr_t`,
+`#!cpp std::string`/other `#!cpp std::basic_string`s (copied once), `basic_json_view` itself, the standard containers
+and other types the library converts (`#!cpp std::vector`, `#!cpp std::map`, `#!cpp std::list`, `#!cpp std::array`,
+`#!cpp std::pair`, `#!cpp std::tuple`, `#!cpp std::optional`, ...), and the types of the `NLOHMANN_DEFINE_TYPE_*` and
+`NLOHMANN_JSON_SERIALIZE_ENUM` macros. Types whose `from_json()` is written for `basic_json` only go through
+[`materialize()`](../../api/basic_json_view/materialize.md)`.get<T>()` instead: the subtree is built into a real
+`basic_json` value first, exactly as [`parse()`](../../api/basic_json/parse.md) would, and converted from there. See
+[A `from_json()` that reads views](generic_code.md#a-from_json-that-reads-views) for how to write a `from_json()` that
+reads views, and [Duplicate keys](differences.md#duplicate-keys) for the one case in which the two ways give different
+results.
+
+Two conversions never copy at all:
+
+- [`get_string()`](../../api/basic_json_view/get_string.md) (equivalently, `#!cpp get<string_view_t>()`) returns a
+  string as a `string_view_t` pointing into the document's [`source()`](../../api/basic_json_document/source.md) text
+  — or, for a string that contains escape sequences, into the document's own buffer of decoded strings — instead of
+  allocating a new `#!cpp std::string`.
+- [`number_token()`](../../api/basic_json_view/number_token.md) returns a number exactly as it was written in the
+  source, e.g. `#!cpp "1.50"`, `#!cpp "1E2"`, or an integer with more digits than any number type holds, instead of
+  rounding it into a `#!cpp double`/`#!cpp int64_t` the way `#!cpp get<T>()` (and
+  [`basic_json::parse()`](../../api/basic_json/parse.md)) would.
+
+Both results are only valid as long as the document and, if it [borrows its source text](lifetime.md), that text.
+
+### `string_view` members
+
+A struct whose member is a `#!cpp std::string_view` (C++17) can be decoded from a view with the
+`NLOHMANN_DEFINE_TYPE_*` macros, and the member is not copied either: for a string without escape sequences it points
+into the document's source text, and for a string with escape sequences into the document's buffer of decoded
+strings. The member is valid as long as the document is alive and, for a document that
+[borrows its source text](lifetime.md) (as [`parse()`](../../api/basic_json_document/parse.md) does for an lvalue
+`#!cpp std::string`, a `#!cpp std::string_view`, or a C string), as long as that text is.
+
+```cpp
+struct record
+{
+    std::string_view name;
+    int count;
+    NLOHMANN_DEFINE_TYPE_INTRUSIVE(record, name, count)
+};
+
+const std::string text = R"({"name": "widget", "count": 3})";
+const json_document doc = json_document::parse(text);  // borrows text
+const record r = doc.root().get<record>();             // r.name points into text, valid while text and doc are
+```
+
+!!! warning "Decoding into a `string_view` from a `json` value dangles at once"
+
+    The same struct also compiles with `#!cpp json::parse(text).get<record>()`, but there `name` points into the
+    temporary `json` value, which is destroyed at the end of the statement. Use a named
+    [`json_document`](../../api/json_document.md), as above, to decode such a struct.
+
+## Writing a view back
+
+[`dump()`](../../api/basic_json_view/dump.md) serializes a view directly from the flat index, without ever building a
+`basic_json` value. It writes object members in document order and every occurrence of a repeated key, so it can
+differ from `#!cpp materialize().dump()`; see [Serialization](differences.md#serialization).
+
+By default, numbers are written the way [`basic_json::dump()`](../../api/basic_json/dump.md) would.
+[`number_format::source`](../../api/basic_json_view/number_format.md) instead copies every number exactly as it was
+written in the source text — a price like `#!cpp 19.90`, a long order or account ID with more digits than any number
+type holds, or a high-precision coordinate — something `basic_json` cannot do at all, since parsing already reduces
+a number to its parsed `#!cpp double`/`#!cpp int64_t` value. Together with an [editable document](editing.md), this
+lets you patch a file without reformatting the rest of it.
+
+[`operator<<`](../../api/basic_json_view/operator_ltlt.md) writes a view to a stream the way `basic_json`'s does, using
+the stream's `width`/`fill` for indentation.
+
+## See also
+
+- [Zero-copy JSON views](index.md) - overview
+- [`get`](../../api/basic_json_view/get.md) - convert a view
+- [`dump`](../../api/basic_json_view/dump.md) - serialize a view
+- [Writing generic code](generic_code.md) - functions and `from_json()` overloads that accept views
+
+## Version history
+
+- Added in version 3.13.0.