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111 lines
4.6 KiB
Markdown
111 lines
4.6 KiB
Markdown
# <small>nlohmann::basic_json_view::</small>get_string
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```cpp
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string_view_t get_string() const;
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```
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Returns the string value as a [`string_view_t`](index.md#member-types), without copying it.
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## Return value
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The string, as a [`string_view_t`](index.md#member-types) that points either into the document's
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[`source()`](../basic_json_document/source.md) text (a string with no escape sequences), or into the document's own
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buffer of decoded strings (a string that contains escape sequences, such as `#!json "\n"` or `#!json "\u00e9"`, which
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had to be decoded once when the document was parsed).
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## Exception safety
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Strong guarantee: there are no changes to the view or the document it refers to.
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## Exceptions
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Throws [`type_error.302`](../../home/exceptions.md#jsonexceptiontype_error302) if the value is not a string; example:
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`"type must be string, but is array"`.
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This exception does not carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics.md) path: the view has no
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`BasicJsonType` value to point at, so the exception is created without one, even if `BasicJsonType` was built with
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`JSON_DIAGNOSTICS` enabled.
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## Complexity
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Constant.
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## Notes
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`basic_json_view` has no `BasicJsonType` value stored anywhere, so unlike `BasicJsonType`, it has no `get_ref()` to
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hand out a reference to a stored `string_t`. `get_string()` (equivalently, [`get<string_view_t>()`](get.md)) is the
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zero-copy alternative: [`BasicJsonType::get_ref<const string_t&>()`](../basic_json/get_ref.md) is its closest
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counterpart, except that it returns a view instead of a reference to a value that must already exist.
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`get_string()` borrows the string: nothing is copied, whereas [`get<std::string>()`](get.md) copies it. For a borrowed
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document, the returned [`string_view_t`](index.md#member-types) is valid as long as the caller's buffer; for an owned
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one, as long as the document.
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The returned [`string_view_t`](index.md#member-types) is valid exactly as long as the view that produced it -- see the
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[validity rules](index.md) of `basic_json_view` -- and, for a string with no escapes, for as long as the document's
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source text, which for a borrowed document must remain valid **and unmodified** for as long as the document exists,
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since floating-point values and unescaped strings are read from the source lazily.
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## Examples
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??? example
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The example below pulls one field out of a JSON text that stands in for a large API response, and shows that no
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`#!cpp std::string` was allocated for it: the returned view still points inside the original buffer. A field that
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contains an escape sequence cannot point into the original text -- it was decoded once into the document's own
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buffer instead -- but still avoids a per-field allocation.
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```cpp
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#include <iostream>
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#include <nlohmann/json_view.hpp>
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#include <string>
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using json_document = nlohmann::json_document;
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using json_view = nlohmann::json_view;
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int main()
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{
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// a "large response" stand-in: only the "id" field is ever read out of it
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const std::string text =
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R"({"id": "8f14e45f-ceea-467e-bb92-963f5e3c7a08", "note": "created via API\n", "payload": "..."})";
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const json_document doc = json_document::parse(text);
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const json_view response = doc.root();
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const json_view::string_view_t id = response["id"].get_string();
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std::cout << id << '\n';
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// no std::string was allocated for "id": its bytes still live inside
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// the original buffer, so id's data lies inside [text.data(),
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// text.data() + text.size())
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const bool id_in_source = id.data() >= text.data() && id.data() + id.size() <= text.data() + text.size();
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std::cout << std::boolalpha << id_in_source << '\n';
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// "note" contains an escape sequence ('\n'), so it was decoded once
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// into the document's own buffer -- get_string() still avoids a copy
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// into a new std::string, but the bytes no longer live inside "text"
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const json_view::string_view_t note = response["note"].get_string();
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const bool note_in_source = note.data() >= text.data() && note.data() + note.size() <= text.data() + text.size();
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std::cout << std::boolalpha << note_in_source << '\n';
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}
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```
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Output:
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```json
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8f14e45f-ceea-467e-bb92-963f5e3c7a08
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true
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false
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```
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## See also
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- [get](get.md) - convert the value to a given type (`#!cpp get<string_view_t>()` is equivalent to this function)
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- [number_token](number_token.md) - a number's token text, without a copy
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- [`BasicJsonType::get_ref`](../basic_json/get_ref.md) - the closest counterpart of `basic_json`
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## Version history
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- Added in version 3.13.0.
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