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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/json_view/lifetime/" rel="canonical"><link href=../ rel=prev><link href=../differences/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.8.0"><title>Ownership and lifetime - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.57a0d79d.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.fb2fffe2.min.css><link rel=stylesheet href=../../../css/custom.css><link rel=stylesheet href=../../../css/brand.css><link rel=stylesheet href=../../../css/fig-sax.css><link rel=stylesheet href=../../../css/fig-jsonview.css><link 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content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Ownership and lifetime </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 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viewbox="0 0 24 24"><paLine truncated
|
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<span class=cp>#include</span><span class=w> </span><span class=cpf><string></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><utility></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json_view.hpp></span>
|
||||
|
||||
<span class=k>using</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json_document</span><span class=p>;</span>
|
||||
|
||||
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
|
||||
<span class=p>{</span>
|
||||
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>boolalpha</span><span class=p>;</span>
|
||||
|
||||
<span class=w> </span><span class=c1>// BORROWED: doc only points into `text`; `text` must outlive `doc`</span>
|
||||
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=w> </span><span class=n>text</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s>{"a": 1}</span><span class=dl>)</span><span class=s>"</span><span class=p>;</span>
|
||||
<span class=w> </span><span class=n>json_document</span><span class=w> </span><span class=n>doc</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json_document</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>text</span><span class=p>);</span>
|
||||
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>doc</span><span class=p>.</span><span class=n>owns_source</span><span class=p>()</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span><span class=w> </span><span class=c1>// false</span>
|
||||
|
||||
<span class=w> </span><span class=c1>// a view is valid as long as the document is alive, has not been</span>
|
||||
<span class=w> </span><span class=c1>// re-parsed (read()) or shrunk (shrink_to_fit()), and -- if borrowed --</span>
|
||||
<span class=w> </span><span class=c1>// the source text is alive</span>
|
||||
<span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json_view</span><span class=w> </span><span class=n>v</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>doc</span><span class=p>.</span><span class=n>root</span><span class=p>();</span>
|
||||
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>v</span><span class=p>.</span><span class=n>is_object</span><span class=p>()</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span>
|
||||
|
||||
<span class=w> </span><span class=c1>// re-parsing the SAME document invalidates views taken before the call;</span>
|
||||
<span class=w> </span><span class=c1>// `v` above must not be used after this line</span>
|
||||
<span class=w> </span><span class=n>doc</span><span class=p>.</span><span class=n>read</span><span class=p>(</span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s>[1, 2, 3]</span><span class=dl>)</span><span class=s>"</span><span class=p>);</span>
|
||||
<span class=w> </span><span class=n>v</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>doc</span><span class=p>.</span><span class=n>root</span><span class=p>();</span><span class=w> </span><span class=c1>// take a fresh view instead</span>
|
||||
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>v</span><span class=p>.</span><span class=n>is_array</span><span class=p>()</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span>
|
||||
|
||||
<span class=w> </span><span class=c1>// moving the document does not invalidate views: the node index is</span>
|
||||
<span class=w> </span><span class=c1>// heap-allocated and does not move with the document object</span>
|
||||
<span class=w> </span><span class=n>json_document</span><span class=w> </span><span class=n>moved</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>move</span><span class=p>(</span><span class=n>doc</span><span class=p>);</span>
|
||||
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>v</span><span class=p>.</span><span class=n>is_array</span><span class=p>()</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=kc>false</span>
|
||||
<span class=kc>true</span>
|
||||
<span class=kc>true</span>
|
||||
<span class=kc>true</span>
|
||||
</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> - overview</li> <li><a href=../../../api/basic_json_document/ ><code>basic_json_document</code></a> - the document that owns the parsed index</li> <li><a href=../../../api/basic_json_document/owns_source/ ><code>owns_source()</code></a> - whether a document borrows its source text</li> </ul> <h2 id=version-history>Version history<a class=headerlink href=#version-history title="Permanent link">¶</a></h2> <ul> <li>Added in version 3.13.0 <span class=unreleased-version title="Not part of a release yet; the latest release is 3.12.0.">unreleased</span>.</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="November 9, 2026 07:12:00 UTC">November 9, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../ class="md-footer__link md-footer__link--prev" aria-label="Previous: Zero-copy JSON views"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Zero-copy JSON views </div> </div> </a> <a href=../differences/ class="md-footer__link md-footer__link--next" aria-label="Next: Differences from basic_json"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Differences from basic_json </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M173.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6m-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2c-2.6-.7-5.5.3-6.2 2.3m44.2-1.7c-2.9.7-4.9 2.6-4.6 4.9.3 2 2.9 3.3 5.9 2.6 2.9-.7 4.9-2.6 4.6-4.6-.3-1.9-3-3.2-5.9-2.9M252.8 8C114.1 8 8 113.3 8 252c0 110.9 69.8 205.8 169.5 239.2 12.8 2.3 17.3-5.6 17.3-12.1 0-6.2-.3-40.4-.3-61.4 0 0-70 15-84.7-29.8 0 0-11.4-29.1-27.8-36.6 0 0-22.9-15.7 1.6-15.4 0 0 24.9 2 38.6 25.8 21.9 38.6 58.6 27.5 72.9 20.9 2.3-16 8.8-27.1 16-33.7-55.9-6.2-112.3-14.3-112.3-110.5 0-27.5 7.6-41.3 23.6-58.9-2.6-6.5-11.1-33.3 2.6-67.9 20.9-6.5 69 27 69 27 20-5.6 41.5-8.5 62.8-8.5s42.8 2.9 62.8 8.5c0 0 48.1-33.6 69-27 13.7 34.7 5.2 61.4 2.6 67.9 16 17.7 25.8 31.5 25.8 58.9 0 96.5-58.9 104.2-114.8 110.5 9.2 7.9 17 22.9 17 46.4 0 33.7-.3 75.4-.3 83.6 0 6.5 4.6 14.4 17.3 12.1C436.2 457.8 504 362.9 504 252 504 113.3 391.5 8 252.8 8M105.2 352.9c-1.3 1-1 3.3.7 5.2 1.6 1.6 3.9 2.3 5.2 1 1.3-1 1-3.3-.7-5.2-1.6-1.6-3.9-2.3-5.2-1m-10.8-8.1c-.7 1.3.3 2.9 2.3 3.9 1.6 1 3.6.7 4.3-.7.7-1.3-.3-2.9-2.3-3.9Line truncated
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@@ -0,0 +1,83 @@
|
||||
# Ownership and lifetime
|
||||
|
||||
A document either **borrows** the text it was parsed from, or **owns** its own copy of it; call [`owns_source()`](https://json.nlohmann.me/api/basic_json_document/owns_source/index.md) to find out which happened. [`parse()`](https://json.nlohmann.me/api/basic_json_document/parse/index.md) decides this from the value category and type of its argument (an lvalue `std::string` is borrowed; an rvalue `std::string` is moved in, owned without a copy; a stream is read into an owned buffer; and so on — see [`parse`'s Notes](https://json.nlohmann.me/api/basic_json_document/parse/#notes) for the full table). [`parse_copy()`](https://json.nlohmann.me/api/basic_json_document/parse_copy/index.md) always owns a copy, regardless of the input.
|
||||
|
||||
A borrowed document depends on your buffer
|
||||
|
||||
If a document borrows its text, that text **must outlive the document** (and every view taken from it) and must stay **unmodified** for as long as the document exists, since floating-point values and unescaped strings are read from it lazily. Reading or writing through a view after the underlying buffer is gone is undefined behavior, exactly as it would be for a dangling `std::string_view`.
|
||||
|
||||
## When a view is valid
|
||||
|
||||
A view is valid only while all of the following hold:
|
||||
|
||||
- the document is alive,
|
||||
- the document has not been re-parsed since the view was taken (with [`read()`](https://json.nlohmann.me/api/basic_json_document/read/index.md) or [`parse()`](https://json.nlohmann.me/api/basic_json_document/parse/index.md) into it),
|
||||
- the document has not had [`shrink_to_fit()`](https://json.nlohmann.me/api/basic_json_document/shrink_to_fit/index.md) called on it since, and
|
||||
- if the document borrows its source text, that text is still alive and unmodified.
|
||||
|
||||
Moving the `basic_json_document<>` itself is fine and does **not** invalidate its views: the index is a separate heap allocation that keeps its address across the move. Take a fresh view from [`root()`](https://json.nlohmann.me/api/basic_json_document/root/index.md) whenever any of the other conditions above was not met. Edits of an [editable document](https://json.nlohmann.me/features/json_view/editing/index.md) keep views valid, too; see [What stays valid](https://json.nlohmann.me/features/json_view/editing/#what-stays-valid-and-what-an-edit-costs).
|
||||
|
||||
Four timelines of a document and its views, each bar showing when an object is alive or valid and when it dangles. Destroying the borrowed text, re-parsing, or shrinking the document invalidates the views taken before; moving the document does not.
|
||||
|
||||
## Why views can dangle
|
||||
|
||||
A view can dangle, exactly like a `std::string_view`: nothing stops you from using it after its document is gone. This is deliberate. A view is two pointers and trivially copyable; making it safe (a `std::shared_ptr` in the document and a `std::weak_ptr` in each view) would make it larger, add reference counting to every copy, and a `lock()` to every access — exactly the costs the view exists to avoid. What is caught is the obvious case: because a view dies with its document, [`root()`](https://json.nlohmann.me/api/basic_json_document/root/index.md) is not callable on a temporary document: `auto v = json_document::parse(text).root();` does not compile. Give the document a name first.
|
||||
|
||||
The same holds for what a view hands out without copying: a string from [`get_string()`](https://json.nlohmann.me/api/basic_json_view/get_string/index.md), a token from [`number_token()`](https://json.nlohmann.me/api/basic_json_view/number_token/index.md), and [`string_view` members](https://json.nlohmann.me/features/json_view/reading/#string_view-members) of decoded structs are valid only as long as the document and, if it borrows, its source text.
|
||||
|
||||
Example: borrowed and owned documents, and when views become invalid
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
#include <string>
|
||||
#include <utility>
|
||||
#include <nlohmann/json_view.hpp>
|
||||
|
||||
using nlohmann::json_document;
|
||||
|
||||
int main()
|
||||
{
|
||||
std::cout << std::boolalpha;
|
||||
|
||||
// BORROWED: doc only points into `text`; `text` must outlive `doc`
|
||||
std::string text = R"({"a": 1})";
|
||||
json_document doc = json_document::parse(text);
|
||||
std::cout << doc.owns_source() << '\n'; // false
|
||||
|
||||
// a view is valid as long as the document is alive, has not been
|
||||
// re-parsed (read()) or shrunk (shrink_to_fit()), and -- if borrowed --
|
||||
// the source text is alive
|
||||
nlohmann::json_view v = doc.root();
|
||||
std::cout << v.is_object() << '\n';
|
||||
|
||||
// re-parsing the SAME document invalidates views taken before the call;
|
||||
// `v` above must not be used after this line
|
||||
doc.read(R"([1, 2, 3])");
|
||||
v = doc.root(); // take a fresh view instead
|
||||
std::cout << v.is_array() << '\n';
|
||||
|
||||
// moving the document does not invalidate views: the node index is
|
||||
// heap-allocated and does not move with the document object
|
||||
json_document moved = std::move(doc);
|
||||
std::cout << v.is_array() << '\n';
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
false
|
||||
true
|
||||
true
|
||||
true
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- [Zero-copy JSON views](https://json.nlohmann.me/features/json_view/index.md) - overview
|
||||
- [`basic_json_document`](https://json.nlohmann.me/api/basic_json_document/index.md) - the document that owns the parsed index
|
||||
- [`owns_source()`](https://json.nlohmann.me/api/basic_json_document/owns_source/index.md) - whether a document borrows its source text
|
||||
|
||||
## Version history
|
||||
|
||||
- Added in version 3.13.0 unreleased.
|
||||
Reference in new issue
Block a user