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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=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/json_view/" rel="canonical"><link href=../types/template_parameters/ rel=prev><link href=../../integration/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Zero-copy JSON views - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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<svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </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> Zero-copy JSON views </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=indigo data-md-color-accent=indigo aria-label="Switch to dark mode" 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12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27h.79l5 5-1.5 1.5-5-5v-.79l-.27-.27A6.52 6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 Line truncated
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<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json_view.hpp></span>
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<span class=k>using</span><span class=w> </span><span class=n>json_document</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json_document</span><span class=p>;</span>
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<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
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<span class=p>{</span>
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<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>
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<span class=w> </span><span class=c1>// json_document is basic_json_document<nlohmann::json>: same value types</span>
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<span class=w> </span><span class=c1>// and containers as the ordinary json specialization</span>
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<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=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s>{"pi": 3.14, "numbers": [1, 2, 3]}</span><span class=dl>)</span><span class=s>"</span><span class=p>);</span>
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<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>root</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>
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<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>root</span><span class=p>().</span><span class=n>materialize</span><span class=p>().</span><span class=n>dump</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>
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<span class=p>}</span>
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</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=kc>true</span>
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<span class=p>{</span><span class=nt>"numbers"</span><span class=p>:[</span><span class=mi>1</span><span class=p>,</span><span class=mi>2</span><span class=p>,</span><span class=mi>3</span><span class=p>],</span><span class=nt>"pi"</span><span class=p>:</span><span class=mf>3.14</span><span class=p>}</span>
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</code></pre></div> </details> <p><a href=../../api/ordered_json_document/ ><code>ordered_json_document</code></a> is the equivalent for <a href=../../api/ordered_json/ ><code>ordered_json</code></a>, just as <a href=../../api/ordered_json/ ><code>ordered_json</code></a> is to <a href=../../api/json/ ><code>json</code></a>.</p> <h2 id=ownership-and-lifetime>Ownership and lifetime<a class=headerlink href=#ownership-and-lifetime title="Permanent link">¶</a></h2> <p>A document either <strong>borrows</strong> the text it was parsed from, or <strong>owns</strong> its own copy of it; call <a href=../../api/basic_json_document/owns_source/ ><code>owns_source()</code></a> to find out which happened. <a href=../../api/basic_json_document/parse/ ><code>parse()</code></a> decides this from the value category and type of its argument (an lvalue <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string</span></code> is borrowed; an rvalue <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string</span></code> is moved in, owned without a copy; a stream is read into an owned buffer; and so on -- see <a href=../../api/basic_json_document/parse/#notes><code>parse</code>'s Notes</a> for the full table). <a href=../../api/basic_json_document/parse_copy/ ><code>parse_copy()</code></a> always owns a copy, regardless of the input.</p> <div class="admonition warning"> <p class=admonition-title>A borrowed document depends on your buffer</p> <p>If a document borrows its text, that text <strong>must outlive the document</strong> (and every view taken from it) and must stay <strong>unmodified</strong> 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 <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string_view</span></code>.</p> </div> <p>A view is valid only while all of the following hold:</p> <ul> <li>the document is alive,</li> <li>the document has not been re-parsed since the view was taken (with <a href=../../api/basic_json_document/read/ ><code>read()</code></a> or <a href=../../api/basic_json_document/parse/ ><code>parse()</code></a> into it),</li> <li>the document has not had <a href=../../api/basic_json_document/shrink_to_fit/ ><code>shrink_to_fit()</code></a> called on it since, and</li> <li>if the document borrows its source text, that text is still alive and unmodified.</li> </ul> <p>Moving the <code>basic_json_document<></code> itself is fine and does <strong>not</strong> invalidate its views: the index is a separate heap allocation that keeps its address across the move. Take a fresh view from <a href=../../api/basic_json_document/root/ ><code>root()</code></a> whenever any of the other conditions above was not met.</p> <p>A view can dangle, exactly like a <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string_view</span></code>: 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 <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>shared_ptr</span></code> in the document and a <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>weak_ptr</span></code> in each view) would make it larger, add reference counting to every copy, and a <code>lock()</code> 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, <a href=../../api/basic_json_document/root/ ><code>root()</code></a> is not callable on a temporary document: <code class=highlight><span class=k>auto</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>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=n>root</span><span class=p>();</span></code> does not compile. Give the document a name first.</p> <details class=example> <summary>Example: borrowed and owned documents, and when views become invalid</summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf><iostream></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><string></span>
|
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<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>json_document</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json_document</span><span class=p>;</span>
|
||||
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<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
|
||||
<span class=p>{</span>
|
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<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>
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<span class=w> </span><span class=c1>// BORROWED: doc only points into `text`; `text` must outlive `doc`</span>
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<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>
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||||
<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>
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||||
<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>
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<span class=w> </span><span class=c1>// a view is valid as long as the document is alive, has not been</span>
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<span class=w> </span><span class=c1>// re-parsed (read()) or shrunk (shrink_to_fit()), and -- if borrowed --</span>
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<span class=w> </span><span class=c1>// the source text is alive</span>
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<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>
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<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>
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||||
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<span class=w> </span><span class=c1>// re-parsing the SAME document invalidates views taken before the call;</span>
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||||
<span class=w> </span><span class=c1>// `v` above must not be used after this line</span>
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<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>
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<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>
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||||
<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>
|
||||
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||||
<span class=w> </span><span class=c1>// moving the document does not invalidate views: the node index is</span>
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<span class=w> </span><span class=c1>// heap-allocated and does not move with the document object</span>
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<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>
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||||
<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>
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<span class=p>}</span>
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||||
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=kc>false</span>
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||||
<span class=kc>true</span>
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||||
<span class=kc>true</span>
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<span class=kc>true</span>
|
||||
</code></pre></div> </details> <h2 id=what-is-the-same-as-parse>What is the same as <code>parse()</code><a class=headerlink href=#what-is-the-same-as-parse title="Permanent link">¶</a></h2> <ul> <li><strong>Accept/reject.</strong> <a href=../../api/basic_json_document/accept/ ><code>accept()</code></a> and <a href=../../api/basic_json_document/parse/ ><code>parse()</code></a> accept and reject exactly the same inputs as <a href=../../api/basic_json/accept/ ><code>basic_json::accept()</code></a>/<a href=../../api/basic_json/parse/ ><code>basic_json::parse()</code></a>, with the same <code>ignore_comments</code> and <code>ignore_trailing_commas</code> options.</li> <li><strong>Errors.</strong> A failing parse throws the same exception -- the same id, message, and position -- because on failure the library's own parser is run on the same bytes to produce the diagnostic. Valid input never pays for this, and the replay builds no values: it costs about a lexing pass up to the error. <code class=highlight><span class=n>allow_exceptions</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=nb>false</span></code> skips the replay and gives a <a href=../../api/basic_json_document/is_discarded/ >discarded</a> document instead of throwing, just as it gives a discarded value for <code class=highlight><span class=n>basic_json</span><span class=o>::</span><span class=n>parse</span><span class=p>()</span></code>.</li> <li><strong>Number classification.</strong> An integer literal that does not fit into the 64-bit integer type becomes a floating-point value, exactly as it does for <code class=highlight><span class=n>basic_json</span><span class=o>::</span><span class=n>parse</span><span class=p>()</span></code>.</li> <li><strong>Macros.</strong> <a href=../../api/macros/json_strict_nul_handling/ ><code>JSON_STRICT_NUL_HANDLING</code></a> and <a href=../../api/macros/json_noexception/ ><code>JSON_NOEXCEPTION</code></a>/<a href=../../api/macros/json_throw_user/ ><code>JSON_THROW_USER</code></a> behave the same way they do for <code><nlohmann/json.hpp></code>. These are the macros that change what the parser accepts or how it reports errors. The others either do not apply (<a href=../../api/macros/json_diagnostics/ ><code>JSON_DIAGNOSTICS</code></a> needs parent pointers in <code>basic_json</code> values) or only affect the values <code>materialize()</code> produces, through <code>BasicJsonType</code>.</li> </ul> <h2 id=what-is-different>What is different<a class=headerlink href=#what-is-different title="Permanent link">¶</a></h2> <ul> <li><strong>Only 64-bit integers.</strong> <code>basic_json_document<BasicJsonType></code> requires <code>BasicJsonType::number_integer_t</code> and <code>number_unsigned_t</code> to both be 64 bits wide; this is a compile-time <code class=highlight><span class=k>static_assert</span></code>.</li> <li><strong>A 4 GiB input limit.</strong> An input of 4294967280 bytes (4 GiB minus 16 bytes) or more throws <a href=../../home/exceptions/#jsonexceptionout_of_range416><code>out_of_range.416</code></a>, a limit <code class=highlight><span class=n>basic_json</span><span class=o>::</span><span class=n>parse</span><span class=p>()</span></code> does not have. It keeps an index entry at 16 bytes: offsets and counts are 32 bits wide. The limit is 16 bytes below 2^32, because the parser reads in 16-byte steps, and a position one step past the end must still fit.</li> <li><strong>A stream is always read to its end.</strong> There is no partial/streaming read of an <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istream</span></code>: the stream is read to its end into a buffer the document owns, and parsing starts after that.</li> <li><strong>No source positions on <code>materialize()</code>.</strong> Even with <a href=../../api/macros/json_diagnostic_positions/ ><code>JSON_DIAGNOSTIC_POSITIONS</code></a> enabled, <a href=../../api/basic_json_view/materialize/ ><code>materialize()</code></a> does not set them. The index has the start offset of every value, but not the end of arrays, objects, and escaped strings, which the positions also need.</li> <li><strong>Objects iterate in document order.</strong> <a href=../../api/basic_json_view/begin/ ><code>begin()</code></a>/ <a href=../../api/basic_json_view/end/ ><code>end()</code></a> and <a href=../../api/basic_json_view/items/ ><code>items()</code></a> visit an object's members in the order they appear in the source text. <code>basic_json</code>'s default <code>object_t</code> is a <code>std::map</code>, which sorts by key, so iterating a <a href=../../api/basic_json_view/materialize/ ><code>materialize()</code></a>d value can print members in a different order than iterating the view they came from.</li> <li><strong>Chained access is safe.</strong> <a href=../../api/basic_json_view/operator%5B%5D/ ><code>operator[]</code></a> with a missing key, an index out of range, or aLine truncated
|
||||
<span class=n>json_view</span><span class=w> </span><span class=n>last</span><span class=p>;</span>
|
||||
<span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=k>auto</span><span class=w> </span><span class=n>item</span><span class=w> </span><span class=o>:</span><span class=w> </span><span class=n>view</span><span class=p>.</span><span class=n>items</span><span class=p>())</span>
|
||||
<span class=p>{</span>
|
||||
<span class=w> </span><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=n>item</span><span class=p>.</span><span class=n>key</span><span class=p>()</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=s>"a"</span><span class=p>)</span>
|
||||
<span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=n>last</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>item</span><span class=p>.</span><span class=n>value</span><span class=p>();</span>
|
||||
<span class=w> </span><span class=p>}</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> <p>If you cannot trust the source text to have unique keys, check <a href=../../api/basic_json_view/size/ ><code>size()</code></a> against the number of distinct keys, or reject duplicates when iterating.</p> <h2 id=getting-values-out-without-copying>Getting values out without copying<a class=headerlink href=#getting-values-out-without-copying title="Permanent link">¶</a></h2> <p><a href=../../api/basic_json_view/get/ ><code>get<T>()</code></a> converts many <code>T</code> directly from the flat index, without ever building a <code>basic_json</code> value for the conversion: <code class=highlight><span class=kt>bool</span></code>, arithmetic types, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>nullptr_t</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string</span></code>/other <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>basic_string</span></code>s (copied once), <code>basic_json</code>/<code>ordered_json</code> (via <a href=../../api/basic_json_view/materialize/ ><code>materialize()</code></a>), <code>basic_json_view</code> itself, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o><</span><span class=n>U</span><span class=o>></span></code>, and <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>map</span></code>/<code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>unordered_map</span></code> with string-like keys. Every other type -- <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>list</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>pair</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>array</span></code>, enumerations, user types with a <code>from_json()</code> -- goes through <a href=../../api/basic_json_view/materialize/ ><code>materialize()</code></a><code>.get<T>()</code> instead: the subtree is built into a real <code>basic_json</code> value first, exactly as <a href=../../api/basic_json/parse/ ><code>parse()</code></a> would, and converted from there.</p> <p>Two conversions never copy at all:</p> <ul> <li><a href=../../api/basic_json_view/get_string/ ><code>get_string()</code></a> (equivalently, <code class=highlight><span class=n>get</span><span class=o><</span><span class=n>string_view_t</span><span class=o>></span><span class=p>()</span></code>) returns a string as a <code>string_view_t</code> pointing into the document's <a href=../../api/basic_json_document/source/ ><code>source()</code></a> text -- or, for a string that contains escape sequences, into the document's own buffer of decoded strings -- instead of allocating a new <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string</span></code>.</li> <li><a href=../../api/basic_json_view/number_token/ ><code>number_token()</code></a> returns a number exactly as it was written in the source, e.g. <code class=highlight><span class=s>"1.50"</span></code>, <code class=highlight><span class=s>"1E2"</span></code>, or an integer with more digits than any number type holds, instead of rounding it into a <code class=highlight><span class=kt>double</span></code>/<code class=highlight><span class=kt>int64_t</span></code> the way <code class=highlight><span class=n>get</span><span class=o><</span><span class=n>T</span><span class=o>></span><span class=p>()</span></code> (and <a href=../../api/basic_json/parse/ ><code>basic_json::parse()</code></a>) would.</li> </ul> <p>Both results are only valid as long as the view -- and, for a string with no escapes, the borrowed source text -- is.</p> <h2 id=writing-a-view-back>Writing a view back<a class=headerlink href=#writing-a-view-back title="Permanent link">¶</a></h2> <p><a href=../../api/basic_json_view/dump/ ><code>dump()</code></a> serializes a view directly from the flat index, without ever building a <code>basic_json</code> value. An object's members are written in document order, not sorted by key, and <em>every</em> occurrence of a repeated key is written, not only the last one -- the same two ways <a href=#what-is-different>iteration</a> already differs from a <a href=../../api/basic_json_view/materialize/ ><code>materialize()</code></a>d value, see <a href=#duplicate-keys>Duplicate keys</a>. <code class=highlight><span class=n>materialize</span><span class=p>().</span><span class=n>dump</span><span class=p>()</span></code> gives a different result in both respects for a <code>json_view</code>.</p> <p>By default, numbers are written the way <a href=../../api/basic_json/dump/ ><code>basic_json::dump()</code></a> would. <a href=../../api/basic_json_view/number_format/ ><code>number_format::source</code></a> instead copies every number exaLine truncated
|
||||
<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>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span>
|
||||
<span class=k>using</span><span class=w> </span><span class=n>json_editable_document</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json_editable_document</span><span class=p>;</span>
|
||||
<span class=k>using</span><span class=w> </span><span class=n>json_editable_view</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json_editable_view</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=c1>// a configuration file, as it might be read from disk</span>
|
||||
<span class=w> </span><span class=k>const</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>{"name": "cache", "host": "db1", "port": 6379, "price": 19.90}</span><span class=dl>)</span><span class=s>"</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>text</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\n\n</span><span class=s>"</span><span class=p>;</span>
|
||||
|
||||
<span class=w> </span><span class=c1>// patch two fields -- "price" is never touched</span>
|
||||
<span class=w> </span><span class=n>json_editable_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_editable_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>doc</span><span class=p>.</span><span class=n>set</span><span class=p>(</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=s>"host"</span><span class=p>,</span><span class=w> </span><span class=s>"db2"</span><span class=p>);</span>
|
||||
<span class=w> </span><span class=n>doc</span><span class=p>.</span><span class=n>set</span><span class=p>(</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=s>"retries"</span><span class=p>,</span><span class=w> </span><span class=mi>3</span><span class=p>);</span>
|
||||
|
||||
<span class=w> </span><span class=c1>// member order (the new member at the end) and the untouched number's</span>
|
||||
<span class=w> </span><span class=c1>// exact spelling survive</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>root</span><span class=p>().</span><span class=n>dump</span><span class=p>(</span><span class=mi>-1</span><span class=p>,</span><span class=w> </span><span class=sc>' '</span><span class=p>,</span><span class=w> </span><span class=nb>false</span><span class=p>,</span><span class=w> </span><span class=n>json_editable_view</span><span class=o>::</span><span class=n>number_format</span><span class=o>::</span><span class=n>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>// the same patch on a plain json value: keys are sorted (object_t is a</span>
|
||||
<span class=w> </span><span class=c1>// std::map), and "price" is rewritten even though the patch never</span>
|
||||
<span class=w> </span><span class=c1>// touched it</span>
|
||||
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>plain</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</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>plain</span><span class=p>[</span><span class=s>"host"</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"db2"</span><span class=p>;</span>
|
||||
<span class=w> </span><span class=n>plain</span><span class=p>[</span><span class=s>"retries"</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mi>3</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>plain</span><span class=p>.</span><span class=n>dump</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=p>{</span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"cache"</span><span class=p>,</span><span class=w> </span><span class=nt>"host"</span><span class=p>:</span><span class=w> </span><span class=s2>"db1"</span><span class=p>,</span><span class=w> </span><span class=nt>"port"</span><span class=p>:</span><span class=w> </span><span class=mi>6379</span><span class=p>,</span><span class=w> </span><span class=nt>"price"</span><span class=p>:</span><span class=w> </span><span class=mf>19.90</span><span class=p>}</span>
|
||||
|
||||
<span class=p>{</span><span class=nt>"name"</span><span class=p>:</span><span class=s2>"cache"</span><span class=p>,</span><span class=nt>"host"</span><span class=p>:</span><span class=s2>"db2"</span><span class=p>,</span><span class=nt>"port"</span><span class=p>:</span><span class=mi>6379</span><span class=p>,</span><span class=nt>"price"</span><span class=p>:</span><span class=mf>19.90</span><span class=p>,</span><span class=nt>"retries"</span><span class=p>:</span><span class=mi>3</span><span class=p>}</span>
|
||||
<span class=p>{</span><span class=nt>"host"</span><span class=p>:</span><span class=s2>"db2"</span><span class=p>,</span><span class=nt>"name"</span><span class=p>:</span><span class=s2>"cache"</span><span class=p>,</span><span class=nt>"port"</span><span class=p>:</span><span class=mi>6379</span><span class=p>,</span><span class=nt>"price"</span><span class=p>:</span><span class=mf>19.9</span><span class=p>,</span><span class=nt>"retries"</span><span class=p>:</span><span class=mi>3</span><span class=p>}</span>
|
||||
</code></pre></div> </details> <h3 id=what-stays-valid-and-what-an-edit-costs>What stays valid, and what an edit costs<a class=headerlink href=#what-stays-valid-and-what-an-edit-costs title="Permanent link">¶</a></h3> <p>The source text itself is <strong>never written</strong>, and the parsed index never moves -- a value keeps the node it was parsed into for as long as it is not itself replaced. So every <a href=../../api/basic_json_view/ >view</a> taken before an edit, including a previously obtained <a href=../../api/basic_json_document/root/ ><code>root()</code></a>, stays valid and, if it still refers to the edited value, sees the edit; a view of a value a later edit drops or replaces just keeps showing what it last held. New values go to storage the document allocates and owns on demand. The one thing an edit does invalidate is the <strong>iterators</strong> taken over an edited array or object: the first time one of its elements is set, appended to, inserted into, or erased, its elements move from the parsed, fixed layout to a growable block of links so that <a href=../../api/basic_json_document/push_back/ ><code>push_back</code></a> can later grow it in amortized constant time -- existing elements are not touched, but an iterator that was walking the old layout no longer matches. A string obtained with <a href=../../api/basic_json_view/get_string/ ><code>get_string()</code></a> is unaffected either way and stays valid across further edits. See <a href=../../api/basic_json_document/#edits><code>basic_json_document</code>'s Edits</a> for the details, and <a href=../../api/basic_json_document/set/#exception-safety><code>set</code>'s Exception safety</a> for what an edit guarantees if it throws (the <em>basic</em> guarantee, not the strong one <code>dump()</code> and the read-only functions provide). How edits are kept in the index is described in the <a href=../../home/architecture/#node-index-of-json-views>architecture overview</a>.</p> <h2 id=images>Images<a class=headerlink href=#images title="Permanent link">¶</a></h2> <p><a href=../../api/basic_json_document/save/ ><code>save()</code></a> writes a document as an <em>image</em>: a byte buffer that <a href=../../api/basic_json_document/load/ ><code>load()</code></a> reads back into a document without parsing -- no lexing, no building the node index, nothing but copying the nodes and pointing the text and the decoded strings at the image. Where <a href=../../api/basic_json_document/parse_copy/ ><code>parse_copy()</code></a> still has to scan the whole input, <a href=../../api/basic_json_document/load/ ><code>load()</code></a> turns that scan into a copy of the node index alone.</p> <p><strong>Why.</strong> A document that is parsed once and then read many times -- a configuration loaded at startup, a template rendered on every request, a large reference dataset a worker process needs in memory -- pays for parsing once but can amortize <a href=../../api/basic_json_document/save/ ><code>save()</code></a>'s cost across every later load. That makes images useful for a cache: save a document the first time it is parsed (to a file, a shared-memory segment, an in-process cache), and <a href=../../api/basic_json_document/load/ ><code>load()</code></a> it on every later use instead of parsing the source text again. They are just as useful for handing a parsed document to another process (or a forked worker) running the same build of the library, since <a href=../../api/basic_json_document/load/ ><code>load()</code></a> turns the transfer into a copy of the node index plus pointers into the received bytes, not a re-parse.</p> <p><strong>Choosing a check.</strong> <a href=../../api/basic_json_document/load/ ><code>load()</code></a> takes an <a href=../../api/basic_json_document/load/#image_check><code>image_check</code></a> that trades validation against speed: <code>image_check::full</code> (the default) checks everything the parser itself guarantees, so a checked image is exactly as safe to read and serialize as a freshly parsed document -- the right choice whenever the image did not come straight from this process's own <a href=../../api/basic_json_document/save/ ><code>save()</code></a>, such as a file or a network peer. <code>image_check::bounds</code> only checks structure and bounds -- cheaper, since it skips scanning the text and the decoded strings -- and fits a cache the process trusts, one it wrote and reads back itself. <code>image_check::none</code> skips validation entirely, for an image trusted as much as the process's own memory. See <a href=../../api/basic_json_document/load/#notes><code>load()</code>'s Notes</a> for exactly what each level does and does not guarantee.</p> <details class=example> <summary>Example: cache a parsed configuration as an image</summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf><cstdint></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><iostream></span>
|
||||
<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><vector></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>json_document</span><span class=w> </span><span class=o>=</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>// a configuration a service parses once and then caches as an image, so</span>
|
||||
<span class=w> </span><span class=c1>// that later requests can load() it instead of parsing the text again</span>
|
||||
<span class=w> </span><span class=k>const</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>{"name": "cache", "host": "db1", "port": 6379, "replicas": ["db2", "db3"]}</span><span class=dl>)</span><span class=s>"</span><span class=p>;</span>
|
||||
<span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>json_document</span><span class=w> </span><span class=n>config</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=c1>// save() turns the parsed document into a byte buffer: a 64-byte header,</span>
|
||||
<span class=w> </span><span class=c1>// the node index, the source text, and the decoded strings</span>
|
||||
<span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>></span><span class=w> </span><span class=n>image</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>config</span><span class=p>.</span><span class=n>save</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>image</span><span class=p>.</span><span class=n>size</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>// the same document always saves to the same bytes</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=p>(</span><span class=n>image</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=n>save</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>// loading the image back needs no parsing, yet dumps exactly what</span>
|
||||
<span class=w> </span><span class=c1>// parsing the text produced</span>
|
||||
<span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>json_document</span><span class=w> </span><span class=n>reloaded</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>load</span><span class=p>(</span><span class=n>image</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=p>(</span><span class=n>reloaded</span><span class=p>.</span><span class=n>root</span><span class=p>().</span><span class=n>dump</span><span class=p>()</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=n>config</span><span class=p>.</span><span class=n>root</span><span class=p>().</span><span class=n>dump</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=mi>316</span>
|
||||
<span class=kc>true</span>
|
||||
<span class=kc>true</span>
|
||||
</code></pre></div> </details> <div class="admonition warning"> <p class=admonition-title>Experimental</p> <p>The image format is versioned but not yet stable, and may change in an incompatible way before it is declared stable. It is little-endian only, and tied to the library build that wrote it -- use it to cache a document or to hand one to another process running the <em>same</em> build, not as a long-term storage format; keep the original <abbr title="JavaScript Object Notation">JSON</abbr> text if a saved document needs to be readable by a future library version.</p> </div> <p>The idea of a document you can read without parsing comes from zero-copy formats such as <a href="https://github.com/google/flatbuffers">FlatBuffers</a> and <a href="https://github.com/yandex/yaff">YaFF</a>; the check <a href=../../api/basic_json_document/load/ ><code>load()</code></a> runs follows the idea of FlatBuffers' Verifier. No code is taken from either.</p> <h2 id=choosing-between-json-ordered_json-the-sax-interface-and-json_view>Choosing between <code>json</code>, <code>ordered_json</code>, the <abbr title="Simple API for XML">SAX</abbr> interface, and <code>json_view</code><a class=headerlink href=#choosing-between-json-ordered_json-the-sax-interface-and-json_view title="Permanent link">¶</a></h2> <table> <thead> <tr> <th></th> <th><a href=../../api/json/ ><code>json</code></a> / <a href=../../api/ordered_json/ ><code>ordered_json</code></a></th> <th><a href=../parsing/sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a></th> <th><a href=../../api/json_document/ ><code>json_document</code></a> / <a href=../../api/json_view/ ><code>json_view</code></a></th> <th><a href=../../api/json_editable_document/ ><code>json_editable_document</code></a> / <a href=../../api/json_editable_view/ ><code>json_editable_view</code></a></th> </tr> </thead> <tbody> <tr> <td><strong>Ownership</strong></td> <td>owns every value</td> <td>owns nothing; you decide what to keep, in your handler</td> <td>borrows or owns the <em>text</em>; the index is always owned by the document</td> <td>same as <code>json_document</code>; edits go to storage the document owns</td> </tr> <tr> <td><strong>Mutability</strong></td> <td>freely mutable</td> <td>not applicable (a one-shot event stream)</td> <td>read-only</td> <td><a href=../../api/basic_json_document/set/ ><code>set</code></a>/<a href=../../api/basic_json_document/push_back/ ><code>push_back</code></a>/<a href=../../api/basic_json_document/insert/ ><code>insert</code></a>/<a href=../../api/basic_json_document/erase/ ><code>erase</code></a> edit in place; the source text is never rewritten</td> </tr> <tr> <td><strong>What you get</strong></td> <td>a full tree you can read, write, and keep as long as you like</td> <td>a sequence of callbacks; whatever your handler builds from them</td> <td>a flat index plus, on demand, <a href=../../api/basic_json_view/materialize/ ><code>materialize()</code></a>d <code>json</code>/<code>ordered_json</code> values for the parts you actually use</td> <td>the same, plus <a href=../../api/basic_json_view/dump/ ><code>dump()</code></a> of an edited document that keeps the member order and, with <a href=../../api/basic_json_view/number_format/ ><code>number_format::source</code></a>, the spelling of every untouched number</td> </tr> <tr> <td><strong>Typical use</strong></td> <td>general-purpose <abbr title="JavaScript Object Notation">JSON</abbr> handling: config, request/response bodies you build or modify, anything you hold onto</td> <td>validating or projecting a text into your own data structure without ever holding the whole thing as <abbr title="JavaScript Object Notation">JSON</abbr></td> <td>large or high-volume input where you only need part of it, or need it repeatedly, and can keep the source text (or a copy) alive for as long as the document lives</td> <td>a document you read, patch a few fields of, and write back -- a configuration file, for instance -- where the rest of it should come back exactly as it was</td> </tr> <tr> <td><strong>Caching/reload</strong></td> <td>not applicable -- re-parse, or roll your own serialization</td> <td>not applicable</td> <td><a href=../../api/basic_json_document/save/ ><code>save()</code></a>/<a href=../../api/basic_json_document/load/ ><code>load()</code></a>: cache the parsed index as an image and reload it without parsing</td> <td>same, saving the document's current -- possibly edited -- state</td> </tr> </tbody> </table> <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=hLine truncated
|
||||
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