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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/" 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=../../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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<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> 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 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0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9Line truncated
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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/" 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-9.8.0"><title>Zero-copy JSON views - 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 rel=stylesheet href=../../css/fig-exceptions.css><link rel=stylesheet href=../../css/fig-pointer.css><link rel=stylesheet href=../../css/fig-charts.css><link rel=stylesheet href=../../css/fig-integration.css><link rel=stylesheet href=../../css/fig-macros.css><script>__md_scope=new URL("../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#zero-copy-json-views class=md-skip> Skip to 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> 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=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 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 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> </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 9Line truncated
|
||||
<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>nlohmann</span><span class=o>::</span><span class=n>json_document</span><span class=p>;</span>
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@@ -53,7 +53,7 @@
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||||
<span class=kc>true</span>
|
||||
<span class=kc>true</span>
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<span class=kc>true</span>
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</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> <p><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. Two more macros add context to the exceptions of views, as they do for those of <code class=highlight><span class=n>basic_json</span></code>:</p> <ul> <li>With <a href=../../api/macros/json_diagnostics/ ><code>JSON_DIAGNOSTICS</code></a>, an exception names the <abbr title="JavaScript Object Notation">JSON</abbr> Pointer path of the value within the document.</li> <li>With <a href=../../api/macros/json_diagnostic_positions/ ><code>JSON_DIAGNOSTIC_POSITIONS</code></a>, an exception names the bytes of the value in the source text.</li> </ul> <p>A view has neither parent pointers nor stored positions, so both are found from the document's index when the exception is thrown. This takes time linear in the size of the document. One exception has neither: <code>key()</code> of an iterator over an array or a primitive value (<a href=../../home/exceptions/#jsonexceptioninvalid_iterator207><code>invalid_iterator.207</code></a>), because a view's iterator does not know the value it iterates over.</p> <p>The other macros only affect the values <code>materialize()</code> produces, through <code>BasicJsonType</code>.</p> </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 <abbr title="Gibibyte (2^30 bytes)">GiB</abbr> input limit.</strong> An input of 4294967280 bytes (4 <abbr title="Gibibyte (2^30 bytes)">GiB</abbr> 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>Fewer source positions after edits.</strong> 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> sets the same poLine truncated
|
||||
</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> <p><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. Two more macros add context to the exceptions of views, as they do for those of <code class=highlight><span class=n>basic_json</span></code>:</p> <ul> <li>With <a href=../../api/macros/json_diagnostics/ ><code>JSON_DIAGNOSTICS</code></a>, an exception names the <abbr title="JavaScript Object Notation">JSON</abbr> Pointer path of the value within the document.</li> <li>With <a href=../../api/macros/json_diagnostic_positions/ ><code>JSON_DIAGNOSTIC_POSITIONS</code></a>, an exception names the bytes of the value in the source text.</li> </ul> <p>A view has neither parent pointers nor stored positions, so both are found from the document's index when the exception is thrown. This takes time linear in the size of the document. One exception has neither: <code>key()</code> of an iterator over an array or a primitive value (<a href=../../home/exceptions/#jsonexceptioninvalid_iterator207><code>invalid_iterator.207</code></a>), because a view's iterator does not know the value it iterates over.</p> <p>The other macros only affect the values <code>materialize()</code> produces, through <code>BasicJsonType</code>.</p> </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 <abbr title="Gibibyte (2^30 bytes)">GiB</abbr> input limit.</strong> An input of 4294967280 bytes (4 <abbr title="Gibibyte (2^30 bytes)">GiB</abbr> 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. For a file, reading it into a <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string</span></code> yourself and parsing that (an lvalue is borrowed) is therefore not more expensive in memory, and was about 6 to 12 % faster than parsLine 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>
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||||
<span class=p>{</span>
|
||||
@@ -171,4 +171,4 @@
|
||||
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=mi>316</span>
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||||
<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 (on big-endian targets, <code>save()</code> and <code>load()</code> do not exist), 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> <p class=nj-table-caption>Each column is one way to work with <abbr title="JavaScript Object Notation">JSON</abbr> in this library, and each row compares one property of them.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/basic_json_document/ ><code>basic_json_document</code></a> - the document that owns the parsLine truncated
|
||||
</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 (on big-endian targets, <code>save()</code> and <code>load()</code> do not exist), 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> <p class=nj-table-caption>Each column is one way to work with <abbr title="JavaScript Object Notation">JSON</abbr> in this library, and each row compares one property of them.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/basic_json_document/ ><code>basic_json_document</code></a> - the document that owns the parsLine truncated
|
||||
Reference in new issue
Block a user