deploy: 94fe31eb9b

home/architecture/index.html

@@ -1,4 +1,4 @@
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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/home/architecture/" rel="canonical"><link href=../design_goals/ rel=prev><link href=../customers/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Architecture - 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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     input[/"input&lt;br&gt;(string, stream,&lt;br&gt;iterator range, file)"/]
     ia["input adapter"]
     lexer["lexer"]
@@ -65,7 +65,7 @@
   16-31: "extra"
   32-63: "off"
   64-95: "len"
-  96-127: "next"</code></pre> <table> <thead> <tr> <th>Bytes</th> <th>Field</th> <th>Type</th> <th>Contents</th> </tr> </thead> <tbody> <tr> <td>0</td> <td><code>kind</code></td> <td><code>uint8_t</code></td> <td>the type, numbered as <a href=../../api/basic_json/value_t/ ><code>value_t</code></a>: 0 null, 1 object, 2 array, 3 string, 4 boolean, 5 signed integer, 6 unsigned integer, 7 float; 10 for a link (see below). There is no node for a discarded value: a <a href=../../api/basic_json_view/is_discarded/ >discarded</a> view has none</td> </tr> <tr> <td>1</td> <td><code>flags</code></td> <td><code>uint8_t</code></td> <td>bits 0-1: where a string's bytes (or a number's token) are (0: the source text, 1: the buffer of decoded strings, for strings with escapes, 2: the edit buffer); bit 2: the value of a boolean; bit 3: moved, bit 4: new, bit 5: linked (see below)</td> </tr> <tr> <td>2-3</td> <td><code>extra</code></td> <td><code>uint16_t</code></td> <td>numbers: the number of integer digits (low byte) and fraction digits (high byte), 255 for more; objects: the number of their key hash index (1-based), or 0; otherwise 0</td> </tr> <tr> <td>4-7</td> <td><code>off</code></td> <td><code>uint32_t</code></td> <td>where the value starts: the first byte after a string's opening quote (or its position in the buffer of decoded strings), the first byte of a number or literal, the bracket of an array or object</td> </tr> <tr> <td>8-11</td> <td><code>len</code></td> <td><code>uint32_t</code></td> <td>strings: the length after decoding; floats and literals: the length of the token; arrays and objects: the number of elements</td> </tr> <tr> <td>12-15</td> <td><code>next</code></td> <td><code>uint32_t</code></td> <td>arrays and objects: the number of nodes of the subtree, including the node itself</td> </tr> </tbody> </table> <ul> <li><strong>Integers</strong> keep their converted value in bytes 8-15 instead of <code>len</code> and <code>next</code>; the length of their token follows from the number of digits in <code>extra</code> (and the sign). Non-negative integers are unsigned integers, as with <a href=../../api/basic_json/parse/ ><code>parse</code></a>.</li> <li><strong>Floats</strong> keep only their token. The digit layout in <code>extra</code> lets the conversion read the digits without scanning the token again, and only when the value is read.</li> <li><strong>Object members</strong> are the node of the key (a string) followed by the nodes of the value.</li> <li><strong>Navigation</strong> needs no pointers: the elements of an array or object follow its node, and the node after a value's subtree is <code>next</code> nodes further for an array or object, and the next node otherwise (<code>document_data::after</code>). Views step from element to element this way and skip whole subtrees in constant time.</li> <li><strong>Offsets</strong> are 32 bits wide, so a document is limited to 4294967279 bytes, 4 GiB minus 16 bytes (a margin below 2^32 for positions one scanner step past the end of the text; <code>out_of_range.416</code>).</li> <li><strong>Large objects</strong> (128 members or more) get a hash index for their keys after parsing (<a href="https://github.com/nlohmann/json/blob/develop/include/nlohmann/detail/view/object_index.hpp"><code>detail/view/object_index.hpp</code></a>): an open-addressing table whose slots hold the distance from the object's node to a key's node, so that a lookup does not compare every key. Of duplicate keys the table leads to the first, as a linear search does. The object's <code>extra</code> holds the number of its table. Only 65,535 tables fit into <code>extra</code>; objects beyond them are searched linearly.</li> </ul> <p>For example, <code class=highlight><span class=p>{</span><span class=nt>"a"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=mi>1</span><span class=p>,</span><span class=w> </span><span class=mf>2.5</span><span class=p>]}</span></code> becomes five nodes. Each node's elements follow it, and <code>next</code> leads from an array or object past its subtree:</p> <pre class=mermaid><code>flowchart LR
+  96-127: "next"</code></pre> <table> <thead> <tr> <th>Bytes</th> <th>Field</th> <th>Type</th> <th>Contents</th> </tr> </thead> <tbody> <tr> <td>0</td> <td><code>kind</code></td> <td><code>uint8_t</code></td> <td>the type, numbered as <a href=../../api/basic_json/value_t/ ><code>value_t</code></a>: 0 null, 1 object, 2 array, 3 string, 4 boolean, 5 signed integer, 6 unsigned integer, 7 float; 10 for a link (see below). There is no node for a discarded value: a <a href=../../api/basic_json_view/is_discarded/ >discarded</a> view has none</td> </tr> <tr> <td>1</td> <td><code>flags</code></td> <td><code>uint8_t</code></td> <td>bits 0-1: where a string's bytes (or a number's token) are (0: the source text, 1: the buffer of decoded strings, for strings with escapes, 2: the edit buffer); bit 2: the value of a boolean; bit 3: moved, bit 4: new, bit 5: linked (see below)</td> </tr> <tr> <td>2-3</td> <td><code>extra</code></td> <td><code>uint16_t</code></td> <td>numbers: the number of integer digits (low byte) and fraction digits (high byte), 255 for more; objects: the number of their key hash index (1-based), or 0; otherwise 0</td> </tr> <tr> <td>4-7</td> <td><code>off</code></td> <td><code>uint32_t</code></td> <td>where the value starts: the first byte after a string's opening quote (or its position in the buffer of decoded strings), the first byte of a number or literal, the bracket of an array or object</td> </tr> <tr> <td>8-11</td> <td><code>len</code></td> <td><code>uint32_t</code></td> <td>strings: the length after decoding; floats and literals: the length of the token; arrays and objects: the number of elements</td> </tr> <tr> <td>12-15</td> <td><code>next</code></td> <td><code>uint32_t</code></td> <td>arrays and objects: the number of nodes of the subtree, including the node itself</td> </tr> </tbody> </table> <ul> <li><strong>Integers</strong> keep their converted value in bytes 8-15 instead of <code>len</code> and <code>next</code>; the length of their token follows from the number of digits in <code>extra</code> (and the sign). Non-negative integers are unsigned integers, as with <a href=../../api/basic_json/parse/ ><code>parse</code></a>.</li> <li><strong>Floats</strong> keep only their token. The digit layout in <code>extra</code> lets the conversion read the digits without scanning the token again, and only when the value is read.</li> <li><strong>Object members</strong> are the node of the key (a string) followed by the nodes of the value.</li> <li><strong>Navigation</strong> needs no pointers: the elements of an array or object follow its node, and the node after a value's subtree is <code>next</code> nodes further for an array or object, and the next node otherwise (<code>document_data::after</code>). Views step from element to element this way and skip whole subtrees in constant time.</li> <li><strong>Offsets</strong> are 32 bits wide, so a document is limited to 4294967279 bytes, 4 <abbr title="Gibibyte (2^30 bytes)">GiB</abbr> minus 16 bytes (a margin below 2^32 for positions one scanner step past the end of the text; <code>out_of_range.416</code>).</li> <li><strong>Large objects</strong> (128 members or more) get a hash index for their keys after parsing (<a href="https://github.com/nlohmann/json/blob/develop/include/nlohmann/detail/view/object_index.hpp"><code>detail/view/object_index.hpp</code></a>): an open-addressing table whose slots hold the distance from the object's node to a key's node, so that a lookup does not compare every key. Of duplicate keys the table leads to the first, as a linear search does. The object's <code>extra</code> holds the number of its table. Only 65,535 tables fit into <code>extra</code>; objects beyond them are searched linearly.</li> </ul> <p>For example, <code class=highlight><span class=p>{</span><span class=nt>"a"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=mi>1</span><span class=p>,</span><span class=w> </span><span class=mf>2.5</span><span class=p>]}</span></code> becomes five nodes. Each node's elements follow it, and <code>next</code> leads from an array or object past its subtree:</p> <pre class=mermaid><code>flowchart LR
     n0["0: object&lt;br&gt;len 1, next 5"]
     n1["1: key a"]
     n2["2: array&lt;br&gt;len 2, next 3"]