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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/api/basic_json/unflatten/" rel="canonical"><link href=../type_name/ rel=prev><link href=../update/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>unflatten - 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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</code></pre></div> <p>The function restores the arbitrary nesting of a <abbr title="JavaScript Object Notation">JSON</abbr> value that has been flattened before using the <a href=../flatten/ ><code>flatten()</code></a> function. The <abbr title="JavaScript Object Notation">JSON</abbr> value must meet certain constraints:</p> <ol> <li>The value must be an object.</li> <li>The keys must be <abbr title="JavaScript Object Notation">JSON</abbr> pointers (see <a href="https://tools.ietf.org/html/rfc6901"><abbr title="Request for Comments">RFC</abbr> 6901</a>)</li> <li>The mapped values must be primitive <abbr title="JavaScript Object Notation">JSON</abbr> types.</li> </ol> <h2 id=return-value>Return value<a class=headerlink href=#return-value title="Permanent link">¶</a></h2> <p>the original <abbr title="JavaScript Object Notation">JSON</abbr> from a flattened version</p> <h2 id=exception-safety>Exception safety<a class=headerlink href=#exception-safety title="Permanent link">¶</a></h2> <p>Strong exception safety: if an exception occurs, the original value stays intact.</p> <h2 id=exceptions>Exceptions<a class=headerlink href=#exceptions title="Permanent link">¶</a></h2> <p>The function can throw the following exceptions:</p> <ul> <li>Throws <a href=../../../home/exceptions/#jsonexceptiontype_error314><code>type_error.314</code></a> if value is not an object</li> <li>Throws <a href=../../../home/exceptions/#jsonexceptiontype_error315><code>type_error.315</code></a> if object values are not primitive</li> <li>Throws <a href=../../../home/exceptions/#jsonexceptiontype_error313><code>type_error.313</code></a> if a key (<abbr title="JavaScript Object Notation">JSON</abbr> pointer) leads to a conflicting nesting; example: <code>"invalid value to unflatten"</code></li> <li>Throws <a href=../../../home/exceptions/#jsonexceptionparse_error106><code>parse_error.106</code></a> if an array index in a key begins with '0'; example: <code>"array index '01' must not begin with '0'"</code></li> <li>Throws <a href=../../../home/exceptions/#jsonexceptionparse_error107><code>parse_error.107</code></a> if a key is not empty and does not begin with a slash (<code>/</code>); example: <code>"JSON pointer must be empty or begin with '/' - was: 'a'"</code></li> <li>Throws <a href=../../../home/exceptions/#jsonexceptionparse_error108><code>parse_error.108</code></a> if a tilde (<code>~</code>) in a key is not followed by <code>0</code> or <code>1</code>; example: <code>"escape character '~' must be followed with '0' or '1'"</code></li> <li>Throws <a href=../../../home/exceptions/#jsonexceptionparse_error109><code>parse_error.109</code></a> if an array index in a key is not a number; example: <code>"array index 'one' is not a number"</code></li> <li>Throws <a href=../../../home/exceptions/#jsonexceptionout_of_range404><code>out_of_range.404</code></a> if a level becomes an array (because one of its keys is <code>0</code>) and another key at that level cannot be an array index; example: <code>"unresolved reference token 'x'"</code></li> </ul> <h2 id=complexity>Complexity<a class=headerlink href=#complexity title="Permanent link">¶</a></h2> <p>Linear in the size of the <abbr title="JavaScript Object Notation">JSON</abbr> value.</p> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">¶</a></h2> <p>Empty objects and arrays are flattened by <a href=../flatten/ ><code>flatten()</code></a> to <code class=highlight><span class=kc>null</span></code> values and cannot unflattened to their original type.</p> <p>A flattened array and a flattened object whose keys are array indices are indistinguishable, because both are described by the same <abbr title="JavaScript Object Notation">JSON</abbr> pointers. A value is therefore restored as an array if and only if one of its keys is the reference token <code>0</code>, and as an object otherwise: <code class=highlight><span class=p>{</span><span class=nt>"2"</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>}</span></code> is restored unchanged, whereas <code class=highlight><span class=p>{</span><span class=nt>"0"</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>}</span></code> is restored as <code class=highlight><span class=p>[</span><span class=mi>1</span><span class=p>]</span></code>. This decision does not depend on the order in which the flattened object is iterated.</p> <p>Apart from these two cases, for a <abbr title="JavaScript Object Notation">JSON</abbr> value <code>j</code>, the following is always true: <code class=highlight><span class=n>j</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>flatten</span><span class=p>().</span><span class=n>unflatten</span><span class=p>()</span></code>.</p> <h2 id=examples>Examples<a class=headerlink href=#examples title="Permanent link">¶</a></h2> <detaLine truncated
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<span class=cp>#include</span><span class=w> </span><span class=cpf><iomanip></span>
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<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json.hpp></span>
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