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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_document/save/" rel="canonical"><link href=../root/ rel=prev><link href=../set/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>save - 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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data-md-component=logo> <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> save </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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<!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_document/save/" rel="canonical"><link href=../root/ rel=prev><link href=../set/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>save - 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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data-md-component=logo> <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> save </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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</code></pre></div> <p>Writes the document as an <em>image</em>: a byte buffer that <a href=../load/ ><code>load</code></a> reads back without parsing. The image holds the node index, the source text (plus, for an edited document, the number tokens edits wrote), and the decoded strings (plus the strings edits wrote) -- everything <a href=../root/ ><code>root()</code></a> needs, with nothing left to parse.</p> <p>An edited document is written in its <em>current</em> state, with its values in document order, the way the library's own parser would have produced them for that <abbr title="JavaScript Object Notation">JSON</abbr> text: a member <a href=../set/ ><code>set</code></a> added goes at the end, an <a href=../erase/ ><code>erase</code></a>d member leaves no trace, and a float that is not finite (<abbr title="Not a Number">NaN</abbr> or positive/negative infinity) becomes null, the same substitution <a href=../../basic_json_view/dump/ ><code>dump()</code></a> makes. The same document always saves to the same bytes -- also across <code>BasicJsonType</code> and <code class=highlight><span class=n>Editable</span></code>, since the image reflects document order and values only, not which specialization produced them.</p> <h2 id=return-value>Return value<a class=headerlink href=#return-value title="Permanent link">&para;</a></h2> <p>The image, as a <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>&gt;</span></code>. Pass it, or a pointer to its data together with its size, to <a href=../load/ ><code>load</code></a> to read the document back.</p> <h2 id=exception-safety>Exception safety<a class=headerlink href=#exception-safety title="Permanent link">&para;</a></h2> <p>Strong guarantee: <code>save()</code> does not modify <code class=highlight><span class=o>*</span><span class=k>this</span></code> (it is <code class=highlight><span class=k>const</span></code>), so if it throws, the document is left exactly as it was, and the partially built image is discarded with the exception.</p> <h2 id=exceptions>Exceptions<a class=headerlink href=#exceptions title="Permanent link">&para;</a></h2> <p>Throws <a href=../../../home/exceptions/#jsonexceptiontype_error320><code>type_error.320</code></a> if the document is <a href=../is_discarded/ >discarded</a> -- a default-constructed document, or one a failed <a href=../parse/ ><code>parse()</code></a>/ <a href=../read/ ><code>read()</code></a> with <code>allow_exceptions == false</code> left discarded.</p> <p>On a big-endian target, throws <code>type_error.320</code> with a different message instead: the image format is little-endian only (see <a href=#notes>Notes</a>).</p> <p>Throws <a href=../../../home/exceptions/#jsonexceptionout_of_range416><code>out_of_range.416</code></a> if the node count, the text, or the decoded strings of the image would individually reach 4 GiB -- the same 32-bit offsets <a href=../parse/#exceptions><code>parse()</code></a> and, for edits, <a href=../set/ ><code>set</code></a>/<a href=../push_back/ ><code>push_back</code></a> are already limited to.</p> <div class="admonition failure"> <p class=admonition-title>Example messages</p> <p><div class=highlight><pre><span></span><code>[json.exception.type_error.320] cannot save a discarded json_document
</code></pre></div> <div class=highlight><pre><span></span><code>[json.exception.type_error.320] json_document images need a little-endian target
</code></pre></div> <div class=highlight><pre><span></span><code>[json.exception.out_of_range.416] images of 4 GiB or more are not supported by json_document