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</code></pre></div> <p>When defined to <code>1</code>, a <code>basic_json</code> value can no longer be constructed from a one-element <code>std::tuple</code> whose element is a reference to that <code>basic_json</code> type, such as <code>std::tuple&lt;json&amp;&gt;</code>, <code>std::tuple&lt;const json&amp;&gt;</code>, or <code>std::tuple&lt;json&amp;&amp;&gt;</code>. These are the tuples created by <code>std::forward_as_tuple(j)</code>.</p> <h2 id=default-definition>Default definition<a class=headerlink href=#default-definition title="Permanent link">&para;</a></h2> <p>The default value is <code>0</code> (disabled — existing behavior is preserved).</p> <div class=highlight><pre><span></span><code><span class=cp>#define JSON_DISABLE_TUPLE_REFERENCE_CONVERSION 0</span>
</code></pre></div> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">&para;</a></h2> <div class="admonition note"> <p class=admonition-title>Background</p> <p>By default, <code>basic_json</code> can be constructed from any <code>std::tuple</code> whose elements can be converted to <abbr title="JavaScript Object Notation">JSON</abbr>; the result is an array. This includes <code>std::tuple&lt;json&amp;&gt;</code>, which becomes a one-element array.</p> <p><code>std::tuple</code> only converts another tuple element by element if its element type cannot be constructed from the whole source tuple. Because <code>json</code> <em>can</em> be constructed from <code>std::tuple&lt;json&amp;&gt;</code>, <code>std::tuple</code> instead converts the whole tuple into a single <code>json</code> value. This has two surprising effects:</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=nb>true</span><span class=p>;</span>