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<p><strong>print-object</strong> <span><i>Standard Generic Function</i></span></p>
<p><strong>Syntax:</strong></p>
<p><span><b>print-object</b></span> <em>object stream → object</em></p>
<p><strong>Method Signatures:</strong></p>
<p><span><b>print-object</b></span> (<em>object standard-object<span><i>) </i></span>stream</em></p>
<p><span><b>print-object</b></span> (<em>object structure-object<span><i>) </i></span>stream</em></p>
<p><strong>Arguments and Values:</strong></p>
<p><span><i>object</i></span>—an <span><i>object</i></span>.</p>
<p><span><i>stream</i></span>—a <span><i>stream</i></span>.</p>
<span><b>print-object</b></span>
<p><strong>Description:</strong></p>
<p>The <span><i>generic function</i></span> <span><b>print-object</b></span> writes the printed representation of <span><i>object</i></span> to <span><i>stream</i></span>. The <span><i>function</i></span> <span><b>print-object</b></span> is called by the <em>Lisp printer</em> ; it should not be called by the user.</p>
<p>Each implementation is required to provide a <span><i>method</i></span> on the <span><i>class</i></span> <span><b>standard-object</b></span> and on the <span><i>class</i></span> <span><b>structure-object</b></span>. In addition, each <span><i>implementation</i></span> must provide <span><i>methods</i></span> on enough other <em>classes</em> so as to ensure that there is always an applicable <span><i>method</i></span>. Implementations are free to add <span><i>methods</i></span> for other <em>classes</em>. Users may write <span><i>methods</i></span> for <span><b>print-object</b></span> for their own <em>classes</em> if they do not wish to inherit an <em>implementation-dependent method</em>.</p>
<p>The <span><i>method</i></span> on the <span><i>class</i></span> <span><b>structure-object</b></span> prints the object in the default #S notation; see Section 22.1.3.12 (Printing Structures).</p>
<p><span><i>Methods</i></span> on <span><b>print-object</b></span> are responsible for implementing their part of the semantics of the <span><i>printer control variables</i></span>, as follows:</p>
<span><b>*print-readably*</b></span>
<p>All methods for <span><b>print-object</b></span> must obey <span><b>*print-readably*</b></span>. This includes both user-defined methods and <span><i>implementation-defined</i></span> methods. Readable printing of <span><i>structures</i></span> and <span><i>standard objects</i></span> is controlled by their <span><b>print-object</b></span> method, not by their <span><b>make-load-form</b></span> <span><i>method</i></span>. <span><i>Similarity</i></span> for these <span><i>objects</i></span> is application dependent and hence is defined to be whatever these <span><i>methods</i></span> do; see Section 3.2.4.2 (Similarity of Literal Objects).</p>
<span><b>*print-escape*</b></span>
<p>Each <span><i>method</i></span> must implement <span><b>*print-escape*</b></span>.</p>
<span><b>*print-pretty*</b></span>
<p>The <span><i>method</i></span> may wish to perform specialized line breaking or other output conditional on the <span><i>value</i></span> of <span><b>*print-pretty*</b></span>. For further information, see (for example) the <span><i>macro</i></span> <span><b>pprint-fill</b></span>. See also Section 22.2.1.4 (Pretty Print Dispatch Tables) and Section 22.2.2 (Examples of using the Pretty Printer).</p>
<span><b>*print-length*</b></span>
<p><span><i>Methods</i></span> that produce output of indefinite length must obey <span><b>*print-length*</b></span>. For further information, see (for example) the <span><i>macros</i></span> <span><b>pprint-logical-block</b></span> and <span><b>pprint-pop</b></span>. See also Section 22.2.1.4 (Pretty Print Dispatch Tables) and Section 22.2.2 (Examples of using the Pretty Printer).</p>
<p><strong>*print-level*</strong></p>
<p>The printer takes care of <strong>*print-level*</strong> automatically, provided that each <span><i>method</i></span> handles exactly one level of structure and calls <span><b>write</b></span> (or an equivalent <span><i>function</i></span>) recursively if there are more structural levels. The printer’s decision of whether an <span><i>object</i></span> has components (and</p>
<p>therefore should not be printed when the printing depth is not less than <strong>*print-level*</strong>) is <span><i>implementation-dependent</i></span>. In some implementations its <span><b>print-object</b></span> <span><i>method</i></span> is not called; in others the <span><i>method</i></span> is called, and the determination that the <span><i>object</i></span> has components is based on what it tries to write to the <span><i>stream</i></span>.</p>
<span><b>*print-circle*</b></span>
<p>When the <span><i>value</i></span> of <span><b>*print-circle*</b></span> is <span><i>true</i></span>, a user-defined <span><b>print-object</b></span> <span><i>method</i></span> can print <span><i>objects</i></span> to the supplied <span><i>stream</i></span> using <span><b>write</b></span>, <span><b>prin1</b></span>, <span><b>princ</b></span>, or <span><b>format</b></span> and expect circularities to be detected and printed using the #<em>n</em># syntax. If a user-defined <span><b>print-object</b></span> <span><i>method</i></span> prints to a <span><i>stream</i></span> other than the one that was supplied, then circularity detection starts over for that <span><i>stream</i></span>. See <span><b>*print-circle*</b></span>.</p>
<p><strong>*print-base*</strong>, <span><b>*print-radix*</b></span>, <span><b>*print-case*</b></span>, <span><b>*print-gensym*</b></span>, and <span><b>*print-array*</b></span></p>
<p>These <span><i>printer control variables</i></span> apply to specific types of <span><i>objects</i></span> and are handled by the <span><i>methods</i></span> for those <span><i>objects</i></span>.</p>
<p>If these rules are not obeyed, the results are undefined.</p>
<p>In general, the printer and the <span><b>print-object</b></span> methods should not rebind the print control variables as they operate recursively through the structure, but this is <span><i>implementation-dependent</i></span>.</p>
<p>In some implementations the <span><i>stream</i></span> argument passed to a <span><b>print-object</b></span> <span><i>method</i></span> is not the original <span><i>stream</i></span>, but is an intermediate <span><i>stream</i></span> that implements part of the printer. <span><i>methods</i></span> should therefore not depend on the identity of this <span><i>stream</i></span>.</p>
<p><strong>See Also:</strong></p>
<p><span><b>pprint-fill</b></span>, <span><b>pprint-logical-block</b></span>, <span><b>pprint-pop</b></span>, <span><b>write</b></span>, <span><b>*print-readably*</b></span>, <span><b>*print-escape*</b></span>, <span><b>*print-pretty*</b></span>, <span><b>*print-length*</b></span>, Section 22.1.3 (Default Print-Object Methods), Section 22.1.3.12 (Printing Structures), Section 22.2.1.4 (Pretty Print Dispatch Tables), Section 22.2.2 (Examples of using the Pretty Printer)</p>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="expanded-reference-print-object">Expanded Reference: print-object<a href="print-object_standard-generic-function.html#expanded-reference-print-object" class="hash-link" aria-label="Direct link to Expanded Reference: print-object" title="Direct link to Expanded Reference: print-object">​</a></h2>
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