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79 lines
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Copyright (C) 2024 Tomas Volf
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<title>Relationship Between SCM and scm_t_bits (Guile Reference Manual)</title>
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<div class="subsubsection-level-extent" id="Relationship-Between-SCM-and-scm_005ft_005fbits">
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Next: <a href="Immediate-Objects.html" accesskey="n" rel="next">Immediate Objects</a>, Up: <a href="The-SCM-Type-in-Guile.html" accesskey="u" rel="up">The SCM Type in Guile</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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<h4 class="subsubsection" id="Relationship-Between-SCM-and-scm_005ft_005fbits-1"><span>9.2.5.1 Relationship Between <code class="code">SCM</code> and <code class="code">scm_t_bits</code><a class="copiable-link" href="#Relationship-Between-SCM-and-scm_005ft_005fbits-1"> ¶</a></span></h4>
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<p>A variable of type <code class="code">SCM</code> is guaranteed to hold a valid Scheme
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object. A variable of type <code class="code">scm_t_bits</code>, on the other hand, may
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hold a representation of a <code class="code">SCM</code> value as a C integral type, but
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may also hold any C value, even if it does not correspond to a valid
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Scheme object.
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</p>
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<p>For a variable <var class="var">x</var> of type <code class="code">SCM</code>, the Scheme object’s type
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information is stored in a form that is not directly usable. To be able
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to work on the type encoding of the scheme value, the <code class="code">SCM</code>
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variable has to be transformed into the corresponding representation as
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a <code class="code">scm_t_bits</code> variable <var class="var">y</var> by using the <code class="code">SCM_UNPACK</code>
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macro. Once this has been done, the type of the scheme object <var class="var">x</var>
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can be derived from the content of the bits of the <code class="code">scm_t_bits</code>
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value <var class="var">y</var>, in the way illustrated by the example earlier in this
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chapter (see <a class="pxref" href="Cheaper-Pairs.html">Cheaper Pairs</a>). Conversely, a valid bit encoding of a
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Scheme value as a <code class="code">scm_t_bits</code> variable can be transformed into the
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corresponding <code class="code">SCM</code> value using the <code class="code">SCM_PACK</code> macro.
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</p>
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