217 lines
24 KiB
HTML
217 lines
24 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<title>2/wal</title>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<meta http-equiv="Content-Language" content="en-gb">
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<link href="inweb.css" rel="stylesheet" rev="stylesheet" type="text/css">
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</head>
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<body>
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<!--Weave of '2/mth' generated by 7-->
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<ul class="crumbs"><li><a href="../webs.html">★</a></li><li><a href="index.html">foundation</a></li><li><a href="index.html#2">Chapter 2: Memory, Streams and Collections</a></li><li><b>Methods</b></li></ul><p class="purpose">General support for something approximating method calls.</p>
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<ul class="toc"><li><a href="#SP1">§1. Method sets</a></li><li><a href="#SP4">§4. Declaring methods</a></li><li><a href="#SP6">§6. Adding methods</a></li><li><a href="#SP7">§7. Calling methods</a></li></ul><hr class="tocbar">
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<p class="inwebparagraph"><a id="SP1"></a><b>§1. Method sets. </b>This section provides a very rudimentary implementation of method calls,
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ordinarily not available in C, but doesn't pretend to offer the full
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functionality of an object-oriented language.
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</p>
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<p class="inwebparagraph">Instead, it's really intended for protocol-based coding patterns. Suppose that
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we have objects of several different structure types, but all of them can
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serve a given purpose — say, all of them contribute an adjective to the
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Inform language. What we want is the ability to take a pointer, which might be
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to an object of any of these types, and to tell the object to do something, or
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ask it a question.
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</p>
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<p class="inwebparagraph">Alternatively, we may have a situation where there are multiple objects of the
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same type which each represent a different way of doing something: for
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example, in the Inweb source code, each different supported programming
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language is represented by an object. These objects need to encapsulate all
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the ways that one language differs from another, and they can do that by
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providing "methods".
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</p>
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<p class="inwebparagraph"><a id="SP2"></a><b>§2. </b>The model is this. If a <code class="display"><span class="extract">typedef struct</span></code> definition includes the line
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<code class="display"><span class="extract">METHOD_CALLS</span></code>, then any instance of that structure can have a queue of
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tagged functions attached to it dynamically: those, we'll call "methods".
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</p>
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<pre class="definitions">
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<span class="definitionkeyword">define</span> <span class="constant">METHOD_CALLS</span>
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<span class="reserved">struct</span><span class="plain"> </span><span class="reserved">method_set</span><span class="plain"> *</span><span class="identifier">methods</span><span class="plain">;</span>
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<span class="definitionkeyword">define</span> <span class="identifier">ENABLE_METHOD_CALLS</span><span class="plain">(</span><span class="identifier">obj</span><span class="plain">)</span>
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<span class="identifier">obj</span><span class="plain">-></span><span class="identifier">methods</span><span class="plain"> = </span><span class="functiontext">Methods::new_set</span><span class="plain">();</span>
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</pre>
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<p class="inwebparagraph"><a id="SP3"></a><b>§3. </b>A "method set" is simply a linked list of methods:
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</p>
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<pre class="display">
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<span class="reserved">typedef</span><span class="plain"> </span><span class="reserved">struct</span><span class="plain"> </span><span class="reserved">method_set</span><span class="plain"> {</span>
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<span class="reserved">struct</span><span class="plain"> </span><span class="reserved">method</span><span class="plain"> *</span><span class="identifier">first_method</span><span class="plain">;</span>
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<span class="constant">MEMORY_MANAGEMENT</span>
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<span class="plain">} </span><span class="reserved">method_set</span><span class="plain">;</span>
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<span class="reserved">method_set</span><span class="plain"> *</span><span class="functiontext">Methods::new_set</span><span class="plain">(</span><span class="reserved">void</span><span class="plain">) {</span>
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<span class="reserved">method_set</span><span class="plain"> *</span><span class="identifier">S</span><span class="plain"> = </span><span class="identifier">CREATE</span><span class="plain">(</span><span class="reserved">method_set</span><span class="plain">);</span>
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<span class="identifier">S</span><span class="plain">-</span><span class="element">>first_method</span><span class="plain"> = </span><span class="identifier">NULL</span><span class="plain">;</span>
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<span class="reserved">return</span><span class="plain"> </span><span class="identifier">S</span><span class="plain">;</span>
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<span class="plain">}</span>
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</pre>
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<p class="inwebparagraph"></p>
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<p class="endnote">The function Methods::new_set is used in <a href="#SP2">§2</a>.</p>
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<p class="endnote">The structure method_set is private to this section.</p>
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<p class="inwebparagraph"><a id="SP4"></a><b>§4. Declaring methods. </b>Each method is a function, though we don't know its type — which is why we
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resort to the desperate measure of storing it as a <code class="display"><span class="extract">void *</span></code> — with an ID
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number attached to it. IDs should be from the <code class="display"><span class="extract">*_MTID</span></code> enumeration set.
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</p>
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<pre class="definitions">
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<span class="definitionkeyword">enum</span> <span class="constant">UNUSED_METHOD_ID_MTID</span><span class="definitionkeyword"> from </span><span class="constant">1</span>
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</pre>
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<p class="inwebparagraph"><a id="SP5"></a><b>§5. </b>The type of a method must neverthess be specified, and we do it with one
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of two macros: one for methods returning an integer, one for void methods,
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i.e., those returning no value.
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</p>
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<p class="inwebparagraph">What these do is to use typedef to give the name <code class="display"><span class="extract">X_type</span></code> to the type of all
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functions sharing the method ID <code class="display"><span class="extract">X</span></code>.
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</p>
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<pre class="definitions">
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<span class="definitionkeyword">define</span> <span class="identifier">IMETHOD_TYPE</span><span class="plain">(</span><span class="identifier">id</span><span class="plain">, </span><span class="identifier">args</span><span class="plain">...)</span>
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<span class="reserved">typedef</span><span class="plain"> </span><span class="reserved">int</span><span class="plain"> (*</span><span class="identifier">id</span><span class="plain">##</span><span class="identifier">_type</span><span class="plain">)(</span><span class="identifier">args</span><span class="plain">);</span>
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<span class="definitionkeyword">define</span> <span class="identifier">VMETHOD_TYPE</span><span class="plain">(</span><span class="identifier">id</span><span class="plain">, </span><span class="identifier">args</span><span class="plain">...)</span>
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<span class="reserved">typedef</span><span class="plain"> </span><span class="reserved">void</span><span class="plain"> (*</span><span class="identifier">id</span><span class="plain">##</span><span class="identifier">_type</span><span class="plain">)(</span><span class="identifier">args</span><span class="plain">);</span>
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</pre>
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<pre class="display">
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<span class="identifier">IMETHOD_TYPE</span><span class="plain">(</span><span class="constant">UNUSED_METHOD_ID_MTID</span><span class="plain">, </span><span class="reserved">text_stream</span><span class="plain"> *</span><span class="identifier">example</span><span class="plain">, </span><span class="reserved">int</span><span class="plain"> </span><span class="identifier">wont_be_used</span><span class="plain">)</span>
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</pre>
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<p class="inwebparagraph"></p>
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<p class="inwebparagraph"><a id="SP6"></a><b>§6. Adding methods. </b>Provided a function has the right type for the ID we're using, we can now
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attach it to an object with a method set, using the <code class="display"><span class="extract">METHOD_ADD</span></code> macro.
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(If the type is wrong, the C compiler will throw errors here.)
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</p>
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<pre class="definitions">
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<span class="definitionkeyword">define</span> <span class="identifier">METHOD_ADD</span><span class="plain">(</span><span class="identifier">upon</span><span class="plain">, </span><span class="identifier">id</span><span class="plain">, </span><span class="identifier">func</span><span class="plain">)</span>
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<span class="functiontext">Methods::add</span><span class="plain">(</span><span class="identifier">upon</span><span class="plain">-></span><span class="identifier">methods</span><span class="plain">, </span><span class="identifier">id</span><span class="plain">, (</span><span class="reserved">void</span><span class="plain"> *) &</span><span class="identifier">func</span><span class="plain">);</span>
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</pre>
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<pre class="display">
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<span class="reserved">typedef</span><span class="plain"> </span><span class="reserved">struct</span><span class="plain"> </span><span class="reserved">method</span><span class="plain"> {</span>
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<span class="reserved">int</span><span class="plain"> </span><span class="identifier">method_id</span><span class="plain">;</span>
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<span class="reserved">void</span><span class="plain"> *</span><span class="identifier">method_function</span><span class="plain">;</span>
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<span class="reserved">struct</span><span class="plain"> </span><span class="reserved">method</span><span class="plain"> *</span><span class="identifier">next_method</span><span class="plain">;</span>
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<span class="constant">MEMORY_MANAGEMENT</span>
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<span class="plain">} </span><span class="reserved">method</span><span class="plain">;</span>
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<span class="reserved">void</span><span class="plain"> </span><span class="functiontext">Methods::add</span><span class="plain">(</span><span class="reserved">method_set</span><span class="plain"> *</span><span class="identifier">S</span><span class="plain">, </span><span class="reserved">int</span><span class="plain"> </span><span class="identifier">ID</span><span class="plain">, </span><span class="reserved">void</span><span class="plain"> *</span><span class="identifier">function</span><span class="plain">) {</span>
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<span class="reserved">method</span><span class="plain"> *</span><span class="identifier">M</span><span class="plain"> = </span><span class="identifier">CREATE</span><span class="plain">(</span><span class="reserved">method</span><span class="plain">);</span>
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<span class="identifier">M</span><span class="plain">-</span><span class="element">>method_id</span><span class="plain"> = </span><span class="identifier">ID</span><span class="plain">;</span>
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<span class="identifier">M</span><span class="plain">-</span><span class="element">>method_function</span><span class="plain"> = </span><span class="identifier">function</span><span class="plain">;</span>
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<span class="identifier">M</span><span class="plain">-</span><span class="element">>next_method</span><span class="plain"> = </span><span class="identifier">NULL</span><span class="plain">;</span>
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<span class="reserved">if</span><span class="plain"> (</span><span class="identifier">S</span><span class="plain">-</span><span class="element">>first_method</span><span class="plain"> == </span><span class="identifier">NULL</span><span class="plain">) </span><span class="identifier">S</span><span class="plain">-</span><span class="element">>first_method</span><span class="plain"> = </span><span class="identifier">M</span><span class="plain">;</span>
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<span class="reserved">else</span><span class="plain"> {</span>
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<span class="reserved">method</span><span class="plain"> *</span><span class="identifier">existing</span><span class="plain"> = </span><span class="identifier">S</span><span class="plain">-</span><span class="element">>first_method</span><span class="plain">;</span>
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<span class="reserved">while</span><span class="plain"> ((</span><span class="identifier">existing</span><span class="plain">) && (</span><span class="identifier">existing</span><span class="plain">-</span><span class="element">>next_method</span><span class="plain">)) </span><span class="identifier">existing</span><span class="plain"> = </span><span class="identifier">existing</span><span class="plain">-</span><span class="element">>next_method</span><span class="plain">;</span>
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<span class="identifier">existing</span><span class="plain">-</span><span class="element">>next_method</span><span class="plain"> = </span><span class="identifier">M</span><span class="plain">;</span>
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<span class="plain">}</span>
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<span class="plain">}</span>
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</pre>
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<p class="inwebparagraph"></p>
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<p class="endnote">The function Methods::add appears nowhere else.</p>
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<p class="endnote">The structure method is private to this section.</p>
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<p class="inwebparagraph"><a id="SP7"></a><b>§7. Calling methods. </b>Method calls are also done with a macro, but it has to come in four variants:
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</p>
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<p class="inwebparagraph"></p>
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<ul class="items"><li>(a) <code class="display"><span class="extract">IMETHOD_CALL</span></code> for a method taking arguments and returning an <code class="display"><span class="extract">int</span></code>,
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</li><li>(b) <code class="display"><span class="extract">IMETHOD_CALLV</span></code> for a method without arguments which returns an <code class="display"><span class="extract">int</span></code>,
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</li><li>(c) <code class="display"><span class="extract">VMETHOD_CALL</span></code> for a method taking arguments and returning nothing,
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</li><li>(d) <code class="display"><span class="extract">VMETHOD_CALLV</span></code> for a method without arguments which returns nothing.
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</li></ul>
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<p class="inwebparagraph">For example:
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</p>
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<p class="inwebparagraph"></p>
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<pre class="display">
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<span class="plain">IMETHOD_CALL(some_object, UNUSED_METHOD_ID_MTID, I"Hello", 17)</span>
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</pre>
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<p class="inwebparagraph">Note that it's entirely possible for the <code class="display"><span class="extract">upon</span></code> object to have multiple methods
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added for the same ID — or none. In the <code class="display"><span class="extract">V</span></code> (void) cases, what we then do is
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to call each of them in turn. In the <code class="display"><span class="extract">I</span></code> (int) cases, we call each in turn, but
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stop the moment any of them returns something other than <code class="display"><span class="extract">FALSE</span></code>, and then
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we put that value into the specified result variable <code class="display"><span class="extract">rval</span></code>.
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</p>
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<p class="inwebparagraph">If <code class="display"><span class="extract">some_object</span></code> has no methods for the given ID, then nothing happens, and
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in the <code class="display"><span class="extract">I</span></code> case, the return value is <code class="display"><span class="extract">FALSE</span></code>.
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</p>
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<p class="inwebparagraph">It will, however, produce a compilation error if <code class="display"><span class="extract">some_object</span></code> is not a pointer
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to a structure which has <code class="display"><span class="extract">METHOD_CALLS</span></code> as part of its definition.
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</p>
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<pre class="definitions">
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<span class="definitionkeyword">define</span> <span class="identifier">IMETHOD_CALL</span><span class="plain">(</span><span class="identifier">rval</span><span class="plain">, </span><span class="identifier">upon</span><span class="plain">, </span><span class="identifier">id</span><span class="plain">, </span><span class="identifier">args</span><span class="plain">...) {</span>
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<span class="identifier">rval</span><span class="plain"> = </span><span class="constant">FALSE</span><span class="plain">;</span>
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<span class="reserved">for</span><span class="plain"> (</span><span class="reserved">method</span><span class="plain"> *</span><span class="identifier">M</span><span class="plain"> = </span><span class="identifier">upon</span><span class="plain">?(</span><span class="identifier">upon</span><span class="plain">-></span><span class="identifier">methods</span><span class="plain">-</span><span class="element">>first_method</span><span class="plain">):</span><span class="identifier">NULL</span><span class="plain">; </span><span class="identifier">M</span><span class="plain">; </span><span class="identifier">M</span><span class="plain"> = </span><span class="identifier">M</span><span class="plain">-</span><span class="element">>next_method</span><span class="plain">)</span>
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<span class="reserved">if</span><span class="plain"> (</span><span class="identifier">M</span><span class="plain">-</span><span class="element">>method_id</span><span class="plain"> == </span><span class="identifier">id</span><span class="plain">) {</span>
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<span class="reserved">int</span><span class="plain"> </span><span class="identifier">method_rval_</span><span class="plain"> = (*((</span><span class="identifier">id</span><span class="plain">##</span><span class="identifier">_type</span><span class="plain">) (</span><span class="identifier">M</span><span class="plain">-</span><span class="element">>method_function</span><span class="plain">)))(</span><span class="identifier">upon</span><span class="plain">, </span><span class="identifier">args</span><span class="plain">);</span>
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<span class="reserved">if</span><span class="plain"> (</span><span class="identifier">method_rval_</span><span class="plain">) {</span>
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<span class="identifier">rval</span><span class="plain"> = </span><span class="identifier">method_rval_</span><span class="plain">;</span>
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<span class="reserved">break</span><span class="plain">;</span>
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<span class="plain">}</span>
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<span class="plain">}</span>
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<span class="plain">}</span>
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<span class="definitionkeyword">define</span> <span class="identifier">IMETHOD_CALLV</span><span class="plain">(</span><span class="identifier">rval</span><span class="plain">, </span><span class="identifier">upon</span><span class="plain">, </span><span class="identifier">id</span><span class="plain">) {</span>
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<span class="identifier">rval</span><span class="plain"> = </span><span class="constant">FALSE</span><span class="plain">;</span>
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<span class="reserved">for</span><span class="plain"> (</span><span class="reserved">method</span><span class="plain"> *</span><span class="identifier">M</span><span class="plain"> = </span><span class="identifier">upon</span><span class="plain">?(</span><span class="identifier">upon</span><span class="plain">-></span><span class="identifier">methods</span><span class="plain">-</span><span class="element">>first_method</span><span class="plain">):</span><span class="identifier">NULL</span><span class="plain">; </span><span class="identifier">M</span><span class="plain">; </span><span class="identifier">M</span><span class="plain"> = </span><span class="identifier">M</span><span class="plain">-</span><span class="element">>next_method</span><span class="plain">)</span>
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<span class="reserved">if</span><span class="plain"> (</span><span class="identifier">M</span><span class="plain">-</span><span class="element">>method_id</span><span class="plain"> == </span><span class="identifier">id</span><span class="plain">) {</span>
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<span class="reserved">int</span><span class="plain"> </span><span class="identifier">method_rval_</span><span class="plain"> = (*((</span><span class="identifier">id</span><span class="plain">##</span><span class="identifier">_type</span><span class="plain">) (</span><span class="identifier">M</span><span class="plain">-</span><span class="element">>method_function</span><span class="plain">)))(</span><span class="identifier">upon</span><span class="plain">);</span>
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<span class="reserved">if</span><span class="plain"> (</span><span class="identifier">method_rval_</span><span class="plain">) {</span>
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<span class="identifier">rval</span><span class="plain"> = </span><span class="identifier">method_rval_</span><span class="plain">;</span>
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<span class="reserved">break</span><span class="plain">;</span>
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<span class="plain">}</span>
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<span class="plain">}</span>
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<span class="plain">}</span>
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<span class="definitionkeyword">define</span> <span class="identifier">VMETHOD_CALL</span><span class="plain">(</span><span class="identifier">upon</span><span class="plain">, </span><span class="identifier">id</span><span class="plain">, </span><span class="identifier">args</span><span class="plain">...)</span>
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<span class="reserved">for</span><span class="plain"> (</span><span class="reserved">method</span><span class="plain"> *</span><span class="identifier">M</span><span class="plain"> = </span><span class="identifier">upon</span><span class="plain">?(</span><span class="identifier">upon</span><span class="plain">-></span><span class="identifier">methods</span><span class="plain">-</span><span class="element">>first_method</span><span class="plain">):</span><span class="identifier">NULL</span><span class="plain">; </span><span class="identifier">M</span><span class="plain">; </span><span class="identifier">M</span><span class="plain"> = </span><span class="identifier">M</span><span class="plain">-</span><span class="element">>next_method</span><span class="plain">)</span>
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<span class="reserved">if</span><span class="plain"> (</span><span class="identifier">M</span><span class="plain">-</span><span class="element">>method_id</span><span class="plain"> == </span><span class="identifier">id</span><span class="plain">)</span>
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<span class="plain">(*((</span><span class="identifier">id</span><span class="plain">##</span><span class="identifier">_type</span><span class="plain">) (</span><span class="identifier">M</span><span class="plain">-</span><span class="element">>method_function</span><span class="plain">)))(</span><span class="identifier">upon</span><span class="plain">, </span><span class="identifier">args</span><span class="plain">);</span>
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<span class="definitionkeyword">define</span> <span class="identifier">VMETHOD_CALLV</span><span class="plain">(</span><span class="identifier">upon</span><span class="plain">, </span><span class="identifier">id</span><span class="plain">)</span>
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<span class="reserved">for</span><span class="plain"> (</span><span class="reserved">method</span><span class="plain"> *</span><span class="identifier">M</span><span class="plain"> = </span><span class="identifier">upon</span><span class="plain">?(</span><span class="identifier">upon</span><span class="plain">-></span><span class="identifier">methods</span><span class="plain">-</span><span class="element">>first_method</span><span class="plain">):</span><span class="identifier">NULL</span><span class="plain">; </span><span class="identifier">M</span><span class="plain">; </span><span class="identifier">M</span><span class="plain"> = </span><span class="identifier">M</span><span class="plain">-</span><span class="element">>next_method</span><span class="plain">)</span>
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<span class="reserved">if</span><span class="plain"> (</span><span class="identifier">M</span><span class="plain">-</span><span class="element">>method_id</span><span class="plain"> == </span><span class="identifier">id</span><span class="plain">)</span>
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<span class="plain">(*((</span><span class="identifier">id</span><span class="plain">##</span><span class="identifier">_type</span><span class="plain">) (</span><span class="identifier">M</span><span class="plain">-</span><span class="element">>method_function</span><span class="plain">)))(</span><span class="identifier">upon</span><span class="plain">);</span>
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</pre>
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