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<body class='hmmessage'><div dir='ltr'>tangent: Shouldn't the methods and header/typeinfo be combined?<br>Or is the header mutable and per-object?<br><br>But, yeah, my point is more to understand, not to question or change.<br><br><br>Thank you,<br> - Jay<br><br><div><div id="SkyDrivePlaceholder"></div><hr id="stopSpelling">From: dragisha@m3w.org<br>Date: Fri, 22 Mar 2013 07:34:32 +0100<br>To: jay.krell@cornell.edu<br>CC: m3devel@elegosoft.com<br>Subject: Re: [M3devel] layout of objects?<br><br>It's not m3front, it's RT0.<div><br></div><div>First data field is at offset 0, and pointer to type information is at -BYTESIZE(POINTER). Type information records are specified in RT0.</div><div><div>
<br><div><div>On Mar 22, 2013, at 5:56 AM, Jay K wrote:</div><br class="ecxApple-interchange-newline"><blockquote><span class="ecxApple-style-span" style="border-collapse:separate;font-family:Helvetica;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;line-height:normal;orphans:2;text-align:-webkit-auto;text-indent:0px;text-transform:none;white-space:normal;widows:2;word-spacing:0px;font-size:medium;"><div class="ecxhmmessage" style="font-size:12pt;font-family:Calibri;"><div dir="ltr">layout of objects?<br><br><br><br>How are Modula-3 objects layed out?<br>i.e. "OBJECT"/"METHODS"/"OVERRIDES"<br>I skimmed m3front and it wasn't obvious.<br><br><br><br>A common way for C++ "objects" to be layed out,<br>in the face of no RTTI and only single inheritance,<br>and virtual functions, is that a pointer to a record<br>of function pointers is first in the record.<br><br><br>Like this:<br><br><br>class Type<br>{<br>virtual void F1();<br>virtual void F2();<br>int data1;<br>int data2;<br>};<br><br><br>ends up lik more this:<br><br><br>struct TypeFunctions<br>{<br> void (*F1)(Type*);<br> void (*F2)(Type*);<br>};<br><br><br>struct Type<br>{<br>TypeFunctions* Functions; /* always first,<br>or at least a fixed offset, and located independent<br>of the size of the data; could also be at "-1" or such */.<br>int data1;<br>int data2;<br>};<br><br><br>Type* x;<br>x->F1();<br><br><br>=><br>x->Functions->F1(x);<br><br><br>Functions added in more derived types go at the end.<br>Ditto for data.<br>In the absence of multiple-inheritance and RTTI, it is simple and predictable.<br>(RTTI makes only small modifications.)<br><br><br>Looking through m3front, it wasn't at all obvious if it works this way.<br><br><br>I would like to declare something in C (or possibly C++, but not likely),<br>such that I might actually recognize the various low level operations<br>and "uncompile" it back to a typeful/typesafe form, like the above C++<br>to C transform.<br><br><br><br>I can't likely uncompile to C++ with virtual functions,<br>because the actual layout in C++ is not guaranteed.<br><br><br><br>Granted, I am being lazy.<br>I should/could compile some small samples.<br>But I might not get the entire story that way.<br><br><br><br>Thanks,<br>- Jay<br><br></div></div></span></blockquote></div><br></div></div></div> </div></body>
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