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258 lines
4.8 KiB
HTML
258 lines
4.8 KiB
HTML
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<HTML
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>VTables and Object-Oriented Assembler</TITLE
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CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK
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TITLE="Programming with Ophis"
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TITLE="Functionals"
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HREF="c953.html"><LINK
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TITLE="Dispatch-on-type and Data-Directed Assembler"
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HREF="x992.html"><LINK
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REL="NEXT"
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TITLE="A final reminder"
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>Programming with Ophis</TH
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WIDTH="10%"
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>Functionals</TD
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><TD
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WIDTH="10%"
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VALIGN="bottom"
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><A
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><DIV
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CLASS="SECTION"
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><H1
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CLASS="SECTION"
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><A
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NAME="AEN1008"
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>VTables and Object-Oriented Assembler</A
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></H1
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><P
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> The usual technique for getting something that looks
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object-oriented in non-object-oriented languages is to fill a
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structure with function pointers, and have those functions take
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the structure itself as an argument. This works just fine in
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assembler, of course (and doesn't really require anything more
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than your traditional jump-indirects), but it's also possible to
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use a lot of the standard optimizations that languages such as C++
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provide.
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</P
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><P
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> The most important of these is the <I
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CLASS="EMPHASIS"
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>vtable</I
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>.
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Each object type has its own vtable, and it's a list of function
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pointers for all the methods that type provides. This is a space
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savings over the traditional structs-with-function-pointers
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approach because when you have many objects of the same class, you
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only have to represent the vtable once. So that all objects may
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be treated identically, the vtable location is traditionally fixed
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as being the first entry in the corresponding structure.
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</P
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><P
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> Virtual method invocation takes an object pointer (traditionally
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called <TT
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CLASS="LITERAL"
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>self</TT
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> or <TT
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CLASS="LITERAL"
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>this</TT
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>) and a
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method index and invokes the approprate method on that object.
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Gee, where have we seen that before?
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</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><PRE
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CLASS="PROGRAMLISTING"
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>sprite'vtable:
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jsr do'jump'table
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.word sprite'init, sprite'update, sprite'render</PRE
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></TD
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></TR
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></TABLE
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><P
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> We mentioned before that vtables are generally the first entries
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in objects. We can play another nasty trick here, paying an
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additional byte per object to have the vtable be not merely a
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pointer to its vtable routine, but an actual jump instruction to
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it. (That is, if an object is at location X, then location X is
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the byte value <TT
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CLASS="LITERAL"
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>$4C</TT
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>,
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representing <TT
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CLASS="LITERAL"
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>JMP</TT
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>, location X+1 is the low byte
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of the vtable, and location X+2 is the high byte of the vtable.)
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Given that, our <TT
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CLASS="LITERAL"
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>invokevirtual</TT
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> function becomes
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very simple indeed:
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</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><PRE
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CLASS="PROGRAMLISTING"
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>invokevirtual:
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sta this
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stx this+1
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jmp (this)</PRE
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></TD
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></TR
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></TABLE
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><P
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> Which, combined with all our previous work here, takes
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the <TT
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CLASS="LITERAL"
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>this</TT
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> pointer in <TT
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CLASS="LITERAL"
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>.AX</TT
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> and
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a method identifier in <TT
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CLASS="LITERAL"
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>.Y</TT
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> and invokes that
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method on that object. Arguments besides <TT
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CLASS="LITERAL"
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>this</TT
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>
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need to be set up before the call
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to <TT
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CLASS="LITERAL"
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>invokevirtual</TT
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>, probably in some global
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argument array somewhere as discussed back in <A
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HREF="c680.html"
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>the Chapter called <I
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>Structured Programming</I
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></A
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>.
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</P
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></DIV
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><DIV
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CLASS="NAVFOOTER"
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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><A
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HREF="x992.html"
|
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ACCESSKEY="P"
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><<< Previous</A
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></TD
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><TD
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WIDTH="34%"
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VALIGN="top"
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HREF="book1.html"
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>Home</A
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><TD
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
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><A
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HREF="x1029.html"
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ACCESSKEY="N"
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>Next >>></A
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></TD
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></TR
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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>Dispatch-on-type and Data-Directed Assembler</TD
|
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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><A
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HREF="c953.html"
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ACCESSKEY="U"
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>Up</A
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></TD
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><TD
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
|
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>A final reminder</TD
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></TR
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></TABLE
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></DIV
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></BODY
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> |