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POTrying to understand complicated use of macros, templates and enums in C++11
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  1. POTrying to understand complicated use of macros, templates and enums in C++11
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    copied!<p>I just saw this video <a href="http://www.youtube.com/watch?v=y71lli8MS8s" rel="nofollow">http://www.youtube.com/watch?v=y71lli8MS8s</a> and find it amazing. The question is about his opcode decoding matrix function:</p> <pre><code>template&lt;u16 op&gt; // Execute a single CPU instruction, defined by opcode "op". void Ins() // With template magic, the compiler will literally synthesize &gt;256 different functions. { // Note: op 0x100 means "NMI", 0x101 means "Reset", 0x102 means "IRQ". They are implemented in terms of "BRK". // User is responsible for ensuring that WB() will not store into memory while Reset is being processed. unsigned addr=0, d=0, t=0xFF, c=0, sb=0, pbits = op&lt;0x100 ? 0x30 : 0x20; // Define the opcode decoding matrix, which decides which micro-operations constitute // any particular opcode. (Note: The PLA of 6502 works on a slightly different principle.) enum { o8 = op/8, o8m = 1 &lt;&lt; (op%8) }; // Fetch op'th item from a bitstring encoded in a data-specific variant of base64, // where each character transmits 8 bits of information rather than 6. // This peculiar encoding was chosen to reduce the source code size. // Enum temporaries are used in order to ensure compile-time evaluation. #define t(s,code) { enum { \ i=o8m &amp; (s[o8]&gt;90 ? (130+" (),-089&lt;&gt;?BCFGHJLSVWZ[^hlmnxy|}"[s[o8]-94]) \ : (s[o8]-" (("[s[o8]/39])) }; if(i) { code; } } /* Decode address operand */ t(" !", addr = 0xFFFA) // NMI vector location t(" *", addr = 0xFFFC) // Reset vector location t("! ,", addr = 0xFFFE) // Interrupt vector location t("zy}z{y}zzy}zzy}zzy}zzy}zzy}zzy}z ", addr = RB(PC++)) t("2 yy2 yy2 yy2 yy2 XX2 XX2 yy2 yy ", d = X) // register index t(" 62 62 62 62 om om 62 62 ", d = Y) t("2 y 2 y 2 y 2 y 2 y 2 y 2 y 2 y ", addr=u8(addr+d); d=0; tick()) // add zeropage-index t(" y z!y z y z y z y z y z y z y z ", addr=u8(addr); addr+=256*RB(PC++)) // absolute address t("3 6 2 6 2 6 286 2 6 2 6 2 6 2 6 /", addr=RB(c=addr); addr+=256*RB(wrap(c,c+1)))// indirect w/ page wrap t(" *Z *Z *Z *Z 6z *Z *Z ", Misfire(addr, addr+d)) // abs. load: extra misread when cross-page t(" 4k 4k 4k 4k 6z 4k 4k ", RB(wrap(addr, addr+d)))// abs. store: always issue a misread /* Load source operand */ t("aa__ff__ab__,4 ____ - ____ ", t &amp;= A) // Many operations take A or X as operand. Some try in t(" knnn 4 99 ", t &amp;= X) // error to take both; the outcome is an AND operation. t(" 9989 99 ", t &amp;= Y) // sty,dey,iny,tya,cpy t(" 4 ", t &amp;= S) // tsx, las t("!!!! !! !! !! ! !! !! !!/", t &amp;= P.raw|pbits; c = t)// php, flag test/set/clear, interrupts t("_^__dc___^__ ed__98 ", c = t; t = 0xFF) // save as second operand t("vuwvzywvvuwvvuwv zy|zzywvzywv ", t &amp;= RB(addr+d)) // memory operand t(",2 ,2 ,2 ,2 -2 -2 -2 -2 ", t &amp;= RB(PC++)) // immediate operand /* Operations that mogrify memory operands directly */ t(" 88 ", P.V = t &amp; 0x40; P.N = t &amp; 0x80) // bit t(" nink nnnk ", sb = P.C) // rol,rla, ror,rra,arr t("nnnknnnk 0 ", P.C = t &amp; 0x80) // rol,rla, asl,slo,[arr,anc] t(" nnnknink ", P.C = t &amp; 0x01) // lsr,sre, ror,rra,asr t("ninknink ", t = (t &lt;&lt; 1) | (sb * 0x01)) t(" nnnknnnk ", t = (t &gt;&gt; 1) | (sb * 0x80)) t(" ! kink ", t = u8(t - 1)) // dec,dex,dey,dcp t(" ! khnk ", t = u8(t + 1)) // inc,inx,iny,isb /* Store modified value (memory) */ t("kgnkkgnkkgnkkgnkzy|J kgnkkgnk ", WB(addr+d, t)) t(" q ", WB(wrap(addr, addr+d), t &amp;= ((addr+d) &gt;&gt; 8))) // [shx,shy,shs,sha?] /* Some operations used up one clock cycle that we did not account for yet */ t("rpstljstqjstrjst - - - -kjstkjst/", tick()) // nop,flag ops,inc,dec,shifts,stack,transregister,interrupts /* Stack operations and unconditional jumps */ t(" ! ! ! ", tick(); t = Pop()) // pla,plp,rti t(" ! ! ", RB(PC++); PC = Pop(); PC |= (Pop() &lt;&lt; 8)) // rti,rts t(" ! ", RB(PC++)) // rts t("! ! /", d=PC+(op?-1:1); Push(d&gt;&gt;8); Push(d)) // jsr, interrupts t("! ! 8 8 /", PC = addr) // jmp, jsr, interrupts t("!! ! /", Push(t)) // pha, php, interrupts /* Bitmasks */ t("! !! !! !! !! ! !! !! !!/", t = 1) t(" ! ! !! !! ", t &lt;&lt;= 1) t("! ! ! !! !! ! ! !/", t &lt;&lt;= 2) t(" ! ! ! ! ! ", t &lt;&lt;= 4) t(" ! ! ! !____ ", t = u8(~t)) // sbc, isb, clear flag t("`^__ ! ! !/", t = c | t) // ora, slo, set flag t(" !!dc`_ !! ! ! !! !! ! ", t = c &amp; t) // and, bit, rla, clear/test flag t(" _^__ ", t = c ^ t) // eor, sre /* Conditional branches */ t(" ! ! ! ! ", if(t) { tick(); Misfire(PC, addr = s8(addr) + PC); PC=addr; }) t(" ! ! ! ! ", if(!t) { tick(); Misfire(PC, addr = s8(addr) + PC); PC=addr; }) /* Addition and subtraction */ t(" _^__ ____ ", c = t; t += A + P.C; P.V = (c^t) &amp; (A^t) &amp; 0x80; P.C = t &amp; 0x100) t(" ed__98 ", t = c - t; P.C = ~t &amp; 0x100) // cmp,cpx,cpy, dcp, sbx /* Store modified value (register) */ t("aa__aa__aa__ab__ 4 !____ ____ ", A = t) t(" nnnn 4 ! ", X = t) // ldx, dex, tax, inx, tsx,lax,las,sbx t(" ! 9988 ! ", Y = t) // ldy, dey, tay, iny t(" 4 0 ", S = t) // txs, las, shs t("! ! ! !! ! ! ! ! !/", P.raw = t &amp; ~0x30) // plp, rti, flag set/clear /* Generic status flag updates */ t("wwwvwwwvwwwvwxwv 5 !}}||{}wv{{wv ", P.N = t &amp; 0x80) t("wwwv||wvwwwvwxwv 5 !}}||{}wv{{wv ", P.Z = u8(t) == 0) t(" 0 ", P.V = (((t &gt;&gt; 5)+1)&amp;2)) // [arr] /* All implemented opcodes are cycle-accurate and memory-access-accurate. * [] means that this particular separate rule exists only to provide the indicated unofficial opcode(s). */ } </code></pre> <p>What I find very confusing is the define statement. When macros get expanded it looks like this <a href="http://codepad.org/bUxdX8MQ" rel="nofollow">http://codepad.org/bUxdX8MQ</a>. But how can <code>'130+" (),-089&lt;&gt;?BCFGHJLSVWZ[^hlmnxy|}"'</code> be legal C++ code? As far as I know combining strings and integers in such fashion is illegal. Also this <code>("zy}z{y}zzy}zzy}zzy}zzy}zzy}zzy}z "[o8]&gt;90)</code> does not make sense at all, unless C++11 added support to access string characters in an array fashion. Hoping to learn something new!</p> <p>EDIT: Thank you to everyone who responded. I was unaware that <code>"blabla"[idx]</code> was allowed in C/C++ now that I do the code makes sense.</p>
 

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