Template%20MetaProgramming%20in%20C

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Template MetaProgramming in C

• Giuseppe Attardi
• Università di Pisa

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Meta Language
Meta Program
Analysis
Generation
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Example

• templateltint igt
• class C
• enum RES i
• cout ltlt Clt2gtRES

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Turing Equivalent language

• template ltint ngt
• struct Fact
• enum RET n Factltn-1gtRET
• template ltgt
• struct Factlt0gt
• enum RET 1
• Factlt5gtRET // 120
• int i 3
• FactltigtRET // does not work (const i works)

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Meta Operators

• template ltbool g, class T, class Egt
• struct IF
• typedef T RET
• template ltclass T, class Egt
• struct IFltfalse, T, Egt
• typedef E RET

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Example

• class CompactingGC
• void GC()
• class MarkAndSweepGC
• void GC()
• IF ltGC COPY, CopyingGC, MarkAndSweepGCgtRET
  gc
• gc.GC()

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Other Operators

• struct FalseType
• struct TrueType
• template ltclass Tgt
• struct isPointer typedef FalseType RET
• template ltclass Tgt
• struct isPointerltTgt
• typedef TrueType RET

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Conditional

• templateltint n1, int n2gt
• struct Max
• enum RET (n1 gt n2) ? n1 n2

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Template Metafunctions

• C(k, n) n! / (k! (n-k)!)
• templateltint k, int ngt
• struct Comb
• enum RET FactltngtRET /
• (FactltkgtRET Factltn-kgtRET)
• cout ltlt Comblt2, 4gtRET ltlt endl

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Representing Data Structures

• template ltint n, class Ugt
• struct List
• enum item n
• typedef U next
• struct Nil
• typedef V Listlt1, Listlt2, Nil()gt gt
• Vitem 1
• Vnextitem 2
• Vnextnext Nil

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Functions on Templates

• template ltclass Lgt
• struct Length
• enum RET 1 LengthltLnextgtRET
• template ltgt
• struct LengthltNilgt enum RET 0

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Possible Uses

• Control loop unrolling
• FORlt3, Bgtloop

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Unrolling vector addition

• inline void addVect(int size, double a, double
  b, double c)
• while (size--)
• c a b

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Template Unrolling

• templateltint sizegt
• inline void addVect(double a, double b, double
  c)
• c a b
• addVectltsize-1gt(a1, b1, c1)
• templateltgt
• inline void addVectlt0gt(double a, double b,
  double c)
• addVectlt3gt(a, b, c)

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Effect of unrolling

• addVectlt3gt(a, b, c)
• c a b
• addVectlt2gt(a1, b1, c1)
• (c1) (a1) (b1)
• addVectlt1gt(a2, b2, c2)
• (c2) (a2) (b2)
• addVectlt0gt(a3, b3, c3)

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Loop Unrolling

• templateltint n, class Bgt
• struct FOR
• static void loop(int m)
• for (int i 0 i lt m/n i)
• UNROLLltn, Bgtiteration(i n)
• for (int i 0 i lt mn i)
• Bbody(n m/n i)

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Loop Unrolling (2)

• template ltint n, class Bgt
• struct UNROLL
• static void iteration(int i)
• Bbody(i)
• UNROLLltn-1, Bgtiteration(i 1)
• template ltclass Bgt
• struct UNROLLlt0, Bgt
• static void iteration(int i)