C++ Template Meta Programming Introduction C++ Template Meta Programming Template Metaprogram- ming A Bad Francesco Nidito Example A Good Example Conclusions Programmazione Avanzata AA 2005/06 Outline C++ Template Meta Programming 1 Introduction Introduction 2 Template Metaprogramming Template Metaprogram- ming 3 A Bad Example A Bad Example A Good 4 A Good Example Example Conclusions 5 Conclusions Reference: K. Czarnecki, U. W. Eisenecker, “Generative Programming: Methods, Tools, and Applications”, Chapter 10 What Is Template Metaprogramming? C++ Template Meta Programming Introduction Template At this moment is quite difficult to give a definition. First Metaprogram- ming define two terms: A Bad Template Example Metaprogramming A Good Example Conclusions What Are Templates? C++ Template Meta Programming Templates are a C++ technique to create Generic Code Introduction Template Some templates are in C++ Standard and can be found in Metaprogram- STL ming A Bad The first version of STL architecture was made by Example A Good Alexander Stepanov Example Some examples of templates in STL are: std::vector, Conclusions std::map and std::list C++ Templates Example: Very Simple Array C++ Template Meta template <typename T, unsigned int L> Programming class VerySimpleArray{ T a[L]; Introduction public: Template Metaprogram- VerySimpleArray(){ ming for(int i=0; i < L; ++i) a[i] = T(); A Bad } Example A Good T& Get(unsigned int i){return a[i];} Example void Set(unsigned int i, T& v){a[i] = v;} Conclusions }; VerySimpleArray<float, 42> vsa; vsa.Set(3, 6.0); C++ Templates Example: Template specialization C++ Template template<typename _Tp, typename _Alloc=allocator<_Tp> > Meta class vector : protected _Vector_base<_Tp, _Alloc> Programming { __glibcpp_class_requires(_Tp, _SGIAssignableConcept) Introduction Template Metaprogram- typedef _Vector_base<_Tp, _Alloc> _Base; ming ... A Bad template <typename _Alloc> Example class vector<bool,_Alloc>: public _Bvector_base<_Alloc> A Good Example { Conclusions public: typedef bool value_type; ... /usr/include/c++/3.3.2/bits/std vector.h /usr/include/c++/3.3.2/bits/std bvector.h What Is Metaprogramming? C++ Template Meta Metaprogramming means writing programs that can Programming modify other programs or themselves The modification can happen at run time or at compile Introduction Template time Metaprogram- ming Compilers are an example of metaprograming: they take a A Bad program in an input language (C, Java...) and produce Example A Good another program (with same semantic) in an output Example language (machine code, bytecode... ) Conclusions Metaprograms have three advantages: Produce code quickly Lift abstraction Produce correct code (if the metaprogram is correct) What Is Template Metaprogramming? C++ Template Meta Programming Introduction Now it is time for the replay, Template Metaprograming Template Metaprogram- is... ming A Bad Producing metaprograms in C++ using templates Example But... wait a second... why do I need Template A Good Example Metaprogramming? Conclusions The Need For Speed C++ Template int Fact(int x){ Meta Programming int acc = 1; for(;x>0;--x) acc*=x; Introduction return acc; Template Metaprogram- } ming A Bad int a, b; Example A Good a = read_int_from_stdin(); Example b = Fact(a); //we compute at run time Conclusions ... b = Fact(5); //we can compute at compile //time but we do not We pay the time to compute something that is a constant! Naive Solution C++ ... Template Meta Programming b = 120; //b = Fact(5); Introduction Template The solution is not elegant. We need to place a lot Magic Metaprogram- ming Numbers in the code. And the magic number does not keep its A Bad meaning (5! = 120 or 42 + 78 = 120 ...). Example We can use a but it is not better: we must provide a A Good #define Example define for all possible value of the input of Fact. Conclusions ... #define FACT_4 (24) #define FACT_5 (120) ... b = FACT_5; Template Metaprogramming Solution C++ Template Meta Programming The general idea is to use the compiler to do some computation at compile time. Introduction Template To do this we need a only a C++ compiler that provides Metaprogram- ming Templates A Bad This can be done because the compiler, when compiles Example A Good Templates, produces code Example If we produce the right code (e.g. the sum of constant Conclusions terms), the compiler optimize and do constant folding But we can do more... First Step C++ Template Meta Programming template< int i> Introduction struct C { Template Metaprogram- enum { RES = i }; ming }; A Bad Example A Good cout <<_ C<2>::RES; Example Conclusions C<2>::RES is substituted by the compiler with 2: it is a cost ant and we can optimize. Back To Factorial C++ Template Meta template<int n> Programming struct Fact { enum { RET = n * Fact<n-1>::RET }; Introduction }; Template Metaprogram- ming template<> A Bad struct Fact<1> { Example A Good enum { RET = 1 }; Example }; Conclusions int b = Fact<5>::RET; // == 120 To do computation we must unroll templates: a kind of recursion Where is The Trick? C++ Template Meta Programming enum { RET = 5 * Fact<4>::RET }; Introduction enum { RET = 5 * 4 * Fact<3>::RET }; Template Metaprogram- enum { RET = 5 * 4 * 3 * Fact<2>::RET }; ming enum { RET = 5 * 4 * 3 * 2 * Fact<1>::RET }; A Bad enum { RET = 5 * 4 * 3 * 2 * 1 }; Example A Good enum { RET = 120 }; Example Conclusions b = 120; //Fact<5>::RET; C++ Template Metaprogramming Operators C++ Template Meta Programming template <bool C, class T, class E> struct IF { Introduction Template typedef T RET; Metaprogram- }; ming A Bad Example template <class T, class E> A Good Example struct IF<false, T, E> { Conclusions typedef E RET; }; C++ Template Metaprogramming Operators C++ Template Meta class CopyingGC { Programming public: void Collect() /*...*/ Introduction Template }; Metaprogram- ming A Bad class MarkAndSweepGC { Example public: A Good void Collect() /*...*/ Example }; Conclusions IF<GC == COPY, CopyingGC, MarkAndSweepGC>::RET gc; gc.Collect(); C++ Template Metaprogramming Functions C++ Template Meta Programming Introduction Template<int n1, int n2> Template struct Max { Metaprogram- ming enum { RET = (n1 > n2) ? n1 : n2 }; A Bad }; Example A Good Example _ cout << Max<42, 6>::RET; //prints 42 Conclusions C++ Template Metaprogramming Code Generation C++ inline int Power(const int x, int n){ Template Meta int p = 1; Programming for(;n>0; --n) p *= x; return p; Introduction } Template Metaprogram- ming A Bad We can specialize Power code for particular cases: Example A Good inline int Power3(const int x){ Example int p = 1; Conclusions p *= x; p *= x; p *= x; return p; } C++ Template Metaprogramming Code Generation C++ template<int n> Template Meta inline int Power(const int x){ Programming return Power<n-1>(x) * x; } Introduction Template Metaprogram- template<> ming A Bad inline int Power<1>(const int x){ Example return 1; A Good } Example Conclusions template<> inline int Power<0>(const int x){ return 1; } cout <<_ Power<4>(m); C++ Template Metaprogramming Code Generation C++ Template Meta _ cout << Power<4>(m); Programming _ cout << Power<3>(m) * m; Introduction Template Metaprogram- cout <<_ Power<2>(m) * m * m; ming A Bad cout <<_ Power<2>(m) * m * m * m; Example A Good Example cout <<_ Power<1>(m) * m * m * m * m; Conclusions cout <<_ 1 * m * m * m * m; cout <<_ m * m * m * m; C++ Template Metaprogramming Loop Unrolling(1) C++ Template Meta Programming template<int n, class B> struct FOR { Introduction static void loop(int m) { Template for (int i = 0; i < m/n; i++) { Metaprogram- ming UNROLL<n, B>::iteration(i * n); A Bad } Example A Good for (int i = 0; i < m%n; i++) { Example B::body(n * m/n + i); Conclusions } } }; C++ Template Metaprogramming Loop Unrolling(2) C++ Template Meta template <int n, class B> Programming struct UNROLL{ static void iteration(int i) { Introduction Template B::body(i); Metaprogram- UNROLL<n-1, B>::iteration(i + 1); ming A Bad } Example }; A Good Example template <class B> Conclusions struct UNROLL<0, B> { static void iteration(int i){ } }; C++ Template Metaprogramming Code Generation C++ Template Meta Programming Introduction We can generate code for multiple purposes: Template Metaprogram- Vector operations ming Math functions A Bad ... Example A Good Exercise: try to write some C++ Template Example Metaprograming functions and control structures (WHILE) Conclusions C++ Template Metaprogramming Data Structures C++ Template Meta Programming Introduction Lisp lovers know very well this code: Template Metaprogram- (cons 1 (cons 2 (cons 3 (cons 4 (cons nil)))) ming A Bad The previous code represents the list: Example (1 2 3 4) A Good Example C++ Template Metaprogrammers have this too... Conclusions C++ Template Metaprogramming Data Structures C++ Template Meta template <int n, class T> Programming struct Cons { enum { item = n }; Introduction typedef T next; Template Metaprogram- }; ming A Bad struct Nil { }; Example A Good typedef Cons<1, Cons<2, Nil()> > V; Example //V::item == 1; Conclusions //V::next::item == 2; //V::next::next == Nil; Exercise: try to create a Tree Total Loop Unroll C++ Template Meta Programming Introduction Template We want to copy all elements of an int array into another Metaprogram- ming We want to do it fast A Bad Example We can use an approach similar to FOR template A Good metastructure Example Conclusions Total Loop Unroll (C++ Code 1) C++ Template Meta Programming template<int N> inline void Copy(int n, int* from, int* to){ Introduction Template for (int i = 0; i < n/N; ++i) { Metaprogram- TMP_COPY_UNROLL<N>::iteration(i * N, from, to);ming A Bad } Example for (int i = 0; i < n%N; ++i) { A Good Example *to++ = *from++; Conclusions } } Total Loop Unroll (C++ Code 2) C++ Template template <int N> Meta struct TMP_COPY_UNROLL { Programming static void iteration(int i, int *f, int* t){ *t++ = *f++; Introduction Template TMP_COPY_UNROLL<N-1>::iteration(i + 1, f, t); Metaprogram- } ming A Bad }; Example A Good template <> Example Conclusions struct TMP_COPY_UNROLL<1> { static void iteration(int i, int* f, int* t){ *t++ = *f++; } }; Performance Results C++ Template Meta We measure the “Execution Time” of various values of Programming unroll and..