Static If I Had a Hammer

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1 Static If I Had a Hammer Andrei Alexandrescu Research Scientist Facebook c 2012 Andrei Alexandrescu, Facebook. 1 / 27

2 In a nutshell (1/2) enum class WithParent { no, yes ; template <class T, WithParent wp> class Tree { class Node {... ; static if (wp == WithParent::yes) { Node * parent_; Node * left_, * right_; T payload_; : : : ; Tree<string, WithParent::yes> dictionary; c 2012 Andrei Alexandrescu, Facebook. 2 / 27

3 In a nutshell (2/2) template <class It> It rotate(it b, It m, It e) if (is_forward_iterator<it>::value) { : : : c 2012 Andrei Alexandrescu, Facebook. 3 / 27

4 Agenda Motivation The static if declaration Template constraints Implementation issues c 2012 Andrei Alexandrescu, Facebook. 4 / 27

5 In a nutshell (1/2) In a nutshell (2/2) Agenda Motivation Whystaticif? Why not staticif? A non-starter: #if The static if declaration Template Constraints Implementation Issues Motivation Time for ritual speakercide c 2012 Andrei Alexandrescu, Facebook. 5 / 27

6 Why static if? Introspection-driven code Acknowledged, encouraged: <traits> Incomplete: no trait-driven conditional compilation Recursive templates impose signature duplication Specialization on arbitrary type properties enable_if incomplete, syntax-heavy, has corner cases Difficulty in defining policy-based designs Opportunistic optimizations depending on type properties c 2012 Andrei Alexandrescu, Facebook. 6 / 27

7 Why not staticif? Would you define a new function for everyif? Makes advanced libraries Easy to implement and debug Easy to understand and use Simple problems should have simple solutions This function only works with numbers This algorithm requires forward iterators I can choose whether this tree has parent pointers c 2012 Andrei Alexandrescu, Facebook. 7 / 27

8 A non-starter: #if Conditional compilation associated with preprocessor Preprocessing step long before semantic analysis Poor integration with the language Impossible to test any trait Or really anything interesting c 2012 Andrei Alexandrescu, Facebook. 8 / 27

9 In a nutshell (1/2) In a nutshell (2/2) Agenda Motivation The static if declaration Basics Details Use inside templates std::pair done right std::tuple done simply Use inside functions Thestaticif declaration Template Constraints Implementation Issues Time for ritual speakercide c 2012 Andrei Alexandrescu, Facebook. 9 / 27

10 Basics Declaration-statement May occur wherever a declaration or statement allowed static if (sizeof(size_t) == 8) { size_t bsr64(size_t bits, size_t amount) {... // or size_t bsr64(size_t bits, size_t amount) { static if (sizeof(size_t) == 8) {... else {... c 2012 Andrei Alexandrescu, Facebook. 10 / 27

11 Details Tested expression is a constant-expression of if-testable type The braces do not introduce a scope For grouping only; add one more layer to introduce a scope No static else needed; else suffices Code on the failing branch tokenized but not compiled Situation similar but not identical to uninstantiated templates c 2012 Andrei Alexandrescu, Facebook. 11 / 27

12 Use inside templates template <class T> struct container {... static if (debug_mode<t>::value) { class const_iterator {... ; static if (std::is_const<t>::value) { typedef const_iterator iterator; else { class iterator {... ; else { class const_iterator {... ; class iterator {... ; ; c 2012 Andrei Alexandrescu, Facebook. 12 / 27

13 std::pair done right template <class T, class U> struct pair { static if (is_empty<t>::value is_empty<u>::value) { union { T first; U second; ; else { T first; U second;... ; c 2012 Andrei Alexandrescu, Facebook. 13 / 27

14 std::tuple done simply template <class... Ts> struct tuple { static if (sizeof...(ts) > 0) {... else { ; c 2012 Andrei Alexandrescu, Facebook. 14 / 27

15 Use inside functions template <class It, class T> void uninitialized_fill(it b, It e, const T& x) { static if (is_same< typename iterator_traits<it>::iterator_category, random_access_iterator_tag>::value) { assert(b <= e); static if (has_trivial_copy_constructor<t>::value) { // Doesn t matter that values were uninitialized std::fill(b, e, x); else { for (; b!= e; ++b) { new(&*b) T(x); c 2012 Andrei Alexandrescu, Facebook. 15 / 27

16 In a nutshell (1/2) In a nutshell (2/2) Agenda Motivation The static if declaration Template Constraints Insufficient Template Constraints But we can do that already! Classes, too Aftermath The concept connection Template Constraints Implementation Issues Time for ritual speakercide c 2012 Andrei Alexandrescu, Facebook. 16 / 27

17 Insufficient uninitialized_fill still unsatisfactory Templates are prone to biting off more than they can chew Any instantiation errors of uninitialized_fill prompt syntax errors inside its body static_assert inside may help, but not with overloading c 2012 Andrei Alexandrescu, Facebook. 17 / 27

18 Template Constraints Any template may have a constraint template <class It, class T> void uninitialized_fill(it b, It e, const T& x) if (is_forward_iterator<it>::value && is_convertible<t, decltype(*b)>::value) {... c 2012 Andrei Alexandrescu, Facebook. 18 / 27

19 But we can do that already! template <class It, class T> typename enable_if< is_forward_iterator<it>::value && is_convertible< T, iterator_traits<it>::value_type >::value >::type uninitialized_fill(it b, It e, const T& x) {... Too important and ubiquitous to leave it to a noisy hack c 2012 Andrei Alexandrescu, Facebook. 19 / 27

20 Classes, too And methods, of course template <class T> class rational if (is_integral<t>::value) {... rational operator-() if (is_signed<t>::value) {... ; c 2012 Andrei Alexandrescu, Facebook. 20 / 27

21 Aftermath Constraint sees symbols in the declaration Essential (terse, too) Definitionless declarations may also be constrained Constraint not part of the signature If constraint false, declaration disappears À la SFINAE c 2012 Andrei Alexandrescu, Facebook. 21 / 27

22 The concept connection Lightweight concept system Concepts expressed as Boolean assertions + Beautifully complements and integrates with traits + Makes concept definition as easy as writing Boolean axioms + Can express constraints as relations between types, not only individual types + Works with numbers too, not only types Not designed for separate compilation of templates Huge positive impact on the D programming language c 2012 Andrei Alexandrescu, Facebook. 22 / 27

23 In a nutshell (1/2) In a nutshell (2/2) Agenda Motivation The static if declaration Template Constraints Implementation Issues Back to conditional compilation Experience Credits Implementation Issues Time for ritual speakercide c 2012 Andrei Alexandrescu, Facebook. 23 / 27

24 Back to conditional compilation How about the dead branches? Possibility: treat like uninstantiated templates + Understood approach, easy to implement Non-portable syntax not possible Possibility: store as brace-balanced tokens More difficult to implement + More flexible, faster compilation speeds +/ static if(false): glorified comment Either way: cannot handle unpaired braces For the better, actually c 2012 Andrei Alexandrescu, Facebook. 24 / 27

25 Experience Already defined, implemented, and used in D Positive experience Backwards compatible Easy to incrementally retrofit code to using it Popular with library writers and users alike c 2012 Andrei Alexandrescu, Facebook. 25 / 27

26 Credits Walter Bright, Herb Sutter, yours truly: N3329 Walter Brown: N3322 Smaller proposal Similarly motivated and defined No template constraints A lot of alignment Merging is a sensible course of action c 2012 Andrei Alexandrescu, Facebook. 26 / 27

27 In a nutshell (1/2) In a nutshell (2/2) Agenda Motivation The static if declaration Template Constraints Implementation Issues Time for ritual speakercide Time for ritual speakercide c 2012 Andrei Alexandrescu, Facebook. 27 / 27

Static If Considered

Document number: N3613 Date: 2013-03-16 Study group: Concepts Reply to: Bjarne Stroustrup Gabriel Dos Reis Andrew Sutton Static If Considered Bjarne