Concepts in C

1
Concepts in C
2
Templates in current C

• C template is typeless
• No language support for constrained generics
• Accidental errors found in instantiation gt
• complex, long, hard to understand error
  messages

3
Example

• ...
• stdlistltintgt intList
• intList.push_back(42) ... intList.push_back(5)
• ...
• stdsort(intList.begin(), intList.end())
• Compiled with gcc

4
Error messages

• /usr/include/c/4.1.3/bits/stl_algo.h In
  function void stdsort(_RandomAccessIterator,
  _RandomAccessIterator) with _RandomAccessIterator
  std_List_iteratorltintgt
• templ.cpp13 instantiated from here
• /usr/include/c/4.1.3/bits/stl_algo.h2713
  error no match for operator- in __last -
  __first
• /usr/include/c/4.1.3/bits/stl_algo.h In
  function void std__final_insertion_sort(_Random
  AccessIterator, _RandomAccessIterator) with
  _RandomAccessIterator std_List_iteratorltintgt
  
• /usr/include/c/4.1.3/bits/stl_algo.h2714
  instantiated from void stdsort(_RandomAccessIte
  rator, _RandomAccessIterator) with
  _RandomAccessIterator std_List_iteratorltintgt
  
• templ.cpp13 instantiated from here

5
Cont.

• /usr/include/c/4.1.3/bits/stl_algo.h2360
  error no match for operator in __first 16
• /usr/include/c/4.1.3/bits/stl_algo.h In
  function void std__insertion_sort(_RandomAccess
  Iterator, _RandomAccessIterator) with
  _RandomAccessIterator std_List_iteratorltintgt
  
• /usr/include/c/4.1.3/bits/stl_algo.h2363
  instantiated from void std__final_insertion_sor
  t(_RandomAccessIterator, _RandomAccessIterator)
  with _RandomAccessIterator std_List_iteratorlt
  intgt
• /usr/include/c/4.1.3/bits/stl_algo.h2714
  instantiated from void stdsort(_RandomAccessIte
  rator, _RandomAccessIterator) with
  _RandomAccessIterator std_List_iteratorltintgt
  
• templ.cpp13 instantiated from here
• /usr/include/c/4.1.3/bits/stl_algo.h2273
  error no match for operator in __first 1

6
Cont 2.

• /usr/include/c/4.1.3/bits/stl_algo.h2363
  instantiated from void std__final_insertion_sor
  t(_RandomAccessIterator, _RandomAccessIterator)
  with _RandomAccessIterator std_List_iteratorlt
  intgt
• /usr/include/c/4.1.3/bits/stl_algo.h2714
  instantiated from void stdsort(_RandomAccessIte
  rator, _RandomAccessIterator) with
  _RandomAccessIterator std_List_iteratorltintgt
  
• templ.cpp13 instantiated from here
• /usr/include/c/4.1.3/bits/stl_algo.h2279
  error no match for operator in __i 1

7
And the reason is

• in the STL implementation?
• The fact sort requires random access iterator
  but list has only forward iterator
• But only this line refers to thattempl.cpp13
  instantiated from here
• /usr/include/c/4.1.3/bits/stl_algo.h2273
  error no match for operator in __first 1

8
C0x

• Well-known problem in industrial application
• In other languages (java, Eiffel, ADA) there are
  constrained generics
• In C0x, there will be concepts

9
The language components

• Concept
• Names requirements against template arguments
• Concept_map
• Semantic relationship between a type and a
  concept
• Requires
• The concrete requirement list agains template
  arguments of a class or function

10
Concept

• List of function signatures and associated types
• Like LessThanComparable concept
• concept LessThanComparablelttypename Tgt
• bool operatorlt(T a, T b)

11
Requires

• LessThanComparable concept in a concrete template
  algorithm
• templatelttypename Tgt
• requires LessThanComparableltTgt
• T min(const T a, const T b)?
• return a lt b ? a b

12
Requires in other way

• If a concept defines constraints on only one
  template parameter, there is a simpler way
• templateltLessThanComparable Tgt
• T min(const T a, const T b)?
• return a lt b ? a b
• Typeof a template

13
A bit more complex example

• concept Regularlttypename Tgt
• // default constructor
• T T()
• // copy constructor
• T T(const T)
• // destructor
• TT()
• // copy assignment
• T operator(T, const T)
• // equality comparison
• bool operator(const T, const T)
• // inequality comparison
• bool operator!(const T, const T)
• // swap
• void swap(T, T)

14
Multi-Type Concept

• Concepts may define requirements on more than one
  types
• concept Convertiblelttypename T, typename Ugt
• operator U(const T)
• And there is a way to express default parameters
• concept EqualtyComparablelttypename T, typename
  U Tgt
• bool operator(T, U)?

15
Example of Multi-Type Concepts

• concept InputIterator lttypename Iter, typename
  Valuegt
• Iter Iter ()
  // default constructor
• Iter Iter (const Iter )
  // copy constructor
• Iter Iter ()
  // destructor
• Iter operator(Iter, const Iter)
  // copy assignment
• Value operator(const Iter)
  // dereference
• Iter operator(Iter)
  // pre- increment
• Iter operator(Iter, int)
  // post- increment
• bool operator(const Iter, const Iter) //
  equality comparison
• bool operator!(const Iter, const Iter ) //
  inequality comparison
• void swap(Iter , Iter )
  // swap

16
Concepts composition

• Regular concept used for InputIterator concept
• concept InputIteratorlttypename Iter, typename
  Valuegt
• requires RegularltItergt
• Value operator(const Iter) // dereference
• Iter operator(Iter) // pre-increment
• Iter operator(Iter, int) // post-increment

17
Concepts refinement

• concept InputIteratorlttypename Iter,typename
  Valuegt RegularltValuegt
• Value operator(const Iter) // dereference
• Iter operator(Iter) //
  pre-increment
• Iter operator(Iter, int) //
  post-increment
• Hint if hierarchy, then refinement,
• Difference in concept_map definition

18
A template using InputIterator concept

• templatelttypename Iter, Regular Vgt
• requires InputIteratorltIter, Vgt
• void print(Iter ?rst, Iter last)?
• while (?rst ! last)?
• stdcout ltlt first
• stdcout ltlt stdendl

19
The real InputIterator concept

• concept InputIterator
• lttypename Iter, typename Value, typename
  Reference
• typename Pointer,
  typename Di?erencegt
• requires RegularltItergt
• requires ConvertibleltReference, Valuegt
• Reference operator(const Iter) //
  dereference
• Iter operator(Iter) //
  pre-increment
• Iter operator(Iter, int) //
  post-increment
• // ...

20
The real template function using InputIterator

• templatelttypename Iter, Regular V, typename R,
• typename P, typename Dgt
• requires InputIteratorltIter, V, R, P, Dgt
• void print(Iter ?rst, Iter last)?
• while (?rst ! last)?
• stdcout ltlt first
• stdcout ltlt stdendl

21
Associated Types

• Associated types
• concept InputIteratorlttypename Itergt
• typename value type
• typename reference
• typename pointer
• typename di?erence type
• requires RegularltItergt
• requires Convertibleltreference type, value
  typegt
• reference operator(const Iter)
• Iter operator(Iter)
• Iter operator(Iter, int)
• // ...

22
Template using InputIterator

• Now we have only one template parameter
• templateltInputIterator Itergt
• requires RegularltItervalue typegt
• void print(Iter ?rst, Iter last)
• while (?rst ! last)
• stdcout ltlt first
• stdcout ltlt stdendl

23
Concept_map

• Semantic relationship between a type and a
  concept
• Example looking for the best student
• struct Student
• stdstring name
• double avg
• ...
• stdlistltStudentgt l
• ...
• stdcout ltlt (stdmin_element(l.begin(),
  l.end())).name ltlt stdendl

24
Concept_map 2

• But operatorlt is not defined on Student
• Non-intrusive solution using concept_map
• concept_map LessThanComparableltDiakgt
• bool operatorlt(Diak a, Diak b)
• return a.atlag lt b.atlag

25
Concept_map 3

• If operatorlt is defined on some type X, we dont
  need to redefine
• concept_map LessThanComparableltXgt
• auto keyword no concept_map is required

26
Other examples for Concept_map

• Suppose InputIterator is auto
• Then this partial concept_map definition is valid
• concept map InputIteratorltchargt
• typedef char value type
• typedef char reference
• typedef char pointer
• typedef stdptrdi?_t di?erence type
• The rest is generated automaticaly

27
More Concept_map

• If implicite (auto) declaration is not valid,
  e.c. we want to define to use int-s, az
  iterator
• concept map InputIteratorltintgt
• typedef int value type
• typedef int reference
• typedef int pointer
• typedef int difference type
• int operator(int x) return x
• Thus, we could inialitize containers.
• stdcopy(1, 101, v.begin())

28
Concept_map template

• concepts and concept_maps can be tempatized
• All the pointers are iterators
• templatelttypename Tgt
• concept_map InputIteratorltTgt
• typedef T value type
• typedef T reference
• typedef T pointer
• typedef std ptrdi? t di?erence type

29
A container adaptor

• With concept, concept_map container adaptors
  could be defined without wrapper classes.
• concept Stacklttypename Xgt
• typename value type
• void push(X, const value type)
• void pop(X)
• value type top(const X)
• bool empty(const X)

30
And its concept_map

• templatelttypename Tgt
• concept_map StackltstdvectorltTgt gt
• typedef T value type
• void push(stdvectorltTgt v, const T x)
• v.push_back(x)
• void pop(std vector ltTgt v)
• v.pop_back()
• T top(const stdvector ltTgt v)
• return v.back()
• bool empty(const stdvector ltTgt v)
• return v.empty()

31
Concept-based Overloading

• Best matching template will be instantiated,
• Better way to implement many STl functions.

32
The advance fv. declarations

• templateltInputIterator Itergt
• void advance(Iter x, Iterdifference_type n)
• while (n gt 0) x --n
• templateltBidirectionalIterator Itergt
• void advance(Iter x, Iterdifference_type n)
• if (n gt 0) while (n gt 0) x --n
• else while (n lt 0) --x n
• templateltRandomAccessIterator Itergt
• void advance(Iter x, Iterdifference_type n)
• x n

33
Negative constraint

• templateltEqualityComparable Tgt
• class dictionary
• // slow, linked-list implementation
• templateltLessThanComparable Tgt
• requires !HashableltTgt
• class dictionaryltTgt
• // balanced binary tree implementation
• templateltHashable Tgt
• class dictionaryltTgt
• // hash table implementation

34
The error messages

• For the first example
• ...
• stdlistltintgt intList
• intList.push_back(42) ... intList.push_back(5)
• ...
• stdsort(intList.begin(), intList.end())
• We got this compiler error
• listconc.cpp In function int main()
• listconc.cpp9 error no matching function for
  call to sort(std_List_iteratorltintgt,
  std_List_iteratorltintgt)
• /home/lupin/local/conceptgcc/bin/../lib/gcc/i686-p
  c-linux-gnu/4.3.0/../../../../include/c/4.3.0/bi
  ts/stl_algo.h2874 note candidates are void
  stdsort(_Iter, _Iter) with _Iter
  std_List_iteratorltintgt ltwhere clausegt
• listconc.cpp9 note no concept map for
  requirement stdMutableRandomAccessIteratorltstd
  _List_iteratorltintgt gt