Higher order programming

1
Higher order programming

• Using C and boost

2
C concepts

• // T must model MyFuncConcept
• templateltclass Tgt
• generic_function(T t)
• t.MyFunc()

3
C concepts

• T is a model of MyFuncConcept if T has a member
  function called MyFunc which takes no arguments

4
C concepts

• struct MyFuncClass void MyFunc() ...
• MyFuncClass mfc
• generic_function(mfc) // Compiles
• int i
• generic_function(i) // Compilation error

5
C example concepts
Concept Assumed valid statment
Default constructible T t
Copyable T t2(t1)
Assignable t1 t2
Convertible to OtherType static_castltOtherTypegt(t)
Output streamable stream ltlt t

6
C concepts

• An assumption about a type parameter
• Compilation error if assumption fails
• Language support for concepts will be added in
  C0X
• template ltDefaultConstructible Tgt ...

7
Polymorphism through concepts

• Compile-time polymorphism through impicit
  interfaces
• templateltclass Tgt void foo(T t)
• ...
• t.bar()
• ...
• Run-time polymorphism through explicit interfaces
• class FooType virtual void bar() 0
• void foo(FooType t)
• ...
• t.bar()
• ...

8
Sidenote boostconcept_check

• Provide easy to read compilation errors
• Earlier compilation errors
• Helps writing understandable code

9
boostconcept_check example

• template ltclass RandomAccessItergt
• void stable_sort(RandomAccessIter first,
• RandomAccessIter last)
• using namespace boost
• function_requireslt RandomAccessIteratorConceptltR
  andomAccessItergt gt()
• typedef typename stditerator_traitsltRandomAcce
  ssItergtvalue_type
• value_type
• function_requireslt LessThanComparableConceptltval
  ue_typegt gt()
• ...

10
Higher order programming

• Passing functions as values
• Having values requires a type

11
C function pointers

• Unintuitive syntax
• Casting between member function pointers
  problematic
• Pointers to virtual functions problematic
• Different compilers behave differentally

12
C concept of a functor

• A class which defines operator()
• struct SomeFunctor
• void operator()() ...
• template ltclass Fgt
• void foo(F f)
• ...
• f()
• ...

13
C concept of a functor

• Some STL algorithms also require internal
  typedefs for
• return_type, first_argument_type,
  second_argument_type
• struct SomeFunctor
• typedef bool return_type
• typedef int first_argument_type
• bool operator()(int x) ...

14
Hand made functor examples

• Can take various amounts of arguments
• struct MyNullaryFunctor
• void operator()()
• struct MyUnaryFunctor
• void operator()(int arg1)

15
Hand made functor examples

• Can hold state
• struct NullaryFunctor
• NullaryFunctor(int state)
• void operator()()
• private
• int iState
• Some STL algorithms requires stateless functors

16
Hand made functor examples

• Can return a value
• struct SomePredicate
• bool operator(int i)()

17
STL algorithm example

• template ltclass InputIterator, class Predicategt
  
• stditerator_traitsltInputIteratorgtdifference_ty
  pe
• stdcount_if(InputIterator first, InputIterator
  last, Predicate pred)
• InputIterator is a model of the Input Iterator
  concept
• Predicate is a model of the Predicate concept
• InputIterator's value type is convertible to
  Predicate's argument type

18
STL functor example

• namespace
• struct MyPredicate
• bool operator()(int i) const return igt0
  ilt9
• stdvectorltintgt vec ...
• unsigned int single_digits stdcount_if(vec.be
  gin(),
• 
  vec.end(),
• 
  MyPredicate())

19
boostfunction

• Generalized callback through functors
• Compatible with free functions, member functions
  and other functors
• Abstracts away callback type
• Intuitive semantics
• Easy to read syntax

20
boostfunction example

• void free_func(int x)
• boostfunctionltvoid(int x)gt callback
  free_func
• callback(10)

21
boostfunction example

• struct MyFunctor void operator()(int x)
• boostfunctionltvoid(int x)gt callback
  MyFunctor()
• callback(10)

22
boostfunction example

• struct MyClass void member_func(int x)
• // Need an instance to call a member function
  variable
• boostfunctionltvoid(MyClass, int x)gt callback
• MyClassmember_func
• MyClass my_class
• callback(my_class, 10)

23
boostfunction semantics

• boostfunctionltvoid(int x)gt callback //
  unitialized
• ...
• if (callback) callback(10)

24
boostfunction usage example

• class OkCancelDialog
• OkCancelDialog(const boostfunctionltvoid()gt
  onOk,
• const boostfunctionltvoid()gt
  onCancel)

25
Recall from RAII presentation

• prepare_something()
• possibly_throwing_call_1()
• possibly_throwing_call_2()
• rollback_preparation()
• rollback_call_1()

26
Recall from RAII presentation

• prepare_something()
• try
• possibly_throwing_call_1()
• try
• possibly_throwing_call_2()
• catch (...)
• rollback_call_1()
• rollback_preparation()
• throw
• catch (...)
• rollback_preparation()
• throw

27
Recall from RAII presentation

• Error-prone to write
• Difficult to read
• Difficult to change
• Poor scalability

28
Recall from RAII presentation

• class ScopeGuard
• ScopeGuard(const boostfunctionltvoid()gt
  rollback)
• mRollBack(rollback)
• ScopeGuard()
• if (mRollBack) mRollBack()
• void Dismiss()
• mRollback boostfunctionltvoid()gt()
• private
• boostfunctionltvoid()gt mRollback

29
Recall from RAII presentation

• prepare_something()
• ScopeGuard preparation_guard(rollback_preparation)
  
• possibly_throwing_call_1()
• ScopeGuard call_1_guard(rollback_call_1)
• possibly_throwing_call_2()
• // Commit
• preparation_guard.Dismiss()
• call_1_guard.Dismiss()

30
Writing custom made functors

• Is boring
• Generates a lot of code
• Takes some time

31
boostbind

• Generates unnamed functors from
• Function pointers
• Member function pointers
• Other functors
• Meant to be used as unnamed temporaries

32
boostbind example

• void foo (int arg1, int arg2, int arg3)
• stdcout ltlt arg1 ltlt ltlt arg2 ltlt ltlt arg3
  ltlt stdendl
• boostbind(foo, _1, _2, _3) (1, 2, 3)
• boostbind(foo, _3, _2, _1) (1, 2, 3)
• boostbind(foo, _1, _1, _1) (20)
• boostbind(foo, _3, _3, _3) (10, 20, 30)
• boostbind(foo, 1, 2, 3) ()
• boostbind(foo, 1, _1, 1) (1, 2, 3)

33
boostbind, whats happening?

• void foo (int arg1, int arg2, int arg3)
• boostbind(foo, 10, _1, 30)
• Whats the return type of the bind expression?
• struct some_super_strange_type_with_loads_of_templ
  ates
• void operator(int x) foo(iArg1, x, iArg3)
• int iArg1, iArg3

34
boostbind example

• namespace
• bool my_pred (int i) return igt0 ilt9
• stdvectorltintgt vec ...
• unsigned int single_digits
• stdcount_if(vec.begin(), vec.end(),
  boostbind(my_pred, _1))

35
boostbind logical operators

• stdvectorltintgt vec ...
• unsigned int single_digits
• stdcount_if(vec.begin(), vec.end(), _1gt0
  _1lt9)

36
boostbind member functions

• struct X
• void SomeFunc(int arg1, int arg2)
• stdvectorltXgt vec
• stdfor_each(vec.begin(), vec.end(),
• boostbind(XSomeFunc, _1, 1, 2))

37
boostbind nested expressions

• stdstring f(stdstring const x) return
  "f(" x ")"
• stdstring g(stdstring const x) return
  "g(" x ")"
• stdstring h(stdstring const x, stdstring
  const y)
• return "h(" x ", " y ")"
• stdstring k() return "k()"
• templateltclass Fgt void test(F f) stdcout ltlt
  f("x", "y") ltlt '\n'
• using namespace boost
• test( bind(f, bind(g, _1)) )
• test( bind(f, bind(h, _1, _2)) )
• test( bind(h, bind(f, _1), bind(g, _1)) )
• test( bind(h, bind(f, _1), bind(g, _2)) )
• test( bind(f, bind(k)) )

38
boostbind semantics

• Bound values are stored by value!
• struct some_super_strange_type_with_loads_of_templ
  ates
• void operator(int x) foo(iArg1, x, iArg3)
• int iArg1, iArg3

39
boostbind references

• void foo(const X arg)
• X x
• boostbind(foo, boostcref(x))()

40
boostbind and shared_ptr

• struct X
• void foo()
• boostshared_ptrltXgt x
• boostbind(Xfoo, x)()

41
bindfunction usage

• Bind
• Pass as unnamed temporaries for templated
  functions
• Bind arguments as state
• Specifically bind instances to member functions
• Reorder arguments
• Function
• Pass as arguments for non-templated functions
• Abstracts away function type
• Store as non-templated variables
• Can be uninitialized

42
Combining bind and function

• function and bind both model the functor concept
• Both are compatible with other functors
• void func(int arg1)
• boostfunctionltvoid()gt call boostbind(func,
  1000)

43
Combining bind and function

• // Non-templated higher order function
• void register_callback(boostfunctionltvoid()gt
  callback)
• void my_callback(int source)
• register_callback(boostbind(my_callback,
  10))

44
Boost lambda

• Similar to bind, larger and more advanced library
• Pushes the limits of the C language
• using namespace boost
• stdvectorltintgt v ...
• stdfor_each(v.begin(), v.end(), if_(_1 2
  0) cout ltlt _1 )
• int a510 int i
• stdfor_each(a, a5, for_loop(var(i)0,
  var(i)lt10, var(i),
• _1var(i) 1))
  
• stdfor_each(v.begin(), v.end(),
• (
• switch_statement( _1,
• case_statementlt0gt(stdcout ltlt
  constant("zero")),
  case_statementlt1gt(stdcout ltlt constant("one")),
  
• default_statement(cout
  ltlt constant("other ") ltlt _1) ),
• cout ltlt constant("\n") )
• )