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  1. #include <utility>
  2. #include <cstddef>
  3. #include <iostream>
  4.  
  5. using namespace std;
  6.  
  7. struct Triangle;
  8. struct Square;
  9.  
  10. struct PolygonVisitor
  11. {
  12.     virtual ~PolygonVisitor() {}
  13.  
  14.     virtual void visit(Triangle& tr) = 0;
  15.     virtual void visit(Square& sq) = 0;
  16. };
  17.  
  18. struct Polygon
  19. {
  20.     virtual void accept(PolygonVisitor& v) = 0;
  21. };
  22.  
  23. struct Triangle : Polygon
  24. {
  25.     void accept(PolygonVisitor& v) override
  26.     {
  27.         v.visit(*this);
  28.     }
  29. };
  30.  
  31. struct Square : Polygon
  32. {
  33.     void accept(PolygonVisitor& v) override
  34.     {
  35.         v.visit(*this);
  36.     }
  37. };
  38.  
  39. template <typename T,
  40.     typename F,
  41.     typename BaseInner,
  42.     typename ArgsT>
  43. struct ComposeVisitor
  44. {
  45.     struct Inner : public BaseInner
  46.     {
  47.         using BaseInner::visit;
  48.         Inner(ArgsT&& args) :
  49.             BaseInner(move(args.second)),
  50.             m_f(move(args.first))
  51.         {
  52.         }
  53.         void visit(T& t) final override
  54.         {
  55.             m_f(t);
  56.         }
  57.     private:
  58.         F m_f;
  59.     };
  60.     ComposeVisitor(ArgsT&& args) :
  61.         m_args(move(args))
  62.     {
  63.     }
  64.     template <typename Tadd,
  65.         typename Fadd>
  66.         ComposeVisitor <
  67.         Tadd,
  68.         Fadd,
  69.         Inner,
  70.         pair < Fadd, ArgsT >> on(Fadd&& f)
  71.     {
  72.         return ComposeVisitor <
  73.             Tadd,
  74.             Fadd,
  75.             Inner,
  76.             pair < Fadd, ArgsT >> (
  77.             make_pair(
  78.             move(f),
  79.             move(m_args)));
  80.     }
  81.     Inner end_visitor()
  82.     {
  83.         return Inner(move(m_args));
  84.     }
  85.  
  86.     ArgsT m_args;
  87. };
  88. template <typename TVisitorBase>
  89. struct EmptyVisitor
  90. {
  91.     struct Inner : public TVisitorBase
  92.     {
  93.         using TVisitorBase::visit;
  94.         Inner(nullptr_t) {}
  95.     };
  96.  
  97.     template <typename Tadd, typename Fadd>
  98.     ComposeVisitor <
  99.         Tadd,
  100.         Fadd,
  101.         Inner,
  102.         pair < Fadd, nullptr_t >> on(Fadd&& f)
  103.     {
  104.         return ComposeVisitor <
  105.             Tadd,
  106.             Fadd,
  107.             Inner,
  108.             pair < Fadd, nullptr_t >> (
  109.             make_pair(
  110.             move(f),
  111.             nullptr));
  112.     }
  113. };
  114. template <typename TVisitorBase>
  115. EmptyVisitor<TVisitorBase> begin_visitor()
  116. {
  117.     return EmptyVisitor<TVisitorBase>();
  118. }
  119.  
  120. int main()
  121. {
  122.     int sides = 0;
  123.  
  124.     auto v = begin_visitor<PolygonVisitor>()
  125.         .on<Triangle>([&sides](Triangle&)
  126.     {
  127.         sides = 3;
  128.     })
  129.         .on<Square>([&sides](Square&)
  130.     {
  131.         sides = 4;
  132.     })
  133.         .end_visitor();
  134.  
  135.     Triangle p;
  136.     p.accept(v);
  137.  
  138.     std::cout << "sides: " << sides << std::endl;
  139. }
  140.  
Compilation error #stdin compilation error #stdout 0s 0KB
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stdin
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Standard input is empty
compilation info 
prog.cpp: In instantiation of ‘struct ComposeVisitor<Triangle, main()::__lambda0, EmptyVisitor<PolygonVisitor>::Inner, std::pair<main()::__lambda0, std::nullptr_t> >::Inner’:
prog.cpp:45:12:   required from ‘struct ComposeVisitor<Square, main()::__lambda1, ComposeVisitor<Triangle, main()::__lambda0, EmptyVisitor<PolygonVisitor>::Inner, std::pair<main()::__lambda0, std::nullptr_t> >::Inner, std::pair<main()::__lambda1, std::pair<main()::__lambda0, std::nullptr_t> > >::Inner’
prog.cpp:133:22:   required from here
prog.cpp:81:11: error: cannot allocate an object of abstract type ‘ComposeVisitor<Triangle, main()::__lambda0, EmptyVisitor<PolygonVisitor>::Inner, std::pair<main()::__lambda0, std::nullptr_t> >::Inner’
     Inner end_visitor()
           ^
prog.cpp:45:12: note:   because the following virtual functions are pure within ‘ComposeVisitor<Triangle, main()::__lambda0, EmptyVisitor<PolygonVisitor>::Inner, std::pair<main()::__lambda0, std::nullptr_t> >::Inner’:
     struct Inner : public BaseInner
            ^
prog.cpp:15:18: note: 	virtual void PolygonVisitor::visit(Square&)
     virtual void visit(Square& sq) = 0;
                  ^
stdout
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Standard output is empty
https://ideone.com/WLmPEs
language:
C++14 (gcc 8.3)
created:
11 years ago
visibility:
public

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