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  1.     #include <iostream>
  2.  
  3.     /// In header
  4.     #include <cassert>
  5.     #include <vector>
  6.  
  7.     template <class, size_t> class BaseT;
  8.  
  9.     class Base {
  10.         template <class, size_t> friend class BaseT;
  11.     public:
  12.  
  13.         int get() const; //     -> Implement: 'int getImpl() const' in Derived
  14.  
  15.         void set(int i); // = 0 -> Implement: 'void setImpl(int i)' in Derived
  16.  
  17.     private:
  18.         struct VTable {
  19.             typedef int (*Getter)(void const*);
  20.             typedef void (*Setter)(void*, int);
  21.  
  22.             VTable(): _get(0), _set(0) {}
  23.  
  24.             Getter _get;
  25.             Setter _set;
  26.         };
  27.  
  28.         static std::vector<VTable>& VT(); // defined in .cpp
  29.  
  30.         explicit Base(size_t v): _v(v) {}
  31.  
  32.         size_t _v;
  33.     }; // class Base
  34.  
  35.     template <class Derived, size_t Index>
  36.     class BaseT: public Base {
  37.     public:
  38.         BaseT(): Base(Index) {
  39.             static bool const _ = Register();
  40.             (void)_;
  41.         }
  42.  
  43.         // Provide default implementation of getImpl
  44.         int getImpl() const { return 0; }
  45.  
  46.         // No default implementation setImpl
  47.  
  48.     private:
  49.         static int Get(void const* b) {
  50.             Derived const* d = static_cast<Derived const*>(b);
  51.             return d->getImpl();
  52.         }
  53.  
  54.         static void Set(void* b, int i) {
  55.             Derived* d = static_cast<Derived*>(b);
  56.             d->setImpl(i);
  57.         }
  58.  
  59.         static bool Register() {
  60.             typedef Base::VTable VTable;
  61.  
  62.             std::vector<VTable>& vt = Base::VT();
  63.  
  64.             if (vt.size() <= Index) {
  65.                 vt.insert(vt.end(), Index - vt.size() + 1, VTable());
  66.             } else {
  67.                 assert(vt[Index]._get == 0 && "Already registered VTable!");
  68.             }
  69.  
  70.             vt[Index]._get = &Get;
  71.             vt[Index]._set = &Set;
  72.  
  73.             return true;
  74.         }
  75.     }; // class BaseT
  76.  
  77.     /// In source
  78.     std::vector<Base::VTable>& Base::VT() {
  79.         static std::vector<VTable> V;
  80.         return V;
  81.     } // Base::VT
  82.  
  83.     int Base::get() const {
  84.         return VT()[_v]._get(this);
  85.     } // Base::get
  86.  
  87.     void Base::set(int i) {
  88.         return VT()[_v]._set(this, i);
  89.     } // Base::set
  90.  
  91.     // 4 must be unique within the hierarchy
  92.     class Derived: public BaseT<Derived, 4> {
  93.          template <class, size_t> friend class BaseT;
  94.     public:
  95.         Derived(): _i(0) {}
  96.  
  97.     private:
  98.         int getImpl() const { return _i; }
  99.  
  100.         void setImpl(int i) { _i = i; }
  101.  
  102.         int _i;
  103.     }; // class Derived
  104.  
  105.  
  106.     /// In action
  107.     void print(Base const& b) { std::cout << b.get() << "\n"; }
  108.     void set(Base& b, int i) { b.set(i); }
  109.  
  110.     int main() {
  111.         Derived d;
  112.         set(d, 4);
  113.         print(d);
  114.     }
Success #stdin #stdout 0.02s 2816KB
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https://ideone.com/AHsZ3
language:
C++ (gcc 8.3)
created:
11 years ago
visibility:
public

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