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  1. #include <iostream>
  2. //////////// Consider that the lines up to the next comment cannot be changed
  3. template< typename S > void F( S y )
  4. {
  5.     std::cout << y <<  " of some type" << std::endl; 
  6. }
  7.  
  8. template<> void F<>( int y )
  9. {
  10.     std::cout << y << " of int type." << std::endl; 
  11. }
  12. //////////// -----------------------------
  13.  
  14.  
  15. class B
  16. {
  17. public:
  18.     virtual ~B() = default;
  19.     virtual void f() const = 0;
  20. };
  21.  
  22. template<typename T>
  23. class D : public B
  24. {
  25. public:
  26.     void f() const override { F(x); }
  27.  
  28.     T x;
  29. };
  30.  
  31. int main()
  32. {
  33.     D<int>  o;
  34.     o.x  =  3;
  35.     B* p = &o;
  36.  
  37.     // Obviously, the following line will not work:
  38.     // F( p->x );
  39.  
  40.     // Obviously, the following will work:
  41.     F( ( (D<int>*) p)->x );
  42.  
  43.     // However, consider that 
  44.     //   1) only p is given and
  45.     //   2) that it is not known that p points to an object of type D<int>.
  46.     // How to modify B or D to invoke the correct instantiation of F, i.e. that for the
  47.     // type of "int"?
  48.  
  49.     p->f();
  50. }
Success #stdin #stdout 0s 15240KB
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Standard input is empty
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3 of int type.
3 of int type.
https://ideone.com/EtbSmd
language:
C++14 (gcc 8.3)
created:
6 years ago
visibility:
public

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