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  1. #include <iostream>
  2. #include <unordered_set>
  3.  
  4. template < typename T > struct magic
  5. {
  6.     magic() : enable_for_each_instance(false) {}
  7.  
  8.     magic( bool b) : enable_for_each_instance(b)
  9.     { if(b) objects.insert( static_cast<T*>(this) ) ; }
  10.  
  11.     magic( const magic& that ) : magic(that.enable_for_each_instance) {}
  12.  
  13.     magic( magic&& that ) : magic(that.enable_for_each_instance) {}
  14.  
  15.     ~magic() { if(enable_for_each_instance) objects.erase( static_cast<T*>(this) ) ; }
  16.  
  17.     template< typename FN > static void for_each_instance( FN fn )
  18.     { for( T* p : objects ) fn(p) ; }
  19.  
  20.     const bool enable_for_each_instance ;
  21.     private: static std::unordered_set<T*> objects ;
  22. };
  23.  
  24. template < typename T > std::unordered_set<T*> magic<T>::objects ;
  25.  
  26. struct A : magic<A>
  27. {
  28.     explicit A( bool enable_magic = false ) : magic<A>(enable_magic) {}
  29.     void foo( int i, char c )
  30.     { std::cout << "A::foo( " << this << ", " << i << ", " << c << " )\n" ; }
  31. };
  32.  
  33. struct B : magic<B>
  34. {
  35.     explicit B( bool enable_magic = false ) : magic<B>(enable_magic) {}
  36.     void bar( double d ) const
  37.     { std::cout << "B::bar( " << this << ", " << d << " )\n" ; }
  38. };
  39.  
  40. int main()
  41. {
  42.     A one(true), x ;
  43.     B two(true), y ;
  44.     {
  45.         A three = one ;
  46.         B four, five = two ;
  47.         A a[1000] ;
  48.         B b[25] ;
  49.         A::for_each_instance( []( A* p ) { p->foo( 99, 'X' ) ; } ) ;
  50.         B::for_each_instance( []( B* p ) { p->bar( 12.8 ) ; } ) ;
  51.     }
  52.     std::cout << "------------------------\n" ;
  53.     A::for_each_instance( []( A* p ) { p->foo( -7, 'Y' ) ; } ) ;
  54.     B::for_each_instance( []( B* p ) { p->bar( 123.45 ) ; } ) ;
  55. }
  56.  
Success #stdin #stdout 0s 3036KB
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stdin
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Standard input is empty
stdout
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A::foo( 0xbfbc3944, 99, X )
A::foo( 0xbfbc3940, 99, X )
B::bar( 0xbfbc3946, 12.8 )
B::bar( 0xbfbc3942, 12.8 )
------------------------
A::foo( 0xbfbc3940, -7, Y )
B::bar( 0xbfbc3942, 123.45 )
https://ideone.com/inxooR
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
public

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