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  1. #include <iostream>
  2. using namespace std;
  3.  
  4. template<class T, class  Pref, class Suf> class  Wrap;
  5. template<class T, class Suf>
  6. class  Call_proxy {
  7. 	T* p;
  8. 	mutable bool own;
  9. 	Suf suffix;
  10. 	Call_proxy(T* pp, Suf su) :p(pp) , own(true) , suffix(su) { }  // restrict creation
  11. 	Call_proxy& operator=(const  Call_proxy&) ;  // prevent assignment
  12. public:
  13. 	template<class  U, class  P, class S> friend class  Wrap;
  14. 	Call_proxy(const  Call_proxy& a) : p(a.p) , own(true) , suffix(a.suffix) { a.own=false; }
  15. 	~Call_proxy() { if (own) suffix() ; }
  16.  
  17. 	//T* operator->() const{ return p;}
  18. 	T& operator->() const{ return *p;}
  19.  
  20. };
  21.  
  22.  
  23. template<class T, class  Pref, class Suf>
  24. class  Wrap {
  25. 	T* p;
  26. 	int* owned;
  27. 	void incr_owned() const{ if(owned) ++*owned; }
  28. 	void decr_owned() const{ if(owned && --*owned == 0) { delete p; delete owned; } }
  29. 	Pref prefix;
  30. 	Suf suffix;
  31. public:
  32. 	Wrap(T& x,  Pref pr, Suf su) :p(&x) , owned(0) , prefix(pr) , suffix(su) { }
  33. 	Wrap(T* pp,  Pref pr, Suf su) :p(pp) , owned(new int(1)) , prefix(pr) , suffix(su) { }
  34. 	Wrap(const  Wrap& a)
  35. 		:p(a.p) , owned(a.owned) , prefix(a.prefix) , suffix(a.suffix) { incr_owned() ; }
  36. 	Wrap& operator=(const  Wrap& a)
  37. 	{
  38. 		a.incr_owned() ;
  39. 		decr_owned() ;
  40. 		p = a.p;
  41. 		owned = a.owned;
  42. 		prefix= a.prefix;
  43. 		suffix= a.suffix;
  44. 		return *this;
  45. 	}
  46. 	~Wrap() { decr_owned() ; }
  47. 	Call_proxy<T,Suf> operator->() const{ prefix() ; return  Call_proxy<T,Suf>(p,suffix) ; }
  48. 	T* direct() const{ return p; } // extract pointer to wrapped object
  49. };
  50.  
  51. class X  { // one user class
  52. public:
  53. 	X() { cout << " make an  X\n"; }
  54. 	~X() { cout << "destroy an  X\n"; }
  55. 	int f() const{ cout << "f()"; return 1; }
  56. 	void g() const{ cout << "g()"; }
  57. };
  58. class Y { // another user class
  59. public:
  60. 	Y() { cout << " make a Y\n"; }
  61. 	~Y() { cout << "destroy a Y\n"; }
  62. 	void h() const{ cout << "h()"; }
  63. };
  64.  
  65. void prefix() { cout << "prefix "; }
  66. void suffix() { cout << " suffix\n"; }
  67.  
  68. struct  Pref { void operator()() const{ cout<< " Pref "; } };
  69. struct Suf { void operator()() const{ cout<< " Suf "; } };
  70.  
  71. template<class T>
  72. struct Shared : public  Wrap<T,Pref, Suf>
  73. {
  74. 	Shared(T& obj) :  Wrap<T,Pref, Suf>(obj,Pref() , Suf()) { }
  75. };
  76.  
  77. template<class T>
  78. struct Tracer : public  Wrap<T,void(*)() ,void(*)()> {
  79. 	Tracer(T& x) :  Wrap<T,void(*)() ,void(*)()>(x,::prefix,::suffix) { }
  80. };
  81.  
  82. int  main()  // test program
  83. {
  84. 	X x;
  85. 	Shared<X> xx(x) ;
  86. 	Tracer<Shared<X>> xxx(xx);
  87.  
  88. 	xx->g();
  89. 	xxx->g();
  90. 	return 0;
  91. }
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 function ‘int main()’:
prog.cpp:88:4: error: result of ‘operator->()’ yields non-pointer result
  xx->g();
    ^
prog.cpp:89:5: error: result of ‘operator->()’ yields non-pointer result
  xxx->g();
     ^
stdout
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Standard output is empty
https://ideone.com/CV93qX
language:
C++14 (gcc 8.3)
created:
11 years ago
visibility:
public

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