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  1. #include <iostream>
  2. #include <memory>
  3.  
  4. class Base
  5. {
  6.     public:
  7.         Base() = default;
  8.         virtual ~Base() = default;
  9.         virtual void run() = 0;
  10. };
  11.  
  12. class Derived1: public Base
  13. {
  14.     public:
  15.         Derived1() = default;
  16.         ~Derived1() override = default;
  17.         void run() override
  18.         {
  19.         	std::cout << "  Derived1";
  20.         }
  21. };
  22.  
  23. class Derived2: public Base
  24. {
  25.     public:
  26.         Derived2() = default;
  27.         ~Derived2() override = default;
  28.         void run() override
  29.         {
  30.         	std::cout << "  Derived2";
  31.         }
  32. };
  33.  
  34. // This function works but increase count
  35. void doGenericCopy(std::shared_ptr<Base> ptr)
  36. {
  37.     ptr->run();
  38.     std::cout << "  Ref count: " << ptr.use_count() << std::endl;
  39. }
  40.  
  41. // This function works without increase count = OK !
  42. void doSpecificD1(std::shared_ptr<Derived1>& ptr)
  43. {
  44.     ptr->run();
  45.     std::cout << "  Ref count: " << ptr.use_count() << std::endl;
  46. }
  47.  
  48. // Compilation error = FAILED !
  49. void doGeneric(std::shared_ptr<Base>& ptr)
  50. {
  51.     ptr->run();
  52.     std::cout << "  Ref count: " << ptr.use_count() << std::endl;
  53. }
  54.  
  55. // Working fine for all Derivate = OK !
  56. template<typename CLASS>
  57. void doGenericTemplate(std::shared_ptr<CLASS>& ptr)
  58. {
  59.     ptr->run();
  60.     std::cout << "  Ref count: " << ptr.use_count() << std::endl;
  61. }
  62.  
  63. void doClassic(Base& ptr)
  64. {
  65.     ptr.run();
  66.     std::cout << std::endl;
  67. }
  68.  
  69. int main()
  70. {
  71.     auto d1 = std::make_shared<Derived1>();
  72.     auto d2 = std::make_shared<Derived2>();
  73.  
  74.     std::cout << "With copy: " << std::endl;
  75.     doGenericCopy(d1);
  76.     doGenericCopy(d2);
  77.  
  78.     std::cout << "Specific: " << std::endl;
  79.     doSpecificD1(d1);
  80.  
  81.     std::cout << "Template: " << std::endl;
  82.     doGenericTemplate(d1);
  83.     doGenericTemplate(d2);
  84.  
  85.     // Compilation issue
  86.     //doGeneric(d1);
  87.  
  88.     {
  89.          std::cout << "Classic: " << std::endl;
  90.          Derived1 c1;
  91.          Derived2 c2;
  92.          doClassic(c1);
  93.          doClassic(c2);
  94.     }
  95. }
  96.  
Success #stdin #stdout 0s 15248KB
comments (?)
stdin 
Standard input is empty
stdout 
With copy: 
  Derived1  Ref count: 2
  Derived2  Ref count: 2
Specific: 
  Derived1  Ref count: 1
Template: 
  Derived1  Ref count: 1
  Derived2  Ref count: 1
Classic: 
  Derived1
  Derived2
https://ideone.com/ZL0v7z
language:
C++14 (gcc 8.3)
created:
7 years ago
visibility:
public

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