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  1. #include <memory>
  2. #include <iostream>
  3.  
  4. struct ABC {
  5.     virtual ~ABC(){}
  6.     virtual int getValue() const { return 42; }
  7. };
  8.  
  9. struct DerivedABC : ABC {
  10.     int getValue() const override { return -1; }
  11. };
  12.  
  13. class XYZFactory {
  14. public:
  15.   virtual ~XYZFactory(){}
  16.   virtual std::unique_ptr<ABC> createABC() const = 0;
  17. };
  18.  
  19. class XYZFactoryImpl : public XYZFactory {
  20. public:
  21.   std::unique_ptr<ABC> createABC() const override {
  22.     return std::make_unique<ABC>();
  23.   }
  24. };
  25.  
  26. class ExtendedXYZFactoryImpl : public XYZFactory {
  27. public:
  28.   std::unique_ptr<ABC> createABC() const override {
  29.     return std::make_unique<DerivedABC>();
  30.   }
  31. };
  32.  
  33. namespace details {  // Or hide this in an anonymous namespace in a .cpp file.
  34.   std::unique_ptr<XYZFactory>& getXYZFactoryInstanceMutable() {
  35.     static std::unique_ptr<XYZFactory> singleton = std::make_unique<XYZFactoryImpl>();
  36.     return singleton;
  37.   }
  38. }
  39.  
  40. const XYZFactory& getXYZFactoryInstance() {
  41.   auto& singleton = details::getXYZFactoryInstanceMutable();
  42.   if (!singleton)
  43.       throw std::runtime_error("No XYZFactory registered");
  44.   return *singleton;
  45. }
  46.  
  47. void setXYZFactoryInstance(std::unique_ptr<XYZFactory> new_factory) {
  48.   details::getXYZFactoryInstanceMutable() = std::move(new_factory);
  49. }
  50.  
  51. int main() {
  52.     auto abc = getXYZFactoryInstance().createABC();
  53.     std::cout << abc->getValue() << "\n";
  54.  
  55.     setXYZFactoryInstance(std::make_unique<ExtendedXYZFactoryImpl>());
  56.  
  57.     auto extended_abc = getXYZFactoryInstance().createABC();
  58.     std::cout << extended_abc->getValue() << "\n";
  59. }
  60.  
Success #stdin #stdout 0s 3464KB
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stdin
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Standard input is empty
stdout
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42
-1
https://ideone.com/TqcGOg
language:
C++14 (gcc 8.3)
created:
7 years ago
visibility:
public

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