trainings/AdvancedCppV2/Presentation/smart_pointers_implementation_details.md

Implementation details

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Implementation details – std::unique_ptr<>

• Just a holding wrapper
• Holds an object pointer
• Constructor copies a pointer
• Call proper delete in destructor
• No copying
• Moving means:
  • Copying original pointer to a new object
  • Setting source pointer to nullptr

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Implementation details – std::shared_ptr<>

• Holds an object pointer
• Holds 2 reference counters:
  • shared pointers count
  • weak pointers count
• Destructor:
  • decrements shared-refs
  • deletes user data when shared-refs == 0
  • deletes reference counters when shared-refs == 0 and weak-refs == 0
• Extra space for a deleter

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Implementation details – std::shared_ptr<>

• Copying means:
  • Copying pointers to the target
  • Incrementing shared-refs

• Moving means:
  • Copying pointers to the target
  • Setting source pointers to nullptr

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Implementation details – std::weak_ptr<>

• Holds an object pointer
• Holds 2 reference counters:
  • shared pointers count
  • weak pointers count
• Destructor:
  • decrements weak-refs
  • deletes reference counters when shared-refs == 0 and weak-refs == 0

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Implementation details – std::weak_ptr<>

• Copying means:
  • Copying pointers to the target
  • Incrementing weak-refs

• Moving means:
  • Copying pointers to the target
  • Setting source pointers to nullptr

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std::weak_ptr<> + std::shared_ptr<>

• Having a shared pointer and a weak pointer

• After removing the shared pointer

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Making a std::shared_ptr<>

• std::shared_ptr<Data> p{new Data};
  • Perform two allocations: one for control block and second for data
  • Before C++17 can make a problem with ordering of operations
  • When all shared_ptr will be deleted but there is some weak_ptr allocated memory for data can be freed.

• auto p = std::make_shared<Data>();
  • Less memory (most likely)
  • Only one allocation
  • Cache-friendly
  • When all shared_ptr will be deleted but there is some weak_ptr allocated memory for data cannot be freed