Month: March 2015

Object Relationships using smart pointers and “Modern C++”

  1. Don’t use raw pointers as class members; that requires managing object life time and manually avoiding dangling pointers
  2. Use smart pointers instead of raw pointers
  3. Don’t use object references as class members as that makes objects of the class non-assignable (which means you cannot store said objects in standard C++ containers)
  4. Using raw pointers and object references to objects stored in standard C++ containers is problematic or impossible. Objects in containers can be relocated (addresses can change) rendering raw pointers and object references invalid.
  5. Best (only feasible?) option is to store std::shared_ptr of the dynamically allocated object in the container.
  6. std::shared_ptr is reference counted std::weak_ptr is not
  7. Use std::make_shared to create an std::shared_ptr to a dynamically created object instance or use std::shared_ptr(new T())
  8. Only way to access members via std::weak_ptr is by calling std::weak_ptr::lock which returns an std::shared_ptr or 0
  9. To create an std::weak_ptr from the this pointer, the class must first derive from std::enable_shared_from_this and then call the shared_from_this function to get an std::shared_ptr pointing to the object. The object in question must be “owned” by an std::shared_ptr or else undefined behaviour results. Also, shared_from_this must not be called from a constructor since the object is yet to be reference counted (i.e. object is not yet “owned” by an std::shared_ptr)
  10. As a result of the previous point, it helps to ensure that objects can only be created via std::shared_ptr by using a public static create function (which returns objects “owned” by an std::shared_ptr) and making constructors private. Note that when constructors are made private, you can no longer use std::make_shared to create a shared pointer. You must use std::shared_ptr(new T()) in this case.

Modern C++ Object Relationships