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Copying a Unique Pointer in a Class
Copying a class that contains a unique pointer as a member requires special consideration. A unique pointer cannot be shared, which necessitates either a deep copy of its contents or a conversion to a shared pointer.
Deep Copying
The deep copy approach creates a new unique pointer and initializes it with a copy of the original pointer's contents:
class A { std::unique_ptr<int> up_; public: A(int i) : up_(new int(i)) {} A(const A& a) : up_(new int(*a.up_)) {} };
Here, A's copy constructor deep copies the int pointed to by the original unique pointer.
Conversion to Shared Pointer
Alternatively, you can convert the unique pointer to a shared pointer, which allows it to be shared among multiple objects:
class B { std::unique_ptr<int> up_; public: B(int i) : up_(new int(i)) {} B(const B& b) { up_ = std::make_shared(*b.up_); } };
In this example, B's copy constructor converts the unique pointer to a shared pointer, enabling multiple copies of B to access the same underlying data.
Other Operators
To work seamlessly with containers, like std::vector, additional operators are typically required:
Move Constructor:
B(B&& b) { up_ = std::move(b.up_); }
Copy Assignment:
B& operator=(const B& b) { up_ = std::make_shared(*b.up_); return *this; }
Move Assignment:
B& operator=(B&& b) { up_ = std::move(b.up_); return *this; }
These operators ensure that objects can be safely assigned and copied in various contexts.
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