Unique Pointers and Copy Constructors

Why does the compiler generate an error when I try to define a copy constructor for a class containing a unique_ptr member?

When a class contains a unique_ptr member, the compiler-generated copy constructor is deleted because unique_ptr is not copyable. The purpose of unique_ptr is to ensure exclusive ownership of the pointed-to resource, so it cannot be copied.

To define a copy constructor for a class with a unique_ptr member, you need to manually implement the copying behavior. This typically involves creating a new unique_ptr in the destination object and copying the pointed-to resource.

Here's an example:

#include <memory>

class MyClass {
public:
  MyClass(const MyClass& other)
    : ptr{std::make_unique<int>(*other.ptr)} {}  

private:
  std::unique_ptr<int> ptr;
};

In the copy constructor, we create a new unique_ptr and initialize it with a copy of the value pointed to by other.ptr. This ensures that the destination object has its own independent copy of the resource.

By manually implementing the copy constructor this way, you can define the appropriate copying behavior for your class while still maintaining the exclusive ownership semantics of unique_ptr.

Copy Semantics and Return Value Optimization

Learn how to control exactly how our objects get copied, and take advantage of copy elision and return value optimization (RVO)

Questions & Answers

Answers are generated by AI models and may not have been reviewed. Be mindful when running any code on your device.

The Rule of Five
I have heard about the "Rule of Five" in C++. What is it, and how does it relate to the Rule of Three mentioned in the lesson?
Copy Elision and Side Effects
What potential issues can arise if I include side effects (e.g., modifying global variables or performing I/O operations) in my copy constructor or destructor?
Copy-and-Swap Idiom
What is the copy-and-swap idiom, and how can it be used to implement the copy assignment operator?
Move Semantics
The lesson mentions that move semantics can be used to make classes more efficient. Can you briefly explain what move semantics are and how they differ from copy semantics?
Compiler Support for Copy Elision
Is copy elision guaranteed to happen in all cases where it is possible, or does it depend on the compiler and optimization settings?
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