How to Implement Copy Constructors for Classes with `unique_ptr` Members in C 11?

Implementing Copy Constructors for Classes with unique_ptr Members

When working with classes that utilize a unique_ptr member variable, implementing a copy constructor becomes crucial. In this article, we will delve into the techniques for creating copy constructors in C 11 for such classes.

As unique_ptr cannot be shared, it's imperative to either deeply copy its contents or convert it to a shared_ptr. The following example demonstrates deep copying:

class A
{
   std::unique_ptr<int> up_;

public:
   A(int i) : up_(new int(i)) {}
   A(const A&amp; a) : up_(new int(*a.up_)) {}
};

In this approach, a new unique_ptr is allocated and assigned a copy of the original pointer's content.

Alternatively, one can utilize a move constructor instead of a copy constructor:

A(A&& a) : up_(std::move(a.up_)) {}

However, this method introduces different semantics, as it transfers ownership of the unique_ptr from the source to the target object.

For completeness, the full set of assignment operators would be:

A& operator=(const A&a)
{
   up_.reset(new int(*a.up_));
   return *this;
}

A& operator=(A&& a)
{
   up_ = std::move(a.up_);
   return *this;
}

Deciding whether to use copy or move constructors depends on the context. If the class is intended to be used in a std::vector, the choice between making the class move-only or copyable is influenced by whether the vector should be the sole owner of the objects.

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