Copy constructor for a class with unique_ptr

by

Since the unique_ptr can not be shared, you need to either deep-copy its content or convert the unique_ptr to a shared_ptr.

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

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

int main()
{
   A a( 42 );
   A b = a;
}

You can, as NPE mentioned, use a move-ctor instead of a copy-ctor but that would result in different semantics of your class. A move-ctor would need to make the member as moveable explicitly via std::move:

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

Having a complete set of the necessary operators also leads to

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

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

If you want to use your class in a std::vector, you basically have to decide if the vector shall be the unique owner of an object, in which case it would be sufficient to make the class moveable, but not copyable. If you leave out the copy-ctor and copy-assignment, the compiler will guide your way on how to use a std::vector with move-only types.

More Related Contents:

  1. Is std::unique_ptr required to know the full definition of T?
  2. Why can I not push_back a unique_ptr into a vector?
  3. make_unique and perfect forwarding
  4. Returning unique_ptr from functions
  5. How do I pass a unique_ptr argument to a constructor or a function?
  6. Differences between std::make_unique and std::unique_ptr with new
  7. How do I use a custom deleter with a std::unique_ptr member?
  8. Differences between unique_ptr and shared_ptr [duplicate]
  9. Is there any use for unique_ptr with array?
  10. How to capture a unique_ptr into a lambda expression?
  11. “Downcasting” unique_ptr to unique_ptr
  12. How to implement make_unique function in C++11? [duplicate]
  13. How do I use unique_ptr for pimpl?
  14. So can unique_ptr be used safely in stl collections?
  15. Exception safety and make_unique
  16. std::unique_ptr, deleters and the Win32 API
  17. How does the custom deleter of std::unique_ptr work?
  18. How can I pass std::unique_ptr into a function
  19. Should I use shared_ptr or unique_ptr
  20. error::make_unique is not a member of ‘std’
  21. shared_ptr is to weak_ptr as unique_ptr is to… what?
  22. Raw pointer lookup for sets of unique_ptrs
  23. unique_ptr and OpenSSL’s STACK_OF(X509)*
  24. How can I compile C++11 code with Orwell Dev-C++?
  25. Why does unique_ptr take two template parameters when shared_ptr only takes one?
  26. Should std::unique_ptr be permitted
  27. stl container with std::unique_ptr’s vs boost::ptr_container
  28. Is there a way to write make_unique() in VS2012?
  29. Does std::bind work with move-only types in general, and std::unique_ptr in particular?
  30. Does C++11 unique_ptr and shared_ptr able to convert to each other’s type?

Leave a Comment