C++ auto_ptr and copy construction

Question

If I have a class

template <typename T>
struct C {
...

private:
  auto_ptr<T> ptr;
};

How do I define the copy constructor for C:

It cannot be

template <typename T>
C<T>::C(const C& other)

because I want to if I copy the auto_ptr from other, I have changed other by removing ownership. Is it legal to define a copy constructor as

template <typename T>
C<T>::C(C& other) {}

Zoli · Answer 1 · 2010-06-21T20:15:10.920

1

If you want to prevent transfer of ownership and/or copying, you can define the copy constructor and assignment operator private, thereby forbidding users of your class from ever copying or assigning your object.

edited Jun 21 '10 at 20:15

answered Jun 21 '10 at 20:07

Zoli

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score 1 · Answer 2 · answered Jun 21 '10 at 20:09

1

Do you actually want to copy your class's state or transfer it? If you want to copy it then you do it just like any other class with pointers in it:

template < typename T >
C<T>::C(C const& other) : ptr(new T(*other.ptr)) {} // or maybe other.ptr->clone()

If you actually want to transfer ownership of the pointer you could do it with a non-const "copy" constructor but I'd recommend you do something that's more obvious at the call site; something that tells people reading the code that ownership has transfered.

answered Jun 21 '10 at 20:09

Edward Strange

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If he wants to transfer he doesn't need to do anything at all. The compiler provides the correct copy constructor automatically in that case - although i would recommend against that too. – Johannes Schaub - litb Jun 21 '10 at 20:23
Interesting. I was sure you were wrong but it appears that the compiler will then create a non-const constructor for C automatically. I'll have to look into the rules that make that happen. At any rate, I still stand by my advice to use something more obvious. – Edward Strange Jun 21 '10 at 20:28

score 0 · Answer 3 · answered Jun 21 '10 at 20:19

There's no such thing as a "standard" copy constructor, but people do expect them to behave a certain way. If your object is going to do any transferring of ownership on copy, then it's a bad idea to make a copy constructor that obscures this fact. A better approach would be to create a method that makes this clear. (You could call it CloneAndTransfer or something similar.)

C++ auto_ptr and copy construction

3 Answers3