Re: copy constructor for class with member pointer

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <>
Sun, 11 Jul 2010 13:53:16 CST
On 11 Jul., 16:28, Sergey Lukoshkin <> wrote:

I faced such a problem. Say I got the class:

class foo


 T* m_ptr;


I need to implement copy constructor. So, the common signature for it
is foo( const& other ). I have to take the pointer from another
object, assign it to this->m_ptr and then make other.m_ptr = 0. But
such a solution discards constness of the other object and signature
of copy constructor should be foo( foo& other ).

Is the approach a correct way to implement the copy constructor for
class with member pointer or not? Loosing constness of argument makes
me hesitating. Thanks.

It depends: In general a copy-constructor with reference
arguments to const should not mutate it's source argument
in an *observable* way. You didn't say whether your
member of type pointer is an implementation detail or

If it is an implementation detail, your type should work
in contexts where the compiler omits to call the copy-
constructor (because the language explicitly provides
freedom to implementations to do so, even, if such an
omission would have observable effects (e.g. if the copy-
constructor outputs to std::cout).

If it is not an implementation detail, you should
declare the copy-constructor as one that takes a
reference to non-const - this is basically the route,
std::auto_ptr decided for. Note that such a decision
will have the effect that you cannot copy from rvalues.

If the latter use-case is important to you and if you
have a C++0x-cappable compiler with support for rvalue,
references, the best decision would probably be to
design your type foo as follows:

class foo {
 foo(foo&& rhs) : m_ptr(rhs.m_ptr) { rhs.m_ptr = 0; }
 foo& operator=(foo&& rhs) { m_ptr = rhs.m_ptr; rhs.m_ptr = 0; return
*this; }
 T* m_ptr;

[With the provision of user-declared move operations
as below, the compiler won't provide the default
versions of the copy operations. If you prefer that
to be more explicit, you could also declare the copy
operations as deleted member functions]

HTH & Greetings from Bremen,

Daniel Kr?gler

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