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Re: A question on "uncopyable" that disable copy constructor

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TH <python152@gmail.com> writes:
> Meyer's Effective C++ book talks about the case where you may want to
> disallow compiler generated copy constructor. The example is:
>
> class HomeForSale {...}
>
> It doesn't make sense to do:
>
> HomeForSale h1(h2); or h1 = h2.
>
> No problem. However, Does disallowing the copy constructor effectively
> prohibit it from putting such object into a container type, as
>
> std::vector<HomeForSale> home_vec;
> home_vec.push_back(h1); // need copy constructor??
>
> It sounds like a severe constraints on the possible use of such object,
> am I missing anything?

A typical example of a class where you don't want an automatically
generated copy-constructor is one that has pointer member which is
deallocated by the destructor; simply copying the pointer member into
a new object would cause obvious problems (as soon as the first of the
two object is destroyed, the pointer would be dangling).

The key point is that _any_ use of a default copy-constructor for such
classes -- _including_ copying by std::vector or whatever -- will
violate the invariants the class relies on, and cause problems.  So
suppressing it for all uses is a feature.  :)

Of course, often one then provides a user copy-constructor instead,
which does the right thing for copying (e.g. making a copy of the
pointed-to object in the above example).

However in some cases that's hard or maybe just annoying to implement
(maybe you'd need to add reference counting for shared resources, etc,
and for the class in question, such complexity would be unjustified by
the expect use).  Then you just disable the auto-generated constructor
and write in the documentation "don't copy this."

Yeah that means you can't put it in std::vector, etc...

However as the decision to make the object uncopyable is usually done
with an eye towards expected usage, presumably such a class is one
where "putting it in a vector" is not expected.  If you _really_ need
a vector of such objects, you can use a vector of pointers or
something instead (obviously you need to be careful and manage the
lifetimes of the pointed-to objects, but oh well).

-miles

-- 
Scriptures, n. The sacred books of our holy religion, as distinguished from
the false and profane writings on which all other faiths are based.

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A question on "uncopyable" that disable copy constructor TH <python152@gmail.com> - 2012-02-22 20:45 -0800
  Re: A question on "uncopyable" that disable copy constructor Ian Collins <ian-news@hotmail.com> - 2012-02-23 18:33 +1300
    Re: A question on "uncopyable" that disable copy constructor Jorgen Grahn <grahn+nntp@snipabacken.se> - 2012-02-23 08:47 +0000
      Re: A question on "uncopyable" that disable copy constructor Ian Collins <ian-news@hotmail.com> - 2012-02-26 08:16 +1300
        Re: A question on "uncopyable" that disable copy constructor Jorgen Grahn <grahn+nntp@snipabacken.se> - 2012-02-25 19:49 +0000
          Re: A question on "uncopyable" that disable copy constructor yatremblay@bel1lin202.(none) (Yannick Tremblay) - 2012-02-27 14:32 +0000
            Re: A question on "uncopyable" that disable copy constructor Jorgen Grahn <grahn+nntp@snipabacken.se> - 2012-02-27 15:54 +0000
              Re: A question on "uncopyable" that disable copy constructor yatremblay@bel1lin202.(none) (Yannick Tremblay) - 2012-02-27 16:15 +0000
                Re: A question on "uncopyable" that disable copy constructor Jorgen Grahn <grahn+nntp@snipabacken.se> - 2012-02-27 20:58 +0000
                Re: A question on "uncopyable" that disable copy constructor Ian Collins <ian-news@hotmail.com> - 2012-02-28 17:18 +1300
                Re: A question on "uncopyable" that disable copy constructor Pavel <pauldontspamtolk@removeyourself.dontspam.yahoo> - 2012-02-28 00:00 -0500
                Re: A question on "uncopyable" that disable copy constructor Ian Collins <ian-news@hotmail.com> - 2012-02-28 20:09 +1300
                Re: A question on "uncopyable" that disable copy constructor Jorgen Grahn <grahn+nntp@snipabacken.se> - 2012-02-28 21:19 +0000
                Re: A question on "uncopyable" that disable copy constructor Pavel <pauldontspamtolk@removeyourself.dontspam.yahoo> - 2012-03-04 01:05 -0500
                Re: A question on "uncopyable" that disable copy constructor 88888 Dihedral <dihedral88888@googlemail.com> - 2012-03-04 18:01 -0800
                Re: A question on "uncopyable" that disable copy constructor Jorgen Grahn <grahn+nntp@snipabacken.se> - 2012-02-28 22:02 +0000
                Re: A question on "uncopyable" that disable copy constructor Ian Collins <ian-news@hotmail.com> - 2012-02-29 11:15 +1300
  Re: A question on "uncopyable" that disable copy constructor Miles Bader <miles@gnu.org> - 2012-02-23 15:14 +0900
    Re: A question on "uncopyable" that disable copy constructor "T.H." <python152@gmail.com> - 2012-02-23 01:51 -0800
  Re: A question on "uncopyable" that disable copy constructor bartek szurgot <basz@no.spam> - 2012-02-24 17:18 +0100
    Re: A question on "uncopyable" that disable copy constructor Juha Nieminen <nospam@thanks.invalid> - 2012-02-25 19:10 +0000
      Re: A question on "uncopyable" that disable copy constructor Pavel <pauldontspamtolk@removeyourself.dontspam.yahoo> - 2012-02-25 17:45 -0500
        Re: A question on "uncopyable" that disable copy constructor Juha Nieminen <nospam@thanks.invalid> - 2012-02-26 09:38 +0000
          Re: A question on "uncopyable" that disable copy constructor Juha Nieminen <nospam@thanks.invalid> - 2012-02-26 09:42 +0000
          Re: A question on "uncopyable" that disable copy constructor Pavel <pauldontspamtolk@removeyourself.dontspam.yahoo> - 2012-02-27 23:23 -0500
      Re: A question on "uncopyable" that disable copy constructor 88888 Dihedral <dihedral88888@googlemail.com> - 2012-02-28 17:01 -0800

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