Re: being copy constructible

"James Kanze" <>
20 Nov 2006 14:47:59 -0500
terry wrote:

Perhaps I can be allowed to ask it again. Do those with
experience think the code below is a breach of the standards.
Does the non-const nature of the copy constructor stop it
being copy constructible in the sense of the standard?

#include <vector>
class S
   S(S& a){} //non-const copy constructor
   bool operator==(const S & rhs){return true;};

int main(int argc, char* argv[])
std::vector<S> u(1);
return 0;

It depends. The non-const-ness doesn't stop the constructor
from being a copy constructor, from the point of view of the
compiler; the compiler will not generate a default copy
constructor for this class, because it considers that the class
already has a copy constructor.

In order to meet the CopyConstructible constraint in the
library, it must be possible to copy const objects, however, and
this constructor doesn't allow that. There is also a
requirement that the result of the copy be "equivalent" to the
original. There is, however, intentionally no definition of
"equivalent", since in many cases, this would depend on the
application---there is no requirement, for example, that the ==
operator be defined, or even, if it is defined, that it
implements the same definition of equivalence; although one
would hope that this is generally the case, it's easy to imagine
cases where the equivalence requirements on copying were
stronger than those for ==. (The case actually arrises
frequently with input iterators, at least in my code.)

James Kanze (GABI Software)
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