Re: Template conversion and default copy constructor
Gene Bushuyev wrote:
"James Kanze" <kanze.james@neuf.fr> wrote in message
news:e9t55s$58s$1@nntp.aioe.org...
Jiang wrote:
All the compilers I tested produced from the following
code executables that printed "2". Shouldn't the
implicitly defined copy constructor be called?
Yes, template constructor is constructor, but it will never
be used as copy constructor, since we need consistent copy
semantics in every TUs.
Correct, but the key to understanding what is occuring here
is to realize that there is no "copying", in the sense of
the standard. So normal function overloading takes place,
and the instantiation of the template is a better match.
(But I'll admit that I had to reread the relevant passages
in the standard several times myself before I came to this
conclusion. Overload resolution is all to often a guessing
game, in which you try to guess what the compiler will do
this time.)
I must admit, what appeared a very simple question at first,
turned out to be more difficult especially in the light of
unfortunate wording of the standard.
Not for the first time.
So my original reply that was written two days ago is wrong.
Interesting that compilers also disagree, and those who write
them are supposed to be the language experts. It looks like
gcc and vc8 got it right after all, and Borland is wrong this
time.
It's also possible that the compiler authors' didn't actually
consider the issues in detail, and that the behavior more or
less fell out from other internal details.
That also indicates that it's a good practice to make all
constructors consistent without "special effects."
As a general rule, I would say that you shouldn't overload
functions if it will make a difference which one would be
called. Trying to know exactly which one overload resolution
will pick is often more of a guessing game than anything else.
With the help of very careful reading of the standard, you can
usually figure out which one should be called, but who wants to
do that kind of analysis every time you see a function call.
Constructors are a special case, because you can't give two
constructors different names just because they do different
things. In at least one case, I've added an extra parameter to
the constructor, just to allow explicit differentiation.
--
James Kanze GABI Software
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