Re: Template conversion and default copy constructor
Jiang wrote:
The signature of the compiler generated copy constructor
here will be Test( Test const& ). But you're right that
?12.8/5 could express it clearer. But I don't think that
there's any question about it. The signature will be Test(
Test const& ) unless at least one base class or member of
class type has a copy constructor whose signature does not
take the const.
Basically, if you consider the two bullets in ?12.8/5, there
are no direct or virtual base classes, so all of the direct
or virtual base classes have the required copy constructor,
and there are no members of class type, so all of the
members of class type have the required copy constructor.
As other rules in standard, the wording is like this:
The implicitly-declared copy constructor ... will be
X::X(const X&)
if
condition 1 here, and
condition 2 here
Otherwise, it will have form
X::X(X &)
Here the word "otherwise" is indeed problematic,
Not really. It's the condition that probably needs clearer
wording.
since the class Test does not have any base classes and
members, then the two specified conditions can not be
satisfied.
I think that's the subtility you're missing. The conditions a
"each [...]" and "for all [...]". Both mean that if there is 1,
then 1 meets the condition, if there are 2, then 2 meet the
condition, and so on. And equally, if there are 0, then 0 meet
the condition. If a class has no non-static data members of
class type, then all zero of its non-static data members of
class type meet the condition.
Thus, the default copy constructor should be the non-const
form. That was what I read it.
I'm 100% sure that this isn't the intent. But I'll admit that
that is mainly because I've known C++ since well before the
standard was adopted, and it has always been this way.
I think that formulating the rule with an unless would be a lot
clearer:
The implicitly-declared copy constructor [...] will be
X::X(const X&)
unless one or more direct or virtual bases B or one or more
members of class type B has a copy constructor whose first
parameter is neither B& or const B&. Otherwise, it will be
X::X(X&)
[...
Not at all. Try this simple test:
class Test { } ;
int
main()
{
Test const t1 ;
Test t2( t1 ) ;
}
The only possible constructor the compiler could use here is
the compiler generated copy constructor. And unless its
signature is: Test::Test( Test const& ), the code won't
compile. (And I'd be very surprised if you find a compiler
where it doesn't compile.)
cl and icl accepted above code, but it wont compile using
como and gcc.
$ como --A test.cpp
Comeau C/C++ 4.3.3 (Jan 13 2004 11:29:09) for MS_WINDOWS_x86
Copyright 1988-2003 Comeau Computing. All rights reserved.
MODE:strict errors C++
"test.cpp", line 9: error: const variable "t1" requires an initializer
--
class "Test" has no explicitly declared default constructor
Test const t1 ;
^
"test.cpp", line 10: warning: variable "t2" was declared but never
referenced
Test t2( t1 ) ;
^
1 error detected in the compilation of "test.cpp".
$ g++ test.cpp
test.cpp: In function `int main()':
test.cpp:9: error: uninitialized const `t1'
The diagnostic is reasonable according to 8.5/p9.
And required. Just goes to show that I should have tested my
code first.
But this diagnostic maybe is irrelative to our discussion,
since if I add the default constructor, all compilers compile
above example without any complain.
Exactly. Change the first line in main to:
Test const t1 = Test() ;
and it compiles fine. More generally, since the problem is only
whether it compiles:
void
f( Test const& t1 )
{
Test t2( t1 ) ;
}
That the signature of the compiler generated copy
constructor is Test( Test const&) here, and that as in the
initial case, overload resolution choses the template.
As I said just now, in the sense of standard, it is
unspecified behavior in my mind.
But I must confess the const form is the better choice
compared with the non-const form.
I'm 100% sure that the const form is the intent. I also think
that after a careful analysis, it is the only way to interpret
the standard. I do think it could have been more clearly
expressed.
--
James Kanze GABI Software
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