Copy constructor with non const rhs arg

Copy constructor with non const rhs arg

[paul moore]

This code does not compile on gcc (3.2.2 on rh9 - 3.3 on suse 9)
 
class test
{
public:
int x;
test(test &rhs)
{
	x = rhx.x;
}
};

test foo();
int main(int argc, char* argv[])
{
	test dd = foo();
	return 0;
}

The test dd line gets hit saying there is no match for test::test(test), the
candidates are test::test(test&)
If I change the copy constructor to 'test(const test&)' it compiles fine.

What am I trying to do? In real life the class contains an auto_ptr - since
the auto_ptr copy constructor updates the rhs you cannot pass it in with
const. (Note the auto_ptr copy constructor itself is declared with a non
const rhs - and code using it compiles fine - this is really puzzling)

Also if I change foo to 'test &foo()' it compiles fine (but of course this
is totally different semantics)
Note also that MSFT vs 2003 compiles this code quite happily.

Re: Copy constructor with non const rhs arg

[Eljay Love-Jensen]

Hi Paul,

I believe a copy constructor requires a const parameter.

But I may be mistaken.  I've been mistaken before.

You can make the auto_ptr member variable mutable.

--Eljay

Re: Copy constructor with non const rhs arg

[Eljay Love-Jensen]

Hi Paul,

How about pass-by-value instead of pass-by-reference?

After all, an auto_ptr as a parameter should always be 
pass-by-value.  That's the semantic of pass-by-value for auto_ptr:  a 
transfer of ownership.

--Eljay

Re: Copy constructor with non const rhs arg

[Eljay Love-Jensen]

hi Paul,

Hmm, I guess pass-by-value a Copy Constructor doth not make.

Solution #1
--------------------------------
#include <memory>

using namespace std;

class Foo
{
public:
     Foo(Foo const& in);
private:
     auto_ptr<int> m;
};

Foo::Foo(Foo const& in)
: m(const_cast<Foo&>(in).m)
{
}

Foo FooFactory();
int main()
{
     Foo f(FooFactory());
     Foo f2 = FooFactory();
}
--------------------------------

Solution #2
--------------------------------
#include <memory>

using namespace std;

class Foo
{
public:
     Foo(Foo const& in);
private:
     mutable auto_ptr<int> m;
};

Foo::Foo(Foo const& in)
: m(in.m)
{
}

Foo FooFactory();
int main()
{
     Foo f(FooFactory());
     Foo f2 = FooFactory();
}
--------------------------------

HTH.
--Eljay

RE: Copy constructor with non const rhs arg

[Tony Wetmore]

(I sent this earlier to Paul but forgot to CC the list...)

I believe the problem here is that "test dd = foo()" is using a
temporary object (returned by foo).  You cannot pass a non-const
reference to that temporary object into test's copy constructor.
Temporary objects are always "const".

You can, of course, pass a const reference to a temporary object.

Similarly, if foo() returns a reference, that reference must point to a
valid (not temporary) object, so passing a reference to THAT object into
test's copy constructor is just fine.

So I think gcc's behavior is correct (though the error text is a bit
unclear).  I don't know what Microsoft's compiler is doing in that case,
but it seems unusual and could probably lead to bad behavior.

---
Tony Wetmore
Raytheon Solipsys
mailto:tony.wetmore@solipsys.com
http://www.solipsys.com
 
 


-----Original Message-----
From: gcc-help-owner@gcc.gnu.org [mailto:gcc-help-owner@gcc.gnu.org] On
Behalf Of Eljay Love-Jensen
Sent: Wednesday, May 12, 2004 12:05 PM
To: paul moore; gcc-help@gcc.gnu.org
Subject: Re: Copy constructor with non const rhs arg


Hi Paul,

I believe a copy constructor requires a const parameter.

But I may be mistaken.  I've been mistaken before.

You can make the auto_ptr member variable mutable.

--Eljay

Re: Copy constructor with non const rhs arg

[llewelly]

"paul moore" <paulmoore100@hotmail.com> writes:

> This code does not compile on gcc (3.2.2 on rh9 - 3.3 on suse 9)
>  
> class test
> {
> public:
> int x;
> test(test &rhs)
> {
> 	x = rhx.x;
> }
> };
> 
> test foo();
> int main(int argc, char* argv[])
> {
> 	test dd = foo();

C++ does not allow binding a temporary to a reference to non-const.

> 	return 0;
> }
> 
> The test dd line gets hit saying there is no match for test::test(test), the
> candidates are test::test(test&)
> If I change the copy constructor to 'test(const test&)' it compiles fine.
> 
> What am I trying to do? In real life the class contains an auto_ptr - since
> the auto_ptr copy constructor updates the rhs you cannot pass it in with
> const. (Note the auto_ptr copy constructor itself is declared with a non
> const rhs - and code using it compiles fine - this is really
> puzzling)

auto_ptr uses a truly hideous trick involving a sort of proxy class
    called auto_ptr_ref. You are probably better off not tryiing to
    emulate it. If you *need* to emulate it, you'll have to look at
    the sources; I can't explain it.

> 
> Also if I change foo to 'test &foo()' it compiles fine (but of course this
> is totally different semantics)
> Note also that MSFT vs 2003 compiles this code quite happily.

M$ supports binding a temporary to a reference to non-const as an extension.

RE: Copy constructor with non const rhs arg

[paul moore]

Thanks to everybody for their input. 
Using mutable worked a treat (first time I have used it in anger - you live
and learn) 

-----Original Message-----
From: llewelly@xmission.xmission.com [mailto:llewelly@xmission.xmission.com]
On Behalf Of llewelly@xmission.com
Sent: Wednesday, May 12, 2004 11:09 AM
To: paul moore
Cc: gcc-help@gcc.gnu.org
Subject: Re: Copy constructor with non const rhs arg

"paul moore" <paulmoore100@hotmail.com> writes:

> This code does not compile on gcc (3.2.2 on rh9 - 3.3 on suse 9)
>  
> class test
> {
> public:
> int x;
> test(test &rhs)
> {
> 	x = rhx.x;
> }
> };
> 
> test foo();
> int main(int argc, char* argv[])
> {
> 	test dd = foo();

C++ does not allow binding a temporary to a reference to non-const.

> 	return 0;
> }
> 
> The test dd line gets hit saying there is no match for 
> test::test(test), the candidates are test::test(test&) If I change the 
> copy constructor to 'test(const test&)' it compiles fine.
> 
> What am I trying to do? In real life the class contains an auto_ptr - 
> since the auto_ptr copy constructor updates the rhs you cannot pass it 
> in with const. (Note the auto_ptr copy constructor itself is declared 
> with a non const rhs - and code using it compiles fine - this is 
> really
> puzzling)

auto_ptr uses a truly hideous trick involving a sort of proxy class
    called auto_ptr_ref. You are probably better off not tryiing to
    emulate it. If you *need* to emulate it, you'll have to look at
    the sources; I can't explain it.

> 
> Also if I change foo to 'test &foo()' it compiles fine (but of course 
> this is totally different semantics) Note also that MSFT vs 2003 
> compiles this code quite happily.

M$ supports binding a temporary to a reference to non-const as an extension.