auto_ptr assignment compile error

Hello everyone,


I posted the code and related compile error below. My analysis below,

1. I think the reason of compile error is, the copy constructor of Goo accepts const reference as input parameter, so the pf member of input parameter is also treated const?
2. And since auto_ptr does not have a copy constructor which accepts const reference auto_ptr as input parameter, the compile error occurs.

My analysis (1) and (2) are both correct?

What makes me confused is it seems the compile error message does not reflect my analysis (1) and (2) above. Any ideas?

Code:

---

#include <memory>

using namespace std;

// Foo is Pimpl class for Goo, suppose.
class Foo {

};

class Goo {

public:

        auto_ptr<Foo> pf;

        Goo (const Goo& input)
        {
                this -> pf = input.pf;
        }
};

---

1>d:\visual studio 2008\projects\test0330\test0330\main.cpp(18) : error C2679: binary '=' : no operator found which takes a right-hand operand of type 'const std::auto_ptr<_Ty>' (or there is no acceptable conversion)
1> with
1> [
1> _Ty=Foo
1> ]
1> d:\program files\microsoft visual studio 9.0\vc\include\memory(689): could be 'std::auto_ptr<_Ty> &std::auto_ptr<_Ty>::operator =<Foo>(std::auto_ptr<_Ty> &) throw()'
1> with
1> [
1> _Ty=Foo
1> ]
1> d:\program files\microsoft visual studio 9.0\vc\include\memory(701): or 'std::auto_ptr<_Ty> &std::auto_ptr<_Ty>::operator =(std::auto_ptr<_Ty> &) throw()'
1> with
1> [
1> _Ty=Foo
1> ]
1> d:\program files\microsoft visual studio 9.0\vc\include\memory(707): or 'std::auto_ptr<_Ty> &std::auto_ptr<_Ty>::operator =(std::auto_ptr_ref<_Ty>) throw()'
1> with
1> [
1> _Ty=Foo
1> ]
1> while trying to match the argument list '(std::auto_ptr<_Ty>, const std::auto_ptr<_Ty>)'
1> with
1> [
1> _Ty=Foo
1> ]


thanks in advance,
George

It looks to me that the compiler tries to tell you that std::auto_ptr doesn't have an assignement operator that takes a const reference to an auto_ptr as rhs ( and indeed there isn't any ).
Kurt

Your analysis is wrong in one way. You say copy constructor but what you should be saying is copy assignment operator. Switch that and that is exactly what the compiler is saying.

No operator= was found that takes a const auto_ptr (and there are no acceptable conversions).

If you initialized the auto_ptr in the initializer list (which you normally should) then you would get a similar error about the copy constructor.

auto_ptr's operator=() function has to set the rhs parameter's contained pointer to NULL, so it obviously can't take a const auto_ptr parameter because then it wouldn't be allowed to change anything.

So the root cause is from statement,

this -> pf = input.pf;

where since input is const, pf is const, but assignment operator of auto_ptr only accepts non-const?


regards,
George

this->pf is not const, but it has no assignment operator that accepts a const reference, which input.pf is (due to that it's being received as a const reference). I'm not sure if it's this that you were hinting at.

>> So the root cause is from statement
Yes, the cause is from that statement, but you would get a similar error if you initialized the auto_ptr in the initializer list.

I would say the root cause is the attempt to transfer ownership of the auto_ptr in the copy constructor.

Thanks Elysia and Daved,


Question answered.

Quote:

---

Originally Posted by Daved

>> So the root cause is from statement
Yes, the cause is from that statement, but you would get a similar error if you initialized the auto_ptr in the initializer list.

I would say the root cause is the attempt to transfer ownership of the auto_ptr in the copy constructor.

---

regards,
George