vector<MyTemplate<MyClass>> not possible?

vector<MyTemplate<MyClass>> not possible?

[Evert, F.K. van]

When I put instances of a template-derived class in a STL vector, g++ gives
me the following error message (Cygwin/g++3.2 as well as Suse8.0/g++2.95.3):

/usr/include/c++/3.2/bits/stl_construct.h:78: passing `const SmartPtr<CX>'
as
   `this' argument of `SmartPtr<T>::operator T*() [with T = CX]' discards
   qualifiers

I have seen similar error messages reported about auto_ptr derived classes
in STL containers, but my template class seems to have all the required
methods. 

The code below compiles and runs fine with VC++ and BC++ (Kylix3/Suse8.0). I
would greatly appreciate any help that you can offer.

Regards,

Frits

===== main file ===========================================================
#include <stdio.h>
#include <vector>

template <class T> class SmartPtr
{
private:
	T* m_pI;
public:

    SmartPtr(T* p = NULL) 
	{
		m_pI = p;
		if (m_pI != NULL)
			m_pI->AddRef();
	}

    ~SmartPtr()
	{
		if (m_pI != NULL)
		{
			T* old = m_pI;
			m_pI = NULL;
			old->Release();
		}
	}

    SmartPtr(const SmartPtr& r) throw() 
	{
		m_pI = r.m_pI;
		if (m_pI != NULL)
			m_pI->AddRef();
	}

    SmartPtr& operator=(const SmartPtr& lp)
    {
		if (m_pI != lp.m_pI)
		{			
			T* pOld = m_pI ;    
			m_pI = lp.m_pI ;    
			if (m_pI != NULL)
			{
				m_pI->AddRef() ;
			}
			if (pOld != NULL)
			{
				pOld->Release() ;       
			}
		}
        return *this;
    }

    T* operator->() { return m_pI; }
	T** operator&() { return &m_pI ;}
    T& operator*()  { return *m_pI; }
	operator T*() { return m_pI ;}

};

class CX {
public:
	CX() { printf("CX::CX()\n"); cnt = 0; }
	~CX() { printf("CX::~CX()\n"); }
	void AddRef() { ++cnt; }
	void Release() { --cnt; if (cnt==0) delete this; }
private:
	long cnt;
};

typedef SmartPtr<CX> XPtr;

int main()
{
	CX* px = new CX();
	XPtr sp(px);
	std::vector<XPtr> v;
	v.push_back(sp);  // THIS IS THE LINE THAT OFFENDS G++
	return 0;
}


F.K. van Evert
Plant Research International
Phone (+31) 317-475957, Fax -423110 

Re: vector<MyTemplate<MyClass>> not possible?

[Sebastian Huber]

Hallo,
you try to use a non-const method on a constant object ...

On Friday 20 September 2002 00:29, Evert, F.K. van wrote:
> When I put instances of a template-derived class in a STL vector, g++ gives
> me the following error message (Cygwin/g++3.2 as well as
> Suse8.0/g++2.95.3):
>
> /usr/include/c++/3.2/bits/stl_construct.h:78: passing `const SmartPtr<CX>'
> as
>    `this' argument of `SmartPtr<T>::operator T*() [with T = CX]' discards
>    qualifiers
>
> I have seen similar error messages reported about auto_ptr derived classes
> in STL containers, but my template class seems to have all the required
> methods.
>
> The code below compiles and runs fine with VC++ and BC++ (Kylix3/Suse8.0).
> I would greatly appreciate any help that you can offer.
>
> Regards,
>
> Frits
>
> ===== main file ===========================================================
> #include <stdio.h>
> #include <vector>
>
> template <class T> class SmartPtr
> {
> private:
> 	T* m_pI;
> public:
>
>     SmartPtr(T* p = NULL)
> 	{
> 		m_pI = p;
> 		if (m_pI != NULL)
> 			m_pI->AddRef();
> 	}
>
>     ~SmartPtr()
> 	{
> 		if (m_pI != NULL)
> 		{
> 			T* old = m_pI;
> 			m_pI = NULL;
> 			old->Release();
> 		}
> 	}
>
>     SmartPtr(const SmartPtr& r) throw()
> 	{
> 		m_pI = r.m_pI;
> 		if (m_pI != NULL)
> 			m_pI->AddRef();
> 	}
>
>     SmartPtr& operator=(const SmartPtr& lp)
>     {
> 		if (m_pI != lp.m_pI)
> 		{
> 			T* pOld = m_pI ;
> 			m_pI = lp.m_pI ;
> 			if (m_pI != NULL)
> 			{
> 				m_pI->AddRef() ;
> 			}
> 			if (pOld != NULL)
> 			{
> 				pOld->Release() ;
> 			}
> 		}
>         return *this;
>     }
>
>     T* operator->() { return m_pI; }
> 	T** operator&() { return &m_pI ;}
>     T& operator*()  { return *m_pI; }
> 	operator T*() { return m_pI ;}

Try: 'operator T*() const { return m_pl; };'
This works because you return a copy of the pointer.

>
> };
>
> class CX {
> public:
> 	CX() { printf("CX::CX()\n"); cnt = 0; }
> 	~CX() { printf("CX::~CX()\n"); }
> 	void AddRef() { ++cnt; }
> 	void Release() { --cnt; if (cnt==0) delete this; }
> private:
> 	long cnt;
> };
>
> typedef SmartPtr<CX> XPtr;
>
> int main()
> {
> 	CX* px = new CX();
> 	XPtr sp(px);
> 	std::vector<XPtr> v;
> 	v.push_back(sp);  // THIS IS THE LINE THAT OFFENDS G++
> 	return 0;
> }
>
>
> F.K. van Evert
> Plant Research International
> Phone (+31) 317-475957, Fax -423110

Re: vector<MyTemplate<MyClass>> not possible?

[Claudio Bley]

>>>>> "FK" == Evert, F K van <F.K.vanEvert@plant.wag-ur.nl> writes:

    FK> When I put instances of a template-derived class in a STL
    FK> vector, g++ gives me the following error message
    FK> (Cygwin/g++3.2 as well as Suse8.0/g++2.95.3):

    FK> /usr/include/c++/3.2/bits/stl_construct.h:78: passing `const
    FK> SmartPtr<CX>' as `this' argument of `SmartPtr<T>::operator
    FK> T*() [with T = CX]' discards qualifiers

    FK> operator T*() { return m_pI ;}

should be: operator T*() const { return m_pI; }

Generally, it's a good idea to declare all member functions `const'
which don't change the state of their class instance.

-- 
Claudio Bley                                                        _ 
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