Template class derived from a Template class???

Norman L Covington -- doubleM@cris.com
Monday, July 22, 1996

Dear List Members:

Environment: MSVC++ 4.1/WinNT 3.51 (Service Pack 4)

I have a base class as follows:

template
class CdMPerson : public CObject
{
// Constructor(s)
public:
  CdMPerson();                      // Default Constructor
  CdMPerson(CdMPerson& rPerson); // Copy Constructor
  ~CdMPerson();

  ...
};

In the same header file (taking CMap... in AfxTempl.h as an example to
follow) I define the Constructor as follows:

template
CdMPerson::CdMPerson()
{
  m_szTitle = _T("");
  m_szFirst = _T("");
  m_szMiddle = _T("");
  m_szLast = _T("");
  m_szSuffix = _T("");
  m_szNameExt = _T("");
  m_pID = new T();
  ASSERT(m_pID != NULL);
}

I then derive a class as follows:

template
class CdMContact : public CdMPerson
{
// Constructor(s)
public:
  CdMContact();                        // Default Constructor
  CdMContact(CdMContact& rContact); // Copy Constructor
  ~CdMContact();
  
  ...
};

The constructor is as follows (It should be noted the template T is used for
the base class CdMPerson rather than within the CdMContact class):

template
CdMContact::CdMContact() : CdMPerson()
{
  m_szJobPosition = _T("");

  m_wPhoneKey = 0x0;
  m_wAddressKey = 0x0;
  m_pmapPhones = NULL;
  m_pmapAddresses = NULL;
}

Again all template declarations and definitions are within the same header file.

Following is the implementation within a derived CPropertySheet to test the
template classes:

  ...

  CdMStrValue* pWContactID1 = new CdMStrValue();
  ASSERT(pWContactID1 != NULL);
  pWContactID1->SetValue(_T("WContactID1"));
  CdMContact pWContact1 = new CdMContact();
  pWContact1->SetID(pWContactID1);
  pWContact1->SetName(_T("Mr"), _T("Adam"), _T("A"), _T("Adamson"), NULL, NULL);
  pWContact1->SetJobPosition(_T("Clerk"));

  ...

My addition reference other than that provided by books online is chapter
14, Mastering Templates, The Most Complete Reference Special Edition Using
Visual C++ 4, Que Publishing. Upon compiling the CPropertySheet derived
class to test these templates I get the following errors:

error C2664: 'CdMContact::CdMContact(class CdMContact &)' : cannot convert
parameter 1 from 'class CdMContact *' to 'class
CdMContact &' (new behavior; please see help)

As depicted above I have copy constructors defined as well as the "default
constructors." Following are the copy construct definitions:

template
CdMPerson::CdMPerson(CdMPerson& rPerson)
{
   ...
}

template
CdMContact::CdMContact(CdMContact& rContact) : CdMPerson(rContact)
{
   ...
}

Obviously because the above errors occurred an following reference to
CdMContact does not compile properly either as illustrated below:

error C2819: type 'CdMContact' does not have an
overloaded member 'operator ->'

error C2227: left of '->SetID' must point to class/struct/union
0
error C2819: type 'CdMContact' does not have an
overloaded member 'operator ->'

error C2227: left of '->SetName' must point to class/struct/union

error C2819: type 'CdMContact' does not have an
overloaded member 'operator ->'

error C2227: left of '->SetJobPosition' must point to class/struct/union

The errors then repeat as above for each contact object instances attempted.

I have looked at the code in the header where the template is
declared/defined and within the derived CPropertySheet class
header/implementation file. I should mention I have other templates working
fine. It is only when I derived a class from a template base class where the
template derived class supplies the derived class with the class type.

Assistance would greatly be appreciated. Thank you for your valuable time in
advance.

Norman
Email: doubleM@concentric.net

---

Ken Nicolson -- kenn@owl.co.uk
Friday, July 26, 1996

[Mini-digest: 3 responses]

On Mon, 22 Jul 1996 17:50:21 -0700, you wrote:

[snip!]

>  CdMContact pWContact1 = new CdMContact();

Surely that line should be:

   CdMContact * pWContact1 = new CdMContact();
                           ^

With the code the way you posted, the error:

>
>error C2664: 'CdMContact::CdMContactCdMStrValue>(class CdMContact &)' : cannot convert
>parameter 1 from 'class CdMContact *' to 'class
>CdMContact &' (new behavior; please see help)
>

makes sense - removing all the template stuff and using, say, int, instead,
the posted code is just:

    int x = new int;

Therefore my suggested fix

    int * x = new int;

looks correct.

HTH

Ken
-----From: bop@gandalf.se

Here's the problem, look below.

>  pWContact1->SetID(pWContactID1);
>  pWContact1->SetName(_T("Mr"), _T("Adam"), _T("A"), _T("Adamson"), NULL,
> NULL);
>  pWContact1->SetJobPosition(_T("Clerk"));
>
> [...snip...]
>
> error C2819: type 'CdMContact' does not have an
> overloaded member 'operator ->'
>
> error C2227: left of '->SetID' must point to class/struct/union
>
> [...snip...]

The missing "->" operator is the clue: You declared a CdMContact
object, not a pointer.  "pWContact1" should be " *pWContact1" !


Bo Persson
bop@gandalf.se
-----From: Sajith Kumar 

 CdMContact pWContact1 = new CdMContact();
or
 CdMContact* pWContact1 = new CdMContact();



Sajith Kumar
sajith@wallop.com
1155 Triton Drive, Suite E
Foster City, CA 94404
(415) 341 1177x227

---

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