C / C++ / MFC

Re: How to declare class Member with template declaration ?

9-Mar-04 4:09

template <class dlg_t="">
class CDialogCreatorHelper
{};

but how to declare members outsude the class body ?

So that I can access to members of class that I will derive from it.

thanks

Antti Keskinen9-Mar-04 5:32

Re: How to declare class Member with template declaration ?

9-Mar-04 5:32

Your declaration lacks the > and < tags. I believe they were removed by the forum script.

Nevertheless, why do you need to declare members of this class outside of it ? In a class declaration, you can define and declare member functions and data variables all inside the class keyword context, you don't need to create a seperate implementation file.

If, however, you MUST have a seperate implementation file, here is an exerpt from MSDN posing a simple example on how to first declare a template class, then define one member function inside it:

// Template class specification
template <class T, int i>
class TestClass 
{
public:
   char buffer[i];
   T testFunc(T* p1 );
};<DIV>

// Defining a member function
template <class T, int i>
T TestClass<T,i>::testFunc(T* p1) 
{
    return *(p1++);
}

Hope this helps

-Antti Keskinen

----------------------------------------------
The definition of impossible is strictly dependant
on what we think is possible.

vgrigor9-Mar-04 5:45

Re: How to declare class Member with template declaration ?

9-Mar-04 5:45

Thanks, it is correct.

can you advice how to derive class from it,
so that base class can have access to private members of derived.

If it would by #define,
or direct substitution -
it works,

but with inheritance I wasnot been able to access members of derived class.

How to do it?

Robert A. T. Káldy9-Mar-04 5:58

Re: How to declare class Member with template declaration ?

9-Mar-04 5:58

1. Base class cannot access to derived class members. Only derived class can access to base class members.

2. Derived classes cannot access private members of base class. Did you mean protected members?

3. Deriving from templatized class goes as following:

template<class T>
class Base
{
protected:
    T base_member;
};

template<class T>
class Derived : public Base<T>
{
...
};

Robert-Antonio

"Love without sex is like a fish without antlers"

vgrigor9-Mar-04 6:09

Re: How to declare class Member with template declaration ?

9-Mar-04 6:09

thanks,
Some other scheme:

class Base1
{
protected: CString base1_member;
};

template<class t="">
class Base_Accessor // to Base1, when instantiated on Derived
{
public:
method()
{
base1_member; //accessed - even not derived

};

};

class Derived : public Base1, public Base_Accessor< ?>{????}
{};

that scheme not works.

Need access to still undefined member, that will
be defined when template will be actually instatiated.

# define so does,
why not template ?

Robert A. T. Káldy9-Mar-04 6:39

Re: How to declare class Member with template declaration ?

9-Mar-04 6:39

I don't understand whar you exactly want.
But generally, you can't access the base1_member from Base_Accessor, if Base_Accessor isn't derived from Base1.

Robert-Antonio

"CRAY is the only computer, which runs an endless loop in just 4 hours"

Antti Keskinen9-Mar-04 6:52

Re: How to declare class Member with template declaration ?

9-Mar-04 6:52

Note here, that access to base1_member can be enabled for the other class if the other class is declared as a friend of the class containing base1_member.

Friend class, if defined, has a complete access to all member variables and methods inside the class where the declaration is placed. See MSDN Library for many good examples on this.

-Antti Keskinen

----------------------------------------------
The definition of impossible is strictly dependant
on what we think is possible.

vgrigor9-Mar-04 20:35

Re: How to declare class Member with template declaration ?

9-Mar-04 20:35

If it would be kinf of macro, it is possible to access to any future member,
due to it became like direct write code in derived class,
whem macro will be inserted.

But for template, that is called macro for compiler I can find how to do it.

Robert A. T. Káldy9-Mar-04 23:52

Re: How to declare class Member with template declaration ?

9-Mar-04 23:52

If you say, that it's possible via macros, show me a example code, please. So I can exactly know, what is your goal.

Robert-Antonio

"A flower walked around a meadow. She saw a beatiful human and plucked off his head."

vgrigor10-Mar-04 0:14

Re: How to declare class Member with template declaration ?

10-Mar-04 0:14

To the class CDialogDerived

lpszTemplateName, m_hDialogTemplate - private member of MFC CDialog

Manual creation of CDialog with custom font:

adding member by macro

# define MACROS \
\
int Create_WithHelper(int IDD, CWnd* pParentWnd, int isModal, int size, CString FontName = "MS Sans Serif")
{

CDialogTemplate dlt;
int nResult;

// load dialog template
if (!dlt.Load(MAKEINTRESOURCE(IDD))) return -1;

// set your own font, for example "Arial", 10 pts.

dlt.SetFont(FontName, size);
// get pointer to the modified dialog template
LPSTR pdata = (LPSTR)GlobalLock(dlt.m_hTemplate);

// let MFC know that you are using your own template

//dlg_T::
//pDialog->
m_lpszTemplateName = NULL;

HINSTANCE hInst = AfxFindResourceHandle(MAKEINTRESOURCE(IDD), RT_DIALOG);

BOOL bResult ;

//pDialog->
m_hDialogTemplate = dlt.m_hTemplate;

if(isModal)
{
nResult =
//pDialog->
DoModal();
}
else
{
bResult =
//pDialog->
CreateIndirect( dlt.m_hTemplate, pParentWnd);
}

GlobalUnlock(dlt.m_hTemplate);

if(m_hWnd == 0 )
ASSERT(0);//for test

//bResult = CreateIndirect(hTemplate, pParentWnd, hInst);
//FreeResource(hTemplate);

return bResult;

};

Antti Keskinen9-Mar-04 6:48

Re: How to declare class Member with template declaration ?

9-Mar-04 6:48

vgrigor wrote:
Need access to still undefined member, that will
be defined when template will be actually instatiated.

You have an undefined member in the base class, to which you need access from a derived class ? That sounds completely crazy, are you developing both the base class and the derived class simultaneously ?

If your base class has a pure virtual method, you can always create an object of the derived class, then cast a pointer into the base class, and call the virtual method. It must be defined in the derived class in order for this call to succeed.

In the case of the derived class (Derived, in this case), it derives from both Base1 and Base_Accessor template. The protected member of the base class is not derived, but it exists in the class derivation chain. This means that if you implement handling functions (Get & Set) for the base class, you can then call these functions to manipulate or get a pointer to the base class protected member, and thus modify it.

If this doesn't help, then could you explain in more detail what you mean with 'access to still undefined member'. If a member function/variable is not declared inside the class context, it does not exist, and thus cannot be accessed in any way. This means that you must at least declare all members of classes which you wish to use. This applies to both templates and standard classes.

Also, when posting a reply, and if writing code, please use a <PRE> </PRE> section to isolate the code. Also, use the 'Formatting' commands found above the smileys below the text areas. This allows you to use e.g. the < and > symbols properly, which are of key importance when writing this type of code. This makes your code much more readable and understandable.

-Antti Keskinen

----------------------------------------------
The definition of impossible is strictly dependant
on what we think is possible.

vgrigor9-Mar-04 20:40

Re: How to declare class Member with template declaration ?

9-Mar-04 20:40

For some template I need some methods be defined.

To define is used other template,
si they work together.

So does in ATL, do you know such a template library?, notI invented this.
They work so.

If member would not private- llosing access right after first inheritance,
all would work?

Antti Keskinen9-Mar-04 7:31

Re: How to declare class Member with template declaration ?

9-Mar-04 7:31

There is a round-about way through this, though it breaks the rules of C++ very harshly. You can create two pure virtual methods into the base class for each variable in the derived class. The first variable sets, the second gets, the variable in question. Then implement these methods in the derived class.

Now, your base class member functions can call the virtual functions to return the variables from the derived class, thus having a rudimentary access into the members of the derived class. However, you can no longer declare objects of the base class, as it has pure virtual methods. Only the derived class is available for instantation.

This approach may, or may not work, as I have not tried it myself. I don't know. But following the C++ specification, it should be possible to implement it.

-Antti Keskinen

----------------------------------------------
The definition of impossible is strictly dependant
on what we think is possible.

Michael Dunn9-Mar-04 13:10

Michael Dunn

9-Mar-04 13:10

How does that break C++ rules? That's precisely how virtual functions were designed to be used. Confused | :confused:

The fact that the abstract base class is a template makes no difference.

--Mike--
Personal stuff:: Ericahist | Homepage
Shareware stuff:: 1ClickPicGrabber | RightClick-Encrypt
CP stuff:: CP SearchBar v2.0.2 | C++ Forum FAQ
----
Four fonts walk into a bar. The bartender says "Hey - get out! We don't want your type in here."

Re: How to declare class Member with template declaration ?

Antti Keskinen9-Mar-04 20:28

Re: How to declare class Member with template declaration ?

9-Mar-04 20:28

Virtual functions are primarily, as far as I know, made so that you can create functions in the base class that can be safely overridden in a derived class if necessary. What we are doing here is creating a base class that is specifically meant for a certain derived class.

If you have a set of get/set function pairs in the base class, you can have only a limited type of derived classes, which follow the guidelines set by the base class. This, by itself, can be considered a breach, as C++ inheritance rules (at least in my book) says that inheritance should be available for all types of derived classes, not just specific ones. In our case, the layout of the base class already dictates some functionalities that must be implemented in the derived class in order for the class hiearchy to compile properly.

Here, the base class has a set of functions that can be used to set and change certain derived-class variables. This type of behaviour, as far as I know, is 'illegal', as if it was supposed to be possible to alter derived class variables directly, the inheritance, by itself, should already allow this. The idea of interfaces (pure abstract base class) used with COM technologies follows the same principles as I have outlined here. I am not saying that COM was illegal, though.

I won't start wrestling about this issue any more than that. In conclusion, it could be said that this is a taste issue: some people think it is 'illegal' while some people think it is completely good and well. It all comes down on what you are trying to accomplish, I guess. I, for myself, see no reason on why you should be able to tamper with derived class members from the base class. Perhaps this is because I have never needed this type of functionality Smile | :)

-Antti Keskinen

----------------------------------------------
The definition of impossible is strictly dependant
on what we think is possible.

vgrigor9-Mar-04 20:47

Re: How to declare class Member with template declaration ?

9-Mar-04 20:47

It need to add functionality to many classes,
not by copy/paste,
bu just inheritance from making extension class,
to all needed classes.
But if it depends from MFC for sample, it need to access to MFC
in base - extension class.

Robert A. T. Káldy9-Mar-04 23:38

Re: How to declare class Member with template declaration ?

9-Mar-04 23:38

I think, that you can't distinguish between interfaces and abstract base classes. Interfaces are only the extreme case of abstract base classes, where the member variables are forbidden.
If you use you own object model, you should simplify it and permit member variables to abstract base classes. But still there remain the "interface functionality" of the abstract base class, so the set-and-get virtual methods should be useful.

Robert-Antonio

"A piece of paper is an ink-lined plane.
An inclined plane is slope up.
A slow pup is a lazy dog.
Q.E.D.: A piece of paper is a lazy dog."

Antti Keskinen10-Mar-04 0:28

Re: How to declare class Member with template declaration ?

10-Mar-04 0:28

Robert A. T. Káldy wrote:
I think, that you can't distinguish between interfaces and abstract base classes

Not so, as I know the difference between these two. Apparently I just didn't explain my ideas clearly enough. An alternative point may be that the original poster is not writing very coherent questions, making it somewhat difficult to guess what he/she is after Smile | :)

The set/get virtual methods can be used. There's nothing wrong there. These methods can be considered an interface into the derived class methods. Just like an interface class is an interface into the derived class's methods, only that in an interface class case, all methods need to be pure virtual, and no member variables are allowed.

I am not going to wrestle about the definition terms, as that would go way off-topic. Let's just say, for peace's sake that I didn't explain myself clearly enough.

-Antti Keskinen

----------------------------------------------
The definition of impossible is strictly dependant
on what we think is possible.

vgrigor9-Mar-04 20:52