C / C++ / MFC

23-Mar-04 5:42

Hi all!

I have a base class, CFeature, from which other classes are derived such as CFeatureLine for example. CFeature has a virtual operator, void draw(), which the derived classes always override.

Creating an instance of the derived class, CFeatureLine *pX=new CFeatureLine(...), then serializing it works fine - no problem.

However, in order to de-serialize I use the following code:

CFeature *pFeature; // i use CFeature so that any derived class may be loaded

ar >> pFeature;

But now whenever the draw function of pFeature is called, the compiler directs control to the base class draw function, and not the derived class. This might seem logical, but it appears that MFC's serialization doesn't instantiate a derived class, and instead instantiates the base class, even though the serialization mechanism is supposed to instantiate the class it was saved as.

One way round this is to store a feature type along with the feature data in order to instantiate the appropriate class upon loading, but this is very ugly, and non-modular.

Do you know of a way to solve this? Any help would be great.

Paul.

Robert A. T. Káldy23-Mar-04 10:30

Robert A. T. Káldy

23-Mar-04 10:30

First: ar >> pFeature will read the pointer value, not the contents of object!
If you want to serialize objects of different types, you have to store a type information with each object, even it seems ugly to you. The stream is only a sequence of bytes, and you haven't any type information, unless you store it explicitly.
You should use the standard MFC serialization through CObject and CRuntimeClass, it is quite modular but IMHO non-elegant. And you must derive your objects from CObject, clearly.
Or you can write your own class factory. Class factory is an object, to that you pass an type ID, and the factory creates an instance of type described by the ID. For details see this article.

Robert-Antonio

"I launched Norton Commander and saw, drive C: on the left, drive C: on the
right...Damn, why I need two drives C:??? So I formatted one..."

_psh_23-Mar-04 21:48

23-Mar-04 21:48

Hi Robert-Antonio,

Issuing "ar >> pFeature" will not read the pointer, because the serialization mechanism uses pointers only - unless you specifically call the Serialize(CArchive &ar) function of a serializable class. So, in fact, "ar >> pFeature" will cause MFC to create a new instance of the class CFeature on the heap (pointed to by pFeature), and load the object into that memory location.

Paul

Ravi Bhavnani23-Mar-04 11:05

Ravi Bhavnani

23-Mar-04 11:05

sntpaul wrote:
Do you know of a way to solve this? Any help would be great

When you serialize an object derived from CFeature, first serialize its signature (a unique id that identifies its class type), then serialize the rest of the object. During deserialization, pass the signature to the factory method CFeature::create (signature), and use the returned CFeature-derived object to deserialize itself.

CString strSignature;
strSignature << *pArchive;
CFeature* pFeature = CFeature::create (strSignature);
ASSERT (pFeature != NULL);
pFeature->serialize (*pArchive);

See this[^] series of articles for detail.

/ravi

My new year's resolution: 2048 x 1536
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_psh_23-Mar-04 23:10

23-Mar-04 23:10

Hi Ravi,

This sounds very promising, but the CFeature does not have a create function! It does have a CreateObject function - are these similar? I'm not sure how to get the signature either, but will look into it more...

Paul

Ravi Bhavnani24-Mar-04 3:43

Ravi Bhavnani

24-Mar-04 3:43

sntpaul wrote:
CFeature does not have a create function

You need to add this method to it. The signature is nothing special - it can be a const int or const CString that's unique to subclasses of CFeature. Btw, this is a typical case of the "factory" design pattern.

// Feature.h
static CFeature* create
  (CString strSignature);

// Feature.cpp
CFeature* CFeature::create
  (CString strSignature)
{
  if (strSignature == CFeature::sigFoo)
     return (new CFooFeature());
  if (strSignature == CFeature::sigBar)
     return (new CBArFeature());
  ...

  // Did you forget to support a subclass?
  ASSERT (FALSE);
  return (NULL);
}

/ravi

My new year's resolution: 2048 x 1536
Home | Articles | Freeware | Music
ravib@ravib.com

_psh_24-Mar-04 3:53

Mixing C++ and C code - HELP

24-Mar-04 3:53

Thanks Ravi,

I will give it a shot

Paul

Member 62566623-Mar-04 5:15

23-Mar-04 5:15

I have created a simple Form project and tried to mix C++ and my existing C code. With problems of course. There is no need for controls on the form to try this example. Here is the important code.

/* BEGIN - don.c */
/* ############################################################ */
/* ### This just a cut-down example of my C code ### */
/* ############################################################ */
/* ### The fact that it is C code and not C++ cannot change ### */
/* ############################################################ */
#include <stdio.h>

extern void showme(int); /* call this routine in Form1.h */

int magiccalc() /* Magic calculation - written in C */
{
showme(3); /* use the integer 3 instead of showing the calculations code */

return(0);
}
/* ############################################################ */
/* END - don.c */

/* BEGIN - Form1.h */
#pragma once

extern int magiccalc(void);

namespace sim50avc
{
using namespace System;
using namespace System::ComponentModel;
using namespace System::Collections;
using namespace System::Windows::Forms;
using namespace System::Data;
using namespace System::Drawing;

///

/// Summary for Form1
///
/// WARNING: If you change the name of this class, you will need to change the
/// 'Resource File Name' property for the managed resource compiler tool
/// associated with all .resx files this class depends on. Otherwise,
/// the designers will not be able to interact properly with localized
/// resources associated with this form.
///

public __gc class Form1 : public System::Windows::Forms::Form
{
public:
Form1(void)
{
int num;

InitializeComponent();
num = magiccalc(); /* Call magiccalc in don.c */
/* num is a meaningless return value */
}

protected:
void Dispose(Boolean disposing)
{
if (disposing && components)
{
components->Dispose();
}
__super::Dispose(disposing);
}

private: System::Windows::Forms::TextBox * inputBox;
private: System::Windows::Forms::TextBox * outputBox;

private:
///

/// Required designer variable.
///

System::ComponentModel::Container * components;

///

/// Required method for Designer support - do not modify
/// the contents of this method with the code editor.
///

void InitializeComponent(void)
{
this->inputBox = new System::Windows::Forms::TextBox();
this->outputBox = new System::Windows::Forms::TextBox();
this->SuspendLayout();
//
// inputBox
//
this->inputBox->Location = System::Drawing::Point(16, 24);
this->inputBox->Multiline = true;
this->inputBox->Name = S"inputBox";
this->inputBox->Size = System::Drawing::Size(256, 88);
this->inputBox->TabIndex = 1;
this->inputBox->Text = S"inputBox";
//
// outputBox
//
this->outputBox->Location = System::Drawing::Point(16, 184);
this->outputBox->Multiline = true;
this->outputBox->Name = S"outputBox";
this->outputBox->Size = System::Drawing::Size(256, 88);
this->outputBox->TabIndex = 2;
this->outputBox->Text = S"";
//
// Form1
//
this->AutoScaleBaseSize = System::Drawing::Size(5, 13);
this->ClientSize = System::Drawing::Size(292, 302);
this->Controls->Add(this->outputBox);
this->Controls->Add(this->inputBox);
this->Name = S"Form1";
this->Text = S"Form1";
this->ResumeLayout(false);

}

public:
void __cdecl showme(int thenum)
{
MessageBox::Show(S"abc", S"Result...");
}

};
}
/* END - Form1.h */

/* Thank You in advance */

Don

Member 62566623-Mar-04 5:20

23-Mar-04 5:20

I can send the project in a zip file if anyone is willing to look at it.

Thanks
Don

Maximilien23-Mar-04 5:44

Maximilien

23-Mar-04 5:44

so, what's the question ? what's the problem ?

maybe you'll get better help in the managed-C++ forum.

Maximilien Lincourt
Your Head A Splode - Strong Bad

Pedro Ruiz23-Mar-04 7:30

Pedro Ruiz

23-Mar-04 7:30

try: extern "C" int magiccalc(void);

Member 62566623-Mar-04 7:36

23-Mar-04 7:36

It's the second line of code in Form1.h

Thanks though

Pedro Ruiz23-Mar-04 7:48

Pedro Ruiz

23-Mar-04 7:48

Well, it's not the same extern "C" than just extern.
In C++ some compilers add extra information to the function names (type of parameters, type of return,..). To avoid this problem, use the extern "C" declaration in all the 'C' functions.

Member 62566623-Mar-04 7:35

23-Mar-04 7:35

Here are the 2 errors.

sim50avc fatal error LNK1284: metadata inconsistent with COFF symbol table: symbol '?magiccalc@@$$FYAHXZ' (0A00002D) mapped to '?magiccalc@@$$J0YAHXZ' (06000001) in don.obj

c:\other\vc_projects\sim50avc\Form1.h(97): warning C4440: calling convention redefinition from '__clrcall ' to '__cdecl ' ignored