C / C++ / MFC

2-Mar-05 17:14

i'm facing a problem with polymorphism currently and need help.

i've 1 base class, let's call it Base, and 2 Derived class, Derive1 and Derive2. Base is NOT an abstract class.

on top of these 2, i've a container class Container that contains a vector of Base pointers. (i.e. vector < Base* > pointer)

now in Container, there's a function called init() that will initialize the vector of pointers and according to the type of class it belongs to.

e.g.
pointer[0] = *(new Base);
pointer[1] = *(new Derive1);
pointer[2] = *(new Derive2); and so on...

the type of classes are created correctly when the constructors are called. however, when i tried to access the pointers again later in my program, the program only treats each pointer as Base and no longer as any derived classes. thus polymorphism cannot work at all... this is crucial to me as i have several classes that's supposed to belong to the vector of pointers. if i use a simple array polymorphism works. but i need the vector....

please help!!

Ryan Binns2-Mar-05 17:17

Ryan Binns

2-Mar-05 17:17

ng kok chuan wrote:
vector < Base* > pointer

ng kok chuan wrote:
pointer[0] = *(new Base);

Are you sure this actually compiles? Shouldn't the second line actually be

pointer[0] = new Base;
etc...

There's no reason why a vector can't be used here.

Ryan

"Punctuality is only a virtue for those who aren't smart enough to think of good excuses for being late" John Nichol "Point Of Impact"

ng kok chuan2-Mar-05 17:23

2-Mar-05 17:23

i was trying to simplify the problem here, which is why i used *(new Base);

what i really have is a while loop:
while (!odyn.IsEOF())
{
node.push_back(*(new MazeNode));
node[node.size()-1].init((string)eqpid, (string)eqptyp, 1);

odyn.MoveNext();
}

and node.push_back() takes a reference, which is why i need to dereference the memory allocated.

the code compiles with no problem, and works without error too. however polymorphism does not work at all. i've tested using a Base array in main() and polymorphism works.

Ryan Binns2-Mar-05 22:27

Ryan Binns

2-Mar-05 22:27

ng kok chuan wrote:
and node.push_back() takes a reference

If it does, then your vector is not a vector of pointers, and that is the prolem. If you want polymorphism to work, your vector will have to store pointers, not objects.

Ryan

"Punctuality is only a virtue for those who aren't smart enough to think of good excuses for being late" John Nichol "Point Of Impact"

ng kok chuan3-Mar-05 14:31

3-Mar-05 14:31

ok i've got the problem solved. silly me.

as you said, i required a vector of node pointers, but when i first started off, i only used a vector of nodes. it worked perfectly fine till i tried to use polymorphism, and this problem arised.

now i've made all the necessary changes and it's ok now. thanks for the reply Smile | :)

David Crow3-Mar-05 2:35

David Crow

3-Mar-05 2:35

ng kok chuan wrote:
was trying to simplify the problem here, which is why i used *(new Base);

When posting, always copy/paste your code verbatim. Re-typing (or simplifying when it is not obvious you are doing so) into the forum will, more often than not, lead to erroneous assumptions and misguided answers.

"Opinions are neither right nor wrong. I cannot change your opinion. I can, however, change what influences your opinion." - David Crow

ng kok chuan2-Mar-05 20:01

2-Mar-05 20:01

hi, anyone please help?? i really need to be able to do both polymorphism and use vectors.

Cedric Moonen2-Mar-05 20:36

Cedric Moonen

2-Mar-05 20:36

There is no problem using polymorphism in a vector. You are probably doing something wrong elsewhere.
Try to post the code of the declaration of the vector, the piece of code in which you add new elements to it and the piece of code where you make use of these elements (and where it seems polymorphism doesn't work).
This will help us to help you Big Grin | :-D

Steen Krogsgaard2-Mar-05 21:40

Steen Krogsgaard

2-Mar-05 21:40

Your methods have to be virtual for polymorphism to work. If you need member data access to be polymorphic, either use access methods or cast your pointer from the array to the correct type using some kind of type information extracted from the pointed-to object using virtual methods.

Did that make any sense? If not, here's an example (lot of stuff leaved out, don't expect it to compile)

<br />
<br />
class Base<br />
{<br />
public:<br />
int m_iInt;<br />
<br />
public:<br />
int virtual GetInt() {return m_iInt);<br />
CString virtual GetClassInfo() {return CString("Base")};<br />
int NonVirtualGetInt(){return m_iInt);<br />
}<br />
<br />
class Derive1 : Base<br />
{<br />
public: <br />
int m_iAnotherInt;<br />
<br />
public:<br />
int virtual GetInt() {return m_iAnotherInt);<br />
CString virtual GetClassInfo() {return CString("Derive1")};<br />
int NonVirtualGetInt(){return m_iAnotherInt);<br />
}<br />
<br />
class Derive2 : Base<br />
{<br />
<br />
public:<br />
int virtual GetInt() {return 2 * m_iInt);<br />
CString virtual GetClassInfo() {return CString("Derive1")};<br />
int NonVirtualGetInt(){return 2 * m_iInt);<br />
}<br />
<br />
int main()<br />
{<br />
Base *pointer[3];<br />
<br />
pointer[0] = new Base;<br />
pointer[1] = new Derive1;<br />
pointer[2] = new Derive2;<br />
<br />
pointer[0]->m_iInt = 1  // access Base.m_iInt<br />
pointer[1]->m_iInt = 10  // access Base.m_iInt although p1 is a Derive1<br />
(CDerive1*)pointer[1]->m_iAnotherInt = 15  // access Derive1.m_iAnotherInt<br />
(CDerive2*)pointer[2]->m_iInt = 100<br />
(CDerive2*)pointer[2]->m_iAnotherInt = 150 // won't work, Derive2 has not m_iAnotherInt<br />
<br />
for (i=0; i<3; i++) <br />
   cout << pointer[i]->GetInt()  // will write 1 (Base.GetInt(), 15 (Derive1.GetInt), 200 (Derive2.GetInt as GetInt is virtual<br />
<br />
for (i=0; i<3; i++)<br />
   cout << pointer[i]->m_iInt // 1, 10, 100, all classe contain m_iInt, but C++ think it's all Base.m_iInt<br />
<br />
for (i=0; i<3; i++)<br />
   cout << pointer[i]->m_iAnotherInt // won't work, m_iAnotherInt not member of Base, and C++ thinks all pointers are Base*<br />
<br />
for (i=0; i<3; i++)<br />
   cout << pointer[i]->NonVirtualGetInt() // returns 1, 10, 100 as Base.NonVirtualInt() is called in all three cases<br />
<br />
for (i=0; i<3; i++) {<br />
   CString cs=pointer[i]->GetClassInfo()<br />
   if (cs == "Base") pointer[i]->NonVirtualGetInt() // Base.NonVirtualGetInt()<br />
   if (cs == "Derive1") (Derive1*)pointer[i]->NonVirtualGetInt() // Derive1.NonVirtualGetInt()<br />
   if (cs == "Derive2") (Derive2*)pointer[i]->NonVirtualGetInt() // Derive2.NonVirutalGetInt()<br />
}<br />

I'm sure you get the general idea. One thing that helps me in these cases is to always keep in mind that the compiler can only act on what it knows - so if it thinks an object is of type Base it will call Base-methods, unless the methods are virtual

Cheers
Steen.

"To claim that computer games influence children is ridiculous. If Pacman had influenced children born in the 80'ies we would see a lot of youngsters running around in dark rooms eating pills while listening to monotonous music"

how to use a video codec to compress avi file

Alton Williams5-Mar-05 4:03

Alton Williams

5-Mar-05 4:03

ng kok chuan wrote:

pointer[0] = *(new Base);<br /> pointer[1] = *(new Derive1);<br /> pointer[2] = *(new Derive2); and so on...

This what is known a late binding

ng kok chuan wrote:
the type of classes are created correctly when the constructors are called. however, when i tried to access the pointers again later in my program, the program only treats each pointer as Base and no longer as any derived classes. thus polymorphism cannot work at all... this is crucial to me as i have several classes that's supposed to belong to the vector of pointers. if i use a simple array polymorphism works. but i need the vector.

How about putting the virtual before the name of the function (in the base class)

ng kok chuan wrote:

vector < Base* > pointer)<br /> pointer[0] = *(new Base);

doesn't make sense
Surely, it shold be

pointer[0] = new Base;

Anyway, I got a sample app to give you an idea. If you e-mail I'll gladly send it to you.

Happy programming
Alton

C++ Wrapper for libCURL

a_diwakar2-Mar-05 17:02

a_diwakar

2-Mar-05 17:02

I am writing C++ wrapper for libCURL to send https requests to the webserver.
Currently i am finding some help on client cert authentication using the same. Please let me know if anybody has done in this.

Mohsen Saad2-Mar-05 15:22

Mohsen Saad

2-Mar-05 15:22

I need to compress an avi file using a video codec in my application
how can I do this

Text to matrix conversion

SeanSheehan2-Mar-05 14:32

SeanSheehan

2-Mar-05 14:32

I'm a relatively new programmer and I was wondering if there was an easy way to read a text file of numbers seperated by spaces into a matrix. Here's what I got:

	int i, j;<br />
	short ImageArray[IMAGE_SIZE_X][IMAGE_SIZE_Y];<br />
<br />
	//Set Filename <br />
	char *FileName = C:\Image.asc;<br />
<br />
	//Open file<br />
	ifstream ImageFile;<br />
	ImageFile.open(FileName);<br />
	<br />
	//Read file data into array<br />
	for(i=0; i<IMAGE_SIZE_Y; ++i)<br />
	{<br />
		for(j=0; j<IMAGE_SIZE_X; j++)<br />
			ImageFile >> ImageArray[i][j];<br />
	}<br />
	//Close file<br />
	ImageFile.close();

I know I'm reading it wrong but I don't know the correct way to do it. Any help would be greatly appreciated.

Small executables

Verolix2-Mar-05 13:49

Verolix

2-Mar-05 13:49

I've seen a few programs out there that are very small in size and actually do something (ex.: .kkreiger)... When I look at their import functions list I see only LoadLibrary and GetProcAddress. The question is: How do you remove all those additional dependencies from your programs (C++)?

Re: Small executables

ThatsAlok2-Mar-05 18:00

ThatsAlok

2-Mar-05 18:00

Yeah read this article about Code optimization

http://www.codeproject.com/tips/aggressiveoptimize.asp

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[Vote One Here, Complete my Survey....]
<h5
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resizing workarea

ahto1112-Mar-05 12:07

ahto111

2-Mar-05 12:07

Hi,

I'm resizing the workarea of Windows in the initialization of my program so that my application will show always, like taskbar.

My problem is that sometimes, not always, the workarea reverts back to the original size.
It usually takes 1-20 seconds after I start my app to happen.
Any ideas on what is causing this behaviour and more importantly how to fix it?

I'm using Visual C++ 6 and my app is dialog based using mfc.

thanks

ahto111 at hotmail

C++ library

Ahmed Adel2-Mar-05 11:33

Ahmed Adel

2-Mar-05 11:33

I use microsoft VC++.
I have a group of header and source files that I want to compile into a library to be linked in my program.
How can i do that in Microsoft Visual Studio 6 ??
Thanks in advance

Re: C++ library

ThatsAlok2-Mar-05 18:10

ThatsAlok

2-Mar-05 18:10

Just Create A Empty Project and Insert all the Header and Source file under it.... Simple

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[Vote One Here, Complete my Survey....]
<h5
alok="" gupta="" <br=""> visit me at http://www.thisisalok.tk

Error?

Larsson2-Mar-05 10:53

Larsson

2-Mar-05 10:53

Hmmm,
I get this error and I dont now what it means!
Can anyone explane this to me, what is I doing wrong?...

TRACE1("Error: no data exchange control with ID 0x%04X.\n", nIDC);
ASSERT(FALSE);
AfxThrowNotSupportedException

Ravi Bhavnani2-Mar-05 10:55

Ravi Bhavnani

2-Mar-05 10:55

Your DDX section refers to a resource id that isn't present in your dialog or view.

/ravi

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Larsson2-Mar-05 10:57

Larsson

2-Mar-05 10:57

Ok and how can I fix this?

Ravi Bhavnani2-Mar-05 11:01

Ravi Bhavnani

2-Mar-05 11:01

Um... add the control to your dialog/view.

/ravi

My new year's resolution: 2048 x 1536
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