C / C++ / MFC

22-Oct-04 7:04

I am trying to create an object of a class(say class1) in a member function of another class(say class2) on the heap. The trouble is it is asserting on that and i cannot proceed further. If I create a normal object(on stack) and call a member function of class1, the function is unable to access the member variables of its own class(class1). It's making me go nuts. I had earlier tested class1 for correctness but now that it is part of this whole application, I don't know what has happened. Please help me. It is urgent.

David Crow22-Oct-04 7:40

22-Oct-04 7:40

AmorphousP wrote:
The trouble is it is asserting on that...

On what? Show us some relevant code.

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

AmorphousP22-Oct-04 9:37

22-Oct-04 9:37

CDns* DnsCl = new CDns;
This is the part where I am creating the object on the heap in a function of another class, CDnsThread. This is the code that asserts.

CDns* DnsCl;
This is the part when it is created on the stack. It doesn't assert but now when I call a public member fuction of the CDns class like

DnsCl->GetSMTPAddress(domain);

the code inside the GetSMTPAddress() function can't access it's own member variables. I tried debugging it and found that the constructor works fine because all the initialization work I do happens correctly. But the first line inside the GetSMTPAddress() function

if (SockStatus = INVALID_SOCKET)<br />
         error handling code...

causes an 'access violation'. This same thing had worked very well some time back when I had tested this module. But now as a part of an application, it's causing troubles. I hope u r able to help.
Thanx

Maximilien22-Oct-04 9:51

Maximilien

22-Oct-04 9:51

CDns* DnsCl = new CDns;

where does it assert ? check what you do in the constructor...

CDns* DnsCl;

this is not allocated at best, the pointer will be 0, at worse, anything ... but DnsCl is invalid ... you need to new it; or remove the *.

Maximilien Lincourt
Your Head A Splode - Strong Bad

AmorphousP22-Oct-04 10:34

22-Oct-04 10:34

Tried removing the *. Didn't work either. Asserted on the Create call only.

David Crow22-Oct-04 9:53

22-Oct-04 9:53

AmorphousP wrote:
CDns* DnsCl = new CDns;
This is the part where I am creating the object on the heap in a function of another class, CDnsThread. This is the code that asserts.

What line of what file is firing the assertion? Have you set a breakpoint in the constructor of CDns and stepped through it?

AmorphousP wrote:
CDns* DnsCl;
This is the part when it is created on the stack. It doesn't assert but now when I call a public member fuction of the CDns class like

DnsCl->GetSMTPAddress(domain);
...
causes an 'access violation'.

I'm not surprised. How is this working without the new operator being called?

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

AmorphousP22-Oct-04 10:29

22-Oct-04 10:29

Actually the object is getting created dynamically now and I tried stepping through the constructor of CDns.

DnsSocket = new CSocket;<br />
	SockStatus = TRUE;<br />
	if (DnsSocket)<br />
	{<br />
		if (!DnsSocket->Create(0, SOCK_DGRAM, 0))

It's asserting on Create. It's here that it gives an access violation. The ASSERT that fires it is

ASSERT(CAsyncSocket::LookupHandle(hSocket, bDead) == NULL);

in the file sockcore.cpp

DavidCrow wrote:
AmorphousP wrote:
CDns* DnsCl;
This is the part when it is created on the stack. It doesn't assert but now when I call a public member fuction of the CDns class like

DnsCl->GetSMTPAddress(domain);
...
causes an 'access violation'.

I'm not surprised. How is this working without the new operator being called?

It was actually a desperate attempt to get the work done although wasn't a very logical choice.

dharani26-Oct-04 22:44

dharani

26-Oct-04 22:44

AmorphousP wrote:
DnsSocket = new CSocket;

What do u do before this line ? You must be calling Create() before the socket object is initialized.

Dharani Babu S

AmorphousP27-Oct-04 9:51

Abstract member functions - strange phenomenon open to discussion:

27-Oct-04 9:51

I actually solved the problem. The trouble was that this code (DnsSocket = new CSocket) is written inside a new secondary thread. As such there was no initialization code for winsock in the thread and that's why the code was asserting on DnsSocket->Create(). Now it works fine

-----------------------------
Nobody got anywhere in this world by simply being content.

Nick Nougat22-Oct-04 6:54

Nick Nougat

22-Oct-04 6:54

Hi!

I recently stumbled over this interesting phenomenon using abstract member functions:

The situation is this: We have an abstract base class that defines an interface. In the constructor of the base, a specific implementation dependent task needs to be carried out, so an abstract

member function is invoked in the constructor. The behaviour of this member function is defined by derived classes.
If no function body is provided for this abstract function in the base class, a linker error occurs (unresolved symbol). If we provide an empty body, in the base, everything links fine and the vtable

lookup correctly resolves the implementation of the deriving class.

The question is this: If the abstract function is resolved at run-time anyway, through use of vtables, why give me a compile-time error if I don't provide a body that will, due to it being an abstract

function, never be used in the first place?!
Is this a Microsoft specific behaviour bug/"feature", or is this standard ISO behaviour?
I compiled this using MS VS .NET 2003

Thanks for any thoughts about this.
Nick

Here is some exaple code for you to check it out:

<br />
#include <stdio.h><br />
class base<br />
{<br />
public:<br />
	base()<br />
	{<br />
		abstractFunction();<br />
	}<br />
	virtual ~base() {};<br />
<br />
	/* if you don't provide the function body<br />
	   with {}, a linker error occurs. Try uncommenting<br />
	   the part of the next line to make it work.<br />
	*/<br />
	virtual void abstractFunction() = 0 /*{}*/;<br />
};<br />
<br />
class derived : public base<br />
{<br />
public:<br />
	derived() {};<br />
	virtual ~derived() {};<br />
<br />
	// should be called by base::base()<br />
	void abstractFunction()<br />
	{<br />
		printf("works\n");<br />
	}<br />
} testInstance;	//create an instance right away...<br />

This code should print "works" to the console, since the

As soon as you uncomment the function body of base::abstractFunction(), everything will link fine and work as expected.

Re: Abstract member functions - strange phenomenon open to discussion:

Roger Allen22-Oct-04 7:13

Roger Allen

22-Oct-04 7:13

You cannot call a virtual function of a derived class from a base class constructor. This is due to the order of construction. Base classes are built first so there is no vtable pointer available to call the correct function in the derived class, which is yet to be constructed.

You need the function body as the virtual call compiles to the base class member function not the derived class one.

You need a two stage construction method to handle what you need to do.

x = new drieved;
x.Initialise();

Where the initialise function calls the correct virtual functions once the object is fully constructed.

If you vote me down, my score will only get lower

Re: Abstract member functions - strange phenomenon open to discussion:

Nick Nougat22-Oct-04 7:58

Nick Nougat

22-Oct-04 7:58

Ah, I see.
That escaped my attention. Quite obvious though, come to think of it. (->silly me)

So the fact that my example code works OK is only by chance and not a regular case and more complicated classes or a release version compile might produce unpredictable results.

Thanks for the comment.
Nick

How can I programmatically install a visual C++ add-in for visual C++?

Behzad Ebrahimi22-Oct-04 5:56

Behzad Ebrahimi

22-Oct-04 5:56

Hi, I want to create an installshield for my Add-in like WndTabs.
http://www.codeproject.com/macro/wndtabs.asp[^]
After copying my add-in to "Microsoft Visual Studio\Common\MSDev98\AddIns" folder, what do I?

Peek stdin

peterchen22-Oct-04 5:50

peterchen

22-Oct-04 5:50

Hi,

Is it possible to "peek" stdin (i.e. check if any data is available) for a Win32 "kind of" console application?

Problem: A thread has to read stdin, but needs to react on an event

we are here to help each other get through this thing, whatever it is Vonnegut jr.
boost your code || Fold With Us! || sighist | doxygen

Re: Peek stdin

David Crow22-Oct-04 6:04

Please help me with MFC Dialogs

22-Oct-04 6:04

See if GetStdHandle(), and ReadConsoleInput() or ReadFile() are of any help.

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

Member 214873122-Oct-04 5:22

Member 2148731

22-Oct-04 5:22

Hi, I just made a program and I put a button in the dialog and called it IDAbout I need to get it so that when this button is clicked it open up another Dialog that I have made called IDD_ABOUTBOX. Can any body please help me write a code for the button so that it opens up the about box?

I am using MFC Dialog based project.

Thanks in advance. Smile | :)

David Crow22-Oct-04 5:33

22-Oct-04 5:33

Have you created a class for the IDD_ABOUTBOX dialog?

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

Member 214873122-Oct-04 5:34

Member 2148731

22-Oct-04 5:34

Yes.

David Crow22-Oct-04 5:37

22-Oct-04 5:37

Ok, have you used ClassWizard to create a handler function for the clicking of the button?

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