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class c{
public:
a();
b();
};
main()
{
class c *pc = 0;
pc->a() //works
}
class c{
int i;
public:
c(0):i=0;
a();
b();
};
main()
{
class c *pc = 0;
pc->a(); //doesn't work fires exception
}
why? 
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Because you are invoking a function (a()) on a pointer that was specifically made invalid (set to 0).
Who is 'General Failure'? And why is he reading my harddisk?!?
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What's the definition of function a()?
Contrary to popular belief, while you can call a member function through a null pointer, it only works if you don't need to access any member variables or virtual functions (essentially, you don't need to dereference this).
If a() uses the i integer in the second example, then you are trying to access a variable through a dereferenced null pointer, which obviously will fall over.
--
Ian Darling
"The moral of the story is that with a contrived example, you can prove anything." - Joel Spolsky
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Bravo![Rose | [Rose]](https://www.codeproject.com/script/Forums/Images/rose.gif)
Live as if your were to die tomorrow. Learn as if you were to live forever.
-Mahatma Gandhi
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Can U clarify on the matter of this. Please, I know what U told is correct but not able to understand 'you don't need to dereference this'.
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SiddharthAtw wrote:
Please, I know what U told is correct but not able to understand 'you don't need to dereference this'.
When you call a member function, that function has access to an intrinsic pointer called "this", which is a pointer the object you called the method on.
So in a hypothetical example:
int main()
{
c myObject;
myObject.a();
c *myPointer = new c;
myPointer->a();
c *myNullPointer = 0;
myNullPointer->a();
}
void c::a()
{
this->i = 1;
i = 1;
}
--
Ian Darling
"The moral of the story is that with a contrived example, you can prove anything." - Joel Spolsky
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Thanx for making it Crystal Clear.
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Not tricky, it is just basic. Second case will not work... how can you initalize the member variable 'i' while declaring it in the class c,u have to use constructor to initialize it. There is nothing to do with null class pointer. In both cases u have to define the functions a() and b() before calling them.
when going gets tough the tough gets going
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I agree with U. But my intention was about null pointer thing. If U have any please tell me
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As far as VC++ 6.0 is concerned it is working i.e. null pointer can be used to call the member function but it is not advisable. it may work in few compilers but it is not standard, u shud always initialize the pointer to some valid memory location of this class C type that is what we call initialization of pointer. it could be any pointer for that matter u shud make it point to a valid memory location of its type. when u go for big programs, in the sense, if u use lot of pointers with huge data and dereference it, u may get into Access violation errors and some ugly errors. Please feed the pointers with valid memory location(food) and ask it to work.
when going gets tough the tough gets going...
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You are using undefined behavior, that is why it doesn't work.
Calling a member function through a null pointer is invalid,
so the fact that two different invalid behaviors are different
is a minor matter of compiler specific details.
To put it another way, both pieces of code are not valid C++,
and consequently the discussion of different behaviors is irrelevant,
Visual Studio 8.0 could give you completely different results (in
fact a good C++ lint program would detect this error and report it
to you.)
If it is your goal to call a class defined function without a
specific instance of the object you need to use a static member
function, viz:
class c { public: static void a(); };
main()
{
c::a();
}
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hello,
here is a prob.
suppose i will print a document. the printer spoller collects all the datas & my program in WIN32 can trap all the datas in the spoller.
here i m getting total no of pages in one document for a particular print job.but i want the following...
1. suppose i will give 2 copies (more than 2 copies) in the
print dialog and put sufficient paper in tray than it is not giving the total no of pages to be printed. WHY /???
2. if any one knows how to calculate the total no of pages printed for a job (with single copy as well as multiple copies) than please help in this regards....
here i m taking ENUMPRINTERS & ENUMJOBS APIs for all the information .
i m getting all the spoller information in ENUMJOBS.except the no of pages ....i m testing in samsung ML-4600 PCL 6 printer.
is the page count is printer specific ? i dont think so ...
otherwise no apis should have a data like pagecount...or so..
if any one knows please try to solve this prob 7 quickly inform me ...
if any information is required from my side ,,,i will help...
regards
skpanda
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Hi all,
i've added an item to the 'File' meun of outlook.
i can set the property 'ShortcutText' to be something like "Ctrl+Shift+M".
if i do that, i see this string near the menu item, when i open the file menu.
problem is when i press 'Ctrl+Shift+M' combination nothing happens?
what should i do?
thanks
Ask not what your application can do for you,
Ask what you can do for your application
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Hai guts.,
I had an problem while indexing an mdb file with dao connection
int CTruckRecord::OpenFiles()
{
try
{
//open the database
Dccdb.Open("\\ibp-naga\\naga\\dbs\\dccclient.mdb");
//open the individual table
TruckTbl.Open("TruckRecord");
//to retrive records from the database table, you should open it
CDaoRecordset::Open(&TruckTbl);
//to set current index for table type record sets,
SetCurrentIndex("TRK_NDX");
}
catch( CDaoException* e )
{
//an exception object for DAO classes which will report error occured
//and delete it afterwards immediately. These are member functions of
//the CException class, the parent class for CDaoException
e->ReportError();
e->Delete();
return FALSE;
}
return TRUE;
}
While executing the above code. The exception handling catch an error at the line
SetCurrentIndex("TRK_NDX");
Display the error as below:
"TRK_NDX@ isn't an index in this table. look in the indexes collection of the tabledef to determine the valid index names"
Any one could say wat this line mention.
Thanx in advance.
bye.,
Shanmuga Sundar.V
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i am student at graduate level and i have a need of projects ideas in clips (A.I).
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A.I?
Stock trading systems,
Power switches
Router links status info
CPU branch predictions
MxN queues (M people in N lines at the register)
Traffic control sytems.
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Do anyone know how to link a Visual C++ program to a interface which is created by Visual basic 6?
i use visual C++ to create a random letter, then i need it to be able to output in my visual basic 6's interface..
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You could always man that massive gun named COM and aim it at your problem...
Who is 'General Failure'? And why is he reading my harddisk?!?
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Write your VC app as dynamic link library (DLL)
and export the function u want.
Sonork 100.41263:Anthony_Yio
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when i compiled some program,
the output in debug show :
"You are not using the latest Platform SDK header/library "
"files. This may prevent the project from building correctly."
failed to compile it.
and the origin code is:
#ifndef WT_EXECUTEINPERSISTENTIOTHREAD
#pragma message("You are not using the latest Platform SDK header/library ")
#pragma message("files. This may prevent the project from buildingcorrectly.")
#endif
could u tell me how to solve my problem and explain what it's in?
Thank u!
Hello World!
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How can I retrieve the content of MFC Active Document Server?
I create my own editor using rich textbox. The editor is also an Activex container.
I can insert one Active Document Server in the rtf documnet. After saving the file as rtf format. I can not retrieve the content of rtf file for conversion to other format.
Can any one help on this regard..
Thanks in advance.
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How can I retrieve the content of MFC Active Document Server?
I create my own editor using rich textbox. The editor is also an Activex container.
I can insert one Active Document Server in the rtf documnet. After saving the file as rtf format. I can not retrieve the content of rtf file for conversion to other format.
Can any one help on this regard..
Thanks in advance.
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I am exporting a function something like show below:
extern "C" DllExport const char* ActivatePlot(CString szPlot);
When I call this function from another DLL or module, the application crashes while returning from the function "ActivatePlot".
From the call-stack I found that the control is in the destructor of CString object and is trying to free the memory using FreeStore().
Can you explain whether we can use CString object in a DllExport functions? And if not, why?
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CString uses dynamic memory allocation to store its strings. When you pass a CString object about, if it needs to re-allocate at any point, it will make use of the current heap manager to allocate the memory. If your doing this in MFC with the MFC library as a shared DLL, you should only have the one heap manager. If your not, then you will find that you are trying to de-allocate an allocation from one heap manager with an allocation from the other - which is probably the problem here. In such cases as this, can you not just pass in the szPlot as a const char * pointer? You need to avoid passing about objects that will do this.
Roger Allen - Sonork 100.10016
If your dead and reading this, then you have no life!
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