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I did this, and it successfully compiled.
Passed a pointer to class Y into class X.
Passed a pointer to class X into class Y.
When I try to use Y* in X, the compiler says Y is undefined.
When I try to use X* in Y, the compiler says X is undefined.
So, when using forward references, where does the compiler eventually get the definition of Y so it can be used as Y* in X? Is there some type of ordering of what is presented to the compiler?
Thanks
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Just to add a little more confusing detail:
If you just reference the object by pointer or reference (either inside the class data or in a function declaration) then you can get away with just a forward reference.
In these cases the size of the object is fixed -- the size of a pointer. And since you're not using the other class, it needs to know nothing about the functions in it...
If you declare an instance of class or override an class -- then you must include the header. In this case, the compiler needs to actually know something about the object to compile (like the size of the other class, what virtual functions it has, ... )
Sorry if this is a little confusing. The gist is -- don't include the header until the object is actually used. And always try to use a forward declaration.... if you can.
Oh, and always use "#pragma once" too
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~<br />
Peter Weyzen<br />
Staff Engineer<br />
<A HREF="http://www.soonr.com">SoonR Inc -- PC Power delivered to your phone</A>
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<blockquote class="FQ"><div class="FQA">Peter Weyzen wrote:</div>Sorry if this is a little confusing. The gist is -- don't include the header until the object is actually used. And always try to use a forward declaration.... if you can.</blockquote>
I always get myself into trouble by not keeping track of #include statements and try to resolve it by adding my #include statement into stdafx.h header.
Sometime it makes even more mess.
My question - is this a good way to code in MFC ?
For extreme - could one do all class forward declarations in stdafx.h?
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I think you'd do the best service if you "crafted" each header to include only what it needs. It should be able to stand on it's own....
But you should decide what that is -- for your project. There's no perfect way to do it.
One of my bosses would always complain about how long the project took to build. So, being the boss that he was, he demanded that all headers got included into stdafx... this caused them all to be precompiled and sped up the build.
The downside was that everything got built all the time as you changed headers....
It wasn't a particularly great style -- not something I thought highly of -- but it was the way we did things at that job.
Do what's works for you and your work environment.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~<br />
Peter Weyzen<br />
Staff Engineer<br />
<A HREF="http://www.soonr.com">SoonR Inc -- PC Power delivered to your phone</A>
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Vaclav_Sal wrote: For extreme - could one do all class forward declarations in stdafx.h?
Vaclav_Sal wrote: My question - is this a good way to code in MFC ?
Answers: No!, and Bad Practice!
Keeping Track of things is one of these things we have to do when writing code. Planning your Code and File Organisation is an important part of your design process.
Regards, 
Bram van Kampen
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Your compiler reads your source file line by line. the #include directive means exactly that: 'include' It is conceptually as if at that point the entire file to be included is copied and pasted to the location of the #include directive. What the compiler digests is your cpp file, with at all levels the #include directives removed and replaced by the contents of the file to be included. This is one of the functions of the Pre-Processor. (Note that the compiler proper knows nothing about header files, it compiles the cpp file after the Pre-Processor has expanded it.) So, the ordering of things depends on the ordering of your #include file directives, and on the order of the statements in your header files.
Similar Sample with Line Numbers:
1 class X;<br />
2 class Y{<br />
3 X* m_pX;<br />
4 };<br />
5 class X{<br />
6 Y* m_pY;<br />
7 };
The Class X is forward declared at line 1
The forward Declaration is used in line 3
The Class Declaration for class Y is completed on line 4
The Class Declaration for class X starts on line 5;
The Class declaration for class X uses class Y on line 6.This can be done without a forward declaration, because class Y is fully declared after line 4.
The Class Declaration for X completes at line 7.
If you try to do anything of the kind of *m_pX before the end of line 7 you get an error, you dereference something that has not been completely declared.
Hope this clarifies things.
N.B. A Note about '#pragma once'
It is not difficult to end up with a 'spagetti' of include files, and at some stage inevitibly a file would be included twice. This gives all sorts of problems, and the compiler usually chokes on this, but this situation is often hard to unravel.
The directive '#pragma once' tells the preprocessor to check if the particular file has already been included, and, if so to ignore the #include directive.
Best of Luck
Bram van Kampen
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The fog begins to clear a bit.
Question: In your example, all the statements are in one file. However, VC++ creates individual header files for each class. To emulate what you have shown, we would have to control of the order in whch the header files are presented to the compiler. Correct? Can we give the compiler the header.h files in the order it needs them while still maintaining the very convenient system of one header file per class? Is there a way to do this?
Thanks
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/////////////////////////////
// Header File X.H
#pragma once;
class Y; //Fwd Decl, Add by Hand
class X{
Y* m_pY;
};
////////////////////////////
////////////////////////////
// Header File Y.H
#pragma once;
class X;; //Fwd Decl, Add by Hand
class Y{
X* m_pX;
};
////////////////////////////
////////////////////////////
// Z.CPP
#include "stdafx.h"
#include "X.H"
#include "Y.H"
// Both Classes X and Y are properly declared here
////////////////////////////
This way things will work whatever the order of the includes. One of the Forwards will technically be redundant, in this example the forward 'class X' This does not matter, the compiler simply ignores redundant forwards.
Note however, that X.H and Y.H on their own are never usable.  For this reason it is probably better to declare both classes in the same header. When you use ClassWizzard to create a new class, the wizzard gives default file names based on the class name. You can change these names, and can direct the generated file to any of your choice. And, Yes, you can direct more than one class definition to a single header file.
Hope this helps,
Bram van Kampen
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I combined the header files and everything works.
First I created class X with X.cpp and X.h
Then created class Y with Y.cpp, but changed the default header file to X.h. So both class X and class Y had their headers in the same file. Forward reference worked.
Both X and Y are created in Z. Z then passes pointers to X and Y by invoking a function of X and a function of Y.
In X, a pointer to Y allows me to execute Y functions.
In Y, a pointer to X allows me to execute X functions.
The only peculiarity is when creating functions in Y. I right click Y in the class window in the workspace, then select ADD MEMBER FUNCTION. The new function is declared in class Y in file X.h, but the implememtation goes into X.cpp. Deleting the skeleton implementtion in X, and pasting it into Y.cpp solves the problem.
Thanks for the help.
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Hi,
I need to define a datatype with a size of 4 bit within a struct.
char has a size of 8 bit and thats the smallest type I know ...
So what else should I do? I really dont know how to search for this, sorry.
btw: I think I saw something like "unsigned short varName:4" in a code-snipet.
Does this mean the varName holds 4bit of data?
Thanks in advance!
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_NielsB wrote: So what else should I do?
Something like:
struct
{
char c:4;
};
"Normal is getting dressed in clothes that you buy for work and driving through traffic in a car that you are still paying for, in order to get to the job you need to pay for the clothes and the car and the house you leave vacant all day so you can afford to live in it." - Ellen Goodman
"To have a respect for ourselves guides our morals; to have deference for others governs our manners." - Laurence Sterne
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DavidCrow wrote: Something like:
struct{ char c:4;};
This is indeed a respected method inkeeping with the language. However it also depends on what the four bit data type is to be used for, Is packing on a Four Bit boundary required.If a lot of manipulating code is to be written, or an array of nibbles needs to be addressed, maybe a more transparent way is Masking and Shifting, and a Raft of Macro's to cover it.
e.g.:
#define LO_NIBBLE(x) ((x)&0xFF)
#define HI_NIBBLE(x) (((x)>4))&0xFF)
#define GET_ARR_ITEM(Arr,N) ((((N)&1)==0)?LO_NIBBLE((Arr)[(N)/2]:HI_NIBBLE((Arr)[(N)/2])
And so on ad infinitum
Even further, depending on the importance and complexity of the grander problem to be solved, (and the Time and resources available)one could conceive of a new class, CNibble with an entire gammet of overloaded operators. Come to think of it, why not also a Class CHalfNibble and CBit.
Hope this is of help,
regards
LateNightsInNewry
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Dear Friends,
I want to programatically change a resource (bitmap) in a dll. Ofcocurse, i do not have a source code of that dll. Can anyone tell me how to do this kind of work.. what APIs are required or if there is any sample article/software you aware of doing such thing; then tell me please.
cheers...
Imtiaz
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See Here[^]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~<br />
Peter Weyzen<br />
Staff Engineer<br />
<A HREF="http://www.soonr.com">SoonR Inc -- PC Power delivered to your phone</A>
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Hello,
I have an Microsoft Outlook style application - a splitter window with a TreeView derived class in the left pane, and , at different times, one of various CView derived class in the right pane.
When the user resized the main application window, eventually the CFormView derived class will display scrollbars. I'd prefer for it to never display scrollbars. Can this be done? How?
Regards,
Sternocera
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If the control bars appear, it means that some working sufrace of that view is not being displayed, so if you want to remove the controlbars, you may lose some working area in your view.
Possible, it is possible, but I don't know why do u want to.
I can think on 2 possibilities.
First one... Change the visible area with SetScrollSize ()
Second one... take the moment you want with WM_SIZE, take a pointer to the scrollbars and disable/make them not visible.
Greetings.
--------
M.D.V.
If something has a solution... Why do we have to worry about?. If it has no solution... For what reason do we have to worry about?
Help me to understand what I'm saying, and I'll explain it better to you
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This has done it for me:
void CsplittertestView::OnInitialUpdate()
{
CFormView::OnInitialUpdate();
GetParentFrame()->RecalcLayout();
ResizeParentToFit();
SIZE mysize; mysize.cx = 1; mysize.cy = 1;
SetScaleToFitSize(mysize);
}
Thanks,
Sternocera
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 I didn't think bout this possibility. 
I love this forum because trying to help one learn new things too 
Greetings.
--------
M.D.V.
If something has a solution... Why do we have to worry about?. If it has no solution... For what reason do we have to worry about?
Help me to understand what I'm saying, and I'll explain it better to you
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I can't find anywhere to add a grid to my dilog window.
Is this due to my older version of c++?
What am I missing here?
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hi there,
i have coded an aplication that talks to a relay board via the parallel port on my pc,it has eight relays on it that effectivly turns things on or off upon recipt of a command.
so what i want to be able to do is set an on and off sequence for each individual relay on the board,using some sort of timer as a trigger.
i think i will need eight timers that are in sync with the sytem clock and will trigger when the values match
can anybody point me in the right dirction as how to achive this
chris
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If you are using MFC, take a look in CWnd::SetTimer (...) and KillTimer (...)
without MFC i have read the function sleep sometimes, but to make a pause I dont know if it triggers code
Greetings.
--------
M.D.V.
If something has a solution... Why do we have to worry about?. If it has no solution... For what reason do we have to worry about?
Help me to understand what I'm saying, and I'll explain it better to you
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Hello
I have written a small application which reads the registry keys present in a .txt file and tries to export each key using regedit, i.e. I read each line of the file (which is a key) and using the commandline for regedit (regedit /e "target file" "source key") I export it into a folder creating a new .reg file for each key read from the file.
Now in my .txt file there are around 11,500 keys. The keys are successfully exported till 5450, but after that calling CreateProcess on regedit returns error no. 1450 (no system resources) and all the subsequent keys are not exported into the target folder.
What could be the reason for this error to come up? And how should I change the logic so that this error doesnt come up and all the 11500 keys are exported as different .reg files into my target folder?
Thanx.
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vipin_nvk wrote: no system resources
Are you closing the processes after each one is used. Windows has a resource limit of 10,000 resource handles 10,000 limit[^](or something like that) per process and if your spawning outside processes, they may count against your main process. I'm not sure how the rules work with spawned processes.
However, be informed that a resource handle can include pens, brushes, windows, etc... and depending on the type of windows or controls being created, you can hit the resource limit pretty quick.
Try closing things that might maintain handles to resources once you are finished with them as it sounds like their being maintained somehow.
Here are some links about resource handle limits...
Concerns about resource handles and Raymonds words on the matter[^]
Window Manager limits[^]
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Hi every one;
This is my first time here. and I faced problem with Visual C++ 2005.
the problem is:
I want to read 6 keys from keyboard as same time. for example (zxcnm,). because I will replace them with dots. that means if the user press c and z it means dot one and dot 3.
may any one help me in this topic. please
Thanks
--------------------------------------------------------------------------------
allooba
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Is this a console or a GUI application? If the former, check out getch(). If the latter, check out the WM_CHAR message.
"Normal is getting dressed in clothes that you buy for work and driving through traffic in a car that you are still paying for, in order to get to the job you need to pay for the clothes and the car and the house you leave vacant all day so you can afford to live in it." - Ellen Goodman
"To have a respect for ourselves guides our morals; to have deference for others governs our manners." - Laurence Sterne
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