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You are talking about a standard virtual function, not a pure virtual function, which is not exaclty the same...
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thanks Tiwari for your reply,
I need little more clarification, if you dont mind.
See, we can achieve the extensibility through a normal inheritance also right . The main reason of inheritance is reusable and extensibility of my base class. Thats what my understanding is. So in this case why we need some function to be declared (make = 0) in base class and again define it in derived class etc etc....we can directly declare and define in the derived class right..
May be one practical example would be appreciated. Or may be a scenario where we cannot go without it.
Thanks
-----------------------------
I am a beginner
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It is used to define an interface that does not have any implementation.
Look at some design patterns - Design Patterns in C#[^].
They all use interfaces.
«_Superman_»
I love work. It gives me something to do between weekends.
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In addition to previous replies, the usage of pure virtual functions is needed:
- when your base class should not be instanciated: if a class contains a pure virtual function, you cannot instanciate it.
- when you want to enforce child classes to override the virtual function: if a child class doesn't override the pure virtual function, it cannot be instanciated.
In that specific case, it makes a lot of sense to use pure virtual functions: if you later add some child classes to the base class, the compiler will make sure you provide the required functions.
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thanks again,
but i guess you are talking about the points/feature or what it is.
May be it will be great if you could give an example where we cannot go without it.
-----------------------------
I am a beginner
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Suppose you are creating a class to log messages. The class first formats the message properly (adding current time for instance). This class can then be implemented by different children depending on where you want to log the message: to a file, to the console, ...
So, the base class looks like:
class CLogger
{
public:
void LogMessage(const string& message)
{
string formattedMessage = FormatMessage(message);
OutputMessage(formattedMessage);
}
protected:
string FormatMessage(const string& message) { }
virtual void OutputMessage(const string& message) = 0;
};
The class cannot be instanciated, because it contains a pure virtual function (OutpuMessage). If you want to create children of this class, you only have to override the OutputMessage function (the other functions are already provided by the base class):
class CConsoleLogger : public CLogger
{
protected:
void OutputMessage(const string& message) { }
};
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#include<iostream>
class IDumpable
{
public :
virtual void dump()=0;
};
class A: public IDumpable
{
int _a;
public:
A(int a):_a(a){}
virtual void dump(){ std::cout << _a << std::endl;}
};
class B: public IDumpable
{
static int _counter;
double _d;
int _id;
public:
B(double d):_d(d){_id=B::_counter++;}
virtual void dump(){ std::cout << "(" << _id << ") " << _d << std::endl;}
};
int B::_counter=0;
void dump( IDumpable * pid[], int size)
{
int i;
for (i=0; i<size; i++)
{
pid[i]->dump();
}
}
void main()
{
IDumpable *pid[3];
pid[0] = new A(10);
pid[1] = new B(5.7);
pid[2] = new B(10.745);
dump(pid, sizeof(pid)/sizeof(pid[0]));
}
Here, the function dump can iterate on the passed array without bothering about the exact type of the array elements.
Note that, once the IDumpable interface is written, we can start coding its consumers (for instance the dump function) even if the actual implementations of the interface are not yet provided.
If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong.
-- Iain Clarke
[My articles]
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For example, if you have a Shape base class in a drawing program with a Draw method the Draw method will be pure virtual - each Shape will need to draw itself but there is no appropriate default behaviour.
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thanks
one of my friend gave me the same example ..could u please give the class diagram, what u meant by exactly....and from the main function how do we call to draw different shape(say triangle or cube or rectangle)....just teh skeleton of the classes and calling methodology if u dont mind
-----------------------------
I am a beginner
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See the message above from CPallini
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Have a look at this extremely well authored article:
Generic Picker Dropdown Control[^]
You see a complex base class, with some virtual methods. To make different kinds of drop downs, You just have to implement some simple virtual methods, and the base class will do the heavy lifting.
It's similar to the Shapes examples that float about, but it's an actual implementation.
Give it a go, make your own picker class in the demo project.
Iain.
I have now moved to Sweden for love (awwww).
If you're in Scandinavia and want an MVP on the payroll (or happy with a remote worker), or need cotract work done, give me a job! http://cv.imcsoft.co.uk/[ ^]
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I want to distinguish between a Shutdown event and a Restart event , what should be done in achieving the same ? As , WM_QUERYENDSESSION doesnot give me any differentiation between Shutdown and Restart. Is there a way to know specifically which one of the two has been initiated ??
Regards,
Kushagra
I hate coding but I luv to develop.
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According to the docs (WM_QUERYENDSESSION Message[^]) it is not possible.
Read the explanation of lParam.
«_Superman_»
I love work. It gives me something to do between weekends.
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Actually I have seen many commercial applications that some how internally know about it ...that is why the question arises ... and If they know it ..we should also be able to device the same 
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Somebodu please help me out in this
Kushagra
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Kushagra Tiwari wrote: and If they know it ..we should also be able to device the same
Perhaps they are using a device driver of some sort (ie, ring 1 or 2).
"Old age is like a bank account. You withdraw later in life what you have deposited along the way." - Unknown
"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons
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No, it's possible : everything is possible...
For undocumented stuff, see Adv. Win32 grp (news://nntp.aioe.org/comp.os.ms-windows.programmer.win32)
(this had been posted in ~1994...)
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Hello people!
I am a new boost user, and I am trying to implement a console for a
game, which binds console commands to functions. These functions can
come from different objects and this is why I decided to try using boost.
I am using Visual Studio 2005 + SP1.
I have something like this:
typedef boost::function1< void, std::vector<std::string>& > funcHolder;
std::map<std::string, funcHolder> commands;
void Console::addCommand(const String &command, funcHolder f)
{
std::map<String, funcHolder>::iterator t = commands.find(command);
if(t == commands.end()) commands.insert( make_pair(command, f) );
else commands[command] = f;
}
All of these things are members of a Console class. So once I added
these, I tried adding this line to the constructor:
addCommand("clear", boost::bind(&Console::clear, this);
void Console::clear( vector<std::string>& args )
{
lines.clear();
}
During compilation I get C4180 warnings. I have heard that those are due
to VS8.0 SP1 compiler bugs, so I ignore them but then I get a following
error:
Project : error PRJ0002 : Error result 1 returned from 'C:\Program
Files\Microsoft Visual Studio 8\VC\bin\cl.exe'.
Compiler basically crashes.
Any ideas what might be wrong?
I thank everyone in advance,
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fley_fly wrote: typedef boost::function1< void, std::vector<std::string>& > funcHolder
With VS2005, you can use this syntax for boost::function - note you don't need to tell it the number of arguments - you just use function declaration syntax...
typedef boost::function< void(std::vector<std::string>&)> funcHolder;
fley_fly wrote: addCommand("clear", boost::bind(&Console::clear, this);
This line's missing a closing bracket at the end.
fley_fly wrote: void Console::addCommand(const String &command, funcHolder f)
I presume String should be std::string?
Finally - how is the function called? It will need a Boost ref wrapper[^] around the vector argument, to make sure a reference gets passed through to the function you want to call for that command.
Java, Basic, who cares - it's all a bunch of tree-hugging hippy cr*p
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Hi,
I want to set "Follow up flag" to some contact items but I am not able to find the method/property to set it.
I am using Outlook XP.
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I have a simple code where a user moves a window and when the user releases the window, a message box is displayed.
here is my code for that.
<br />
void CFldWnd::OnLButtonDown(UINT nFlags, CPoint point)<br />
{ <br />
SendMessage(WM_NCLBUTTONDOWN, HTCAPTION);<br />
CWnd::OnLButtonDown(nFlags, point);<br />
}<br />
<br />
void CFldWnd::OnLButtonUp(UINT nFlags, CPoint point)<br />
{<br />
<br />
MessageBox("Button up"); <br />
CWnd::OnLButtonUp(nFlags, point);<br />
<br />
}<br />
the window moving part works fine but when i release the left mouse button, messagebox is not displayed. but when i double click on the window, message box is displayed.
please help!!!! thanx in advance!
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Try listening for WM_NCLBUTTONUP, but this would trigger also when the user simply clicks somewhere in the non-client area of the window.
> The problem with computers is that they do what you tell them to do and not what you want them to do. <
> Sometimes you just have to hate coding to do it well. <
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If the mouse is over the window, it will receive WM_LBUTTONUP, if the mouse is outside - it will not. To overcome this situation, you need to 'SetCapture' on WM_LBUTTONDOWN and 'ReleaseCapture' on WM_LBUTTONUP...
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TThank u 4 ur quick responses.. i tried both of them but the problem still exists.. I commented SendMessage(WM_NCLBUTTONDOWN,HTCATION); and put some variable assigning code there and both LBUTTONDOWN and LBUTTONUP triggers..
when put back SendMessage(WM_NCLBUTTONDOWN,HTCATION);, LBUTTONUP does not trigger..
i dont know wat to do..
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The mouse up message does happen - you can check this with Spy++.
But the window moving stuff happens in response to the WM_NCLBUTTONDOWN message you've manually sent. That starts a message loop looking for mouse messages that is not the MFC message loops. It moves the window until you let go of the mouse, or press escape.
If you want your window to be movable with the mouse, a far more elegant way of doing it is responding to the WM_NCHITTEST message.
class CMyMovableDialog : public CDialog
{
...
afx_msg LRESULT OnNcHitTest(CPoint point);
...
};
...
ON_WM_NCHITTEST()
...
LRESULT CMyMovableDialog::OnNcHitTest(CPoint point)
{
return HTCAPTION;
}
I hope that helps,
Iain.
I have now moved to Sweden for love (awwww).
If you're in Scandinavia and want an MVP on the payroll (or happy with a remote worker), or need cotract work done, give me a job! http://cv.imcsoft.co.uk/[ ^]
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