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Dear Chris, the first winner,
thank you very much, it works!!! (no sun today here...)
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Dear Mike,
thank you very much, it works!!! (no sun today here...)
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Hello,
Class 'Base' has abstract operations that I want to call in the 'Base' constructor; the implementation of the abstract operations should be in Class 'Derived'.
It doesn't work. How can I solve this?
I tried the approach of not using abstract base operations as well. Then only the base operations but not the derived operations will be called.
class Base {
public:
Base(){
a();
b();
};
virtual ~Base();
protected:
virtual void a() = 0;
virtual void b() = 0;
};
class Derived {
protected:
void a(){ }
void b(){ }
};
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Oh sorry, I forgot to implement the inheritance in the example code. But it's existing in the real code.
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boogalo79 wrote: Then only the base operations but not the derived operations will be called
how can you say that only the Base class member functions are called, when they are pure virtual function (no body to define them...) 
TOXCCT >>> GEII power
[VisualCalc 3.0 updated ][Flags Beginner's Guide new! ]
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You will not get the result you hope for if you call a virtual function from a constructor. It's because the virtual table isn't fully set up until the contructor call returns. There is nothing you can do about it. Try something like:
class Base {
public:
base(){}
virtual void Init() { a(); b();}
};
...or make an object factory class which does just that.
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Thank you,
I implemented your suggestion. Now it's working.
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Oh..  really good one. ![Rose | [Rose]](https://codeproject.freetls.fastly.net/script/Forums/Images/rose.gif)
Nice talking to you. 
If you judge people, you have no time to love them. -- Mother Teresa
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Even after you've implemented toxcct's solution, calling virtual functions in a constructor is bad practice and a recipe for disaster. Consider the following code:
#include "stdafx.h"
#include <iostream>
using namespace std;
class Base
{
public:
Base()
{
LogClass();
}
virtual LogClass()
{
cout << "Base" << endl;
}
};
class Derived : public Base
{
public:
virtual LogClass()
{
cout << "Derived" << endl;
}
};
int main(int argc, char* argv[])
{
Derived d;
d.LogClass();
return 0;
}
The output of this program is:
Base
Derived
And not, as you may expect:
Derived
Derived
There are languages where the second is produced in code like this (like C# I believe); they've got it wrong. The reason for this behaviour is that the constructor of a class builds the environment the member functions of that class require; they setup the classes’ invariants. For example for a logging class it may open a file and set a member variable to the file handle; all the classes' functions expect it. This means it is wrong to call a classes member functions before its constructor has been called. In our example the class Derived is only partially constructed; the Derived constructor hasn't been called yet so we can't call its member functions; the virtual function call mechanism makes sure this can't happen accidently.
Steve
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The compiler adds a default constructor to a class that does contain any constructor. Here also the constructor adds a default constructor to the class Derived. In inheritance, when th e derived class object is created,
first the base class constructor will be called and then the derived class construcot will be called. Here the base class constructor involves a call to the local LogClass() hence first 'Base' will be printed and then d.LogClass() prints 'Derived'.
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Hi,
Im pretty new to C++.. been messing with the standard library for a while..
but im in need of some help here.. i apologise i dont really have decent code
to show.. as i said im new... Anyway.. the problem is im getting some strange imput from the command line console some after the accepting socket...
heres what i get...
C:\>winsock
Listening on Socket
USER test
q▲∟¼q↔¼q^↔¼qzv½qPëV
PASS test
qi ¼q2è½q►=½q╡↔¼qµ@½qUSER test
q▲∟¼q↔¼q^↔¼qzv½qPëV
Listening on Socket
Yeah.. my computer beeps and everything.. some strange mating call of FTP i presume... basically i was wondering what that is.. and how to recieve data in only regular text format after sending data... so i can process the login and other ftp commands...
heres what i get in the client so far for this code...
[R] Connecting to 127.0.0.1 -> IP=127.0.0.1 PORT=21
[R] Connected to 127.0.0.1
[R] 220 Welcome to my Custom FTP
[R] USER test
[R] 331 Password Required
[R] PASS (hidden)
[R] 230 Welcome to my Custom FTP.
[R] SYST
[R] QUIT
please have a look at the code... im using devc++ and it does compile ok...
without further wait.. heres what ive got so far...
#include <winsock2.h>
#include <string.h>
#include <iostream.h>
using namespace std;
int accept(SOCKET AcceptSocket, SOCKET ListenSocket);
int main() {
int sk;
WSAData wsaData;
WORD wVersionRequested;
wVersionRequested=MAKEWORD(2, 0);
WSAStartup( wVersionRequested, &wsaData );
sockaddr_in service;
service.sin_family = AF_INET;
service.sin_addr.s_addr = inet_addr("127.0.0.1");
service.sin_port = htons(21);
int test;
while(1){
SOCKET ListenSocket;
ListenSocket = socket(AF_INET, SOCK_STREAM, 0);
if (bind( ListenSocket,
(SOCKADDR*) &service,
sizeof(service)) == SOCKET_ERROR) {
cout << "bind() failed.\n";
closesocket(ListenSocket);
return 1;
}
cout << "Listening on Socket\n";
SOCKET AcceptSocket;
sk=listen( ListenSocket, 10 );
AcceptSocket = accept( ListenSocket, NULL, NULL );
test=accept(AcceptSocket, ListenSocket);
#ifdef WIN32
Sleep( 5000 );
#endif
if(test==2){break;}
}
WSACleanup(); return 0;
}
int accept (SOCKET AcceptSocket, SOCKET ListenSocket) {
int bytesSent;
int bytesRecv;
int test;
char sendbuf[1024] = "220 Welcome to my Custom FTP\n";
char* recvbuf = "";
char user[32];
char pass[32];
bytesSent=send( AcceptSocket, sendbuf, strlen(sendbuf), 0 );
while(1) { bytesRecv=recv( AcceptSocket, user, strlen(user), 0 ); break;}
#ifdef WIN32
Sleep( 50 );
#endif
char newsendbuf[1024]="331 Password Required\n";
bytesSent=send( AcceptSocket, newsendbuf, strlen(newsendbuf), 0 );
while(1) { bytesRecv=recv( AcceptSocket, pass, strlen(pass), 0 ); break;}
//HERE IS WHERE I COUT USER AND PASS and wierd result
cout << user << "\n" << pass << "\n";
Sleep( 50 );
char newendbuf[1024]="230 Welcome to my Custom FTP.\n";
bytesSent=send( AcceptSocket, newendbuf, strlen(newendbuf), 0 );
Sleep( 50 );
closesocket(ListenSocket);
}
// ANY HELP IS APPRECIATED THNX IN ADVANCE 
-- modified at 10:26 Monday 29th May, 2006
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hello
I've found my silly mistake
please help me find error in my code :
class Base
{
public :
virtual ~Base() {};
virtual void getX() const
{
cout<<endl<<"Inside Base : "<<m_X<<endl;
};
protected :
int m_X;
};
class Derived1 : public Base
{
public :
Derived1()
:Base()
{
m_X = 100;
};
virtual void getX() const
{
cout<<endl<<"Inside Derived1 : "<<m_X<<endl;
};
};
int _tmain(int argc, _TCHAR* argv[])
{
try
{
throw Derived1();
}
catch (Base *pError)
{
pError->getX();
}
return 0;
}
sorry for stupid question
I am pretty tired, need a good sleep
-- modified at 9:42 Monday 29th May, 2006
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big_denny_200 wrote: virtual void getX() const {
cout<<endl<<"inside base="" :="" "<<m_x<<endl;
}<b="">;
don't close your functions definitions with semicolons (';').
at last, just curious... don't you understand what the compiler says when complaining ?! 
i have no compiler to test here, so, sorry.
TOXCCT >>> GEII power
[VisualCalc 3.0 updated ][Flags Beginner's Guide new! ]
-- modified at 9:45 Monday 29th May, 2006
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no 
why not use semicolons ?
thanks
-- modified at 9:34 Monday 29th May, 2006
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big_denny_200 wrote: why not use semicolons ?
and you didn't answer my questions more.
* What are the compiler errors ?
TOXCCT >>> GEII power
[VisualCalc 3.0 updated ][Flags Beginner's Guide new! ]
-- modified at 9:45 Monday 29th May, 2006
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toxcct wrote: because the C++ doesn't want them here
In fact, this won't change anything. You can let them there. They will simply be ignored.
Cédric Moonen
Software developer
Charting control
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Cedric have u seen my post? I think that is the reason for his assertion?
SaRath
"D on't blindly follow rules you read somewhere without verifying that it makes sense for your situation!"
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a function ending with ";" is simply compiling with visual studio .net and visual studio 6 C++ compiler.
SaRath
"D on't blindly follow rules you read somewhere without verifying that it makes sense for your situation!"
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it is not the problem with a ";"
problem is with ur try catch block;
big_denny_200 wrote: catch (Base *pError)
u r catching a pointer to the throwing object but inside the
try block u r doing
big_denny_200 wrote: throw Derived1(); // when I add this line I get assert
so inside the throw block do as follows
Derived1 d; // when I add this line I get assert
throw &d;
or
throw &Derived1(); // when I add this line I get assert
u can also resolve if u change the catch block to pass by value instead pass by reference
SaRath
"D on't blindly follow rules you read somewhere without verifying that it makes sense for your situation!"
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SaRath C wrote: Derived1 d; // when I add this line I get assert
throw &d;
Never do this as it’s seriously flawed. The object d is on the stack but the stack is unwound when an exception is caught. The upshot is that when you catch an exception with such code the exception handler gets a pointer to an object which no longer exists. It's the same situation as returning a reference or pointer to a local variable from a function.
Steve
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You're throwing a Derived1 but catching a Base*. If you want to catch a Base* you need code like this:
try
{
throw new Derived1();
}
catch (Base *pError)
{
pError->getX();
delete pError;
}
A better was is not to throw a pointer. Use code like this:
try
{
throw Derived1();
}
catch (const Base &e)
{
e.getX();
}
Steve
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Probably you have seen "winrar" to add "Add to archive" type menuitem to default right-click pop-up menu. I want to add this feature to my file conversion software. After some searching on the internet, I think I have to modify the Windows Explorer context menu. I don't know how to do it. Does anyone know how to do it?
Tamal Saha,
Student,
Bangladesh University of Engineering and Technology.
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