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How to create an initialised object declared as a class member variable?

P: n/a
CFF
I am working on a VC6++ project that involve an object to be initialised
by a 'this' pointer pointing to another object. I encountered, however,
a syntax error. I wonder if someone can help. See the code below. What I
couldn't figure out is it is OK with

CMyClassA B2(this);

in fileB.cpp but a SYNTAX ERROR with

CMyClassA m_B2(this);

in fileB.h. What should I do in order to create a initialised object of
CMyClassA if I really need a class member variable (m_B2) rather than a local (B2)?

Thanks for any help.
/////////////////////////// fileA.h ///////////////////////////
//
// forward declaration
class CMyClassB;

// CMyClassA declaration
class CMyClassA{
public:
CMyClassB* m_ptA;

public:
CMyClassA();
CMyClassA(CMyClassB* ptB);
};
/////////////////////////// fileA.cpp /////////////////////////
//
#include "fileA.h"

// constructor 1
CMyClassA::CMyClassA(void){
// ... whatever ...
}

// constructor 2
CMyClassA::CMyClassA(CMyClassB* ptB){
m_ptA = ptB;
}
/////////////////////////// fileB.h ///////////////////////////
//
#include "fileA.h"

class CMyClassB{
public:
CMyClassA m_B1; // no problem
CMyClassA m_B2(this); // syntax error : 'this', WHY???

public:
CMyClassB(void);
};
/////////////////////////// fileB.cpp //////////////////////////

#include "fileB.h"

CMyClassB::CMyClassB(void){
CMyClassA B1; // no problem
CMyClassA B2(this); // no problem
}
int main(void){
// ... whatever ...
return 1;
}
Jul 22 '05 #1
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5 Replies


P: n/a

Try this


/////////////////////////// fileB.h ///////////////////////////
//
#include "fileA.h"

class CMyClassB{
public:
CMyClassA m_B1; // no problem
CMyClassA m_B2; // we won't call constructor here

public:
CMyClassB(void);
};
/////////////////////////// fileB.cpp //////////////////////////

#include "fileB.h"

CMyClassB::CMyClassB(void)
: m_B2(this) // we will call it here instead ( hope there is no problem with passing 'this' pointing to not fully constructed object ){
CMyClassA B1; // no problem
CMyClassA B2(this); // no problem
}
int main(void){
// ... whatever ...
return 1;
}

Jul 22 '05 #2

P: n/a
CFF wrote:

/////////////////////////// fileB.h ///////////////////////////
//
#include "fileA.h"

class CMyClassB{
public:
CMyClassA m_B1; // no problem
CMyClassA m_B2(this); // syntax error : 'this', WHY???


Because this is not the way initializations are specified in C++
What you need is a constructor for CMyClassB, which does the initialization
CMyClassB() : m_B2( this ) {}
--
Karl Heinz Buchegger
kb******@gascad.at
Jul 22 '05 #3

P: n/a
Karl Heinz Buchegger wrote:

CFF wrote:

/////////////////////////// fileB.h ///////////////////////////
//
#include "fileA.h"

class CMyClassB{
public:
CMyClassA m_B1; // no problem
CMyClassA m_B2(this); // syntax error : 'this', WHY???


Because this is not the way initializations are specified in C++
What you need is a constructor for CMyClassB, which does the initialization

CMyClassB() : m_B2( this ) {}


Oh. I see you already have a constructor. So add the initialization
to that constructor
--
Karl Heinz Buchegger
kb******@gascad.at
Jul 22 '05 #4

P: n/a
CFF
"kamil" <ka*******@xxxpoczta.onet.pl> wrote in message news:<#5*************@TK2MSFTNGP12.phx.gbl>...
Try this


/////////////////////////// fileB.h ///////////////////////////
//
#include "fileA.h"

class CMyClassB{
public:
CMyClassA m_B1; // no problem
CMyClassA m_B2; // we won't call constructor here

public:
CMyClassB(void);
};
/////////////////////////// fileB.cpp //////////////////////////

#include "fileB.h"

CMyClassB::CMyClassB(void)
: m_B2(this) // we will call it here instead ( hope there is no problem

with passing 'this' pointing to not fully constructed object )
{
CMyClassA B1; // no problem
CMyClassA B2(this); // no problem
}
int main(void){
// ... whatever ...
return 1;
}


Thank you for your help. I've tried it out and it works. But ... I am
getting a warning message saying

"warning C4355: 'this' : used in base member initializer list"

when complied. I am a bit worry about this. Any idea about what's the
problem with it? Does it cause any undersirable effect? Thanks again.
Jul 22 '05 #5

P: n/a
I think as long as you don't try to manipulate the half-constructed
object, you're fine. I believe I do this very thing in some piece of
software I wrote, and I have seen no ill effects. The compiler is just
reminding you... you can always disable the warning with a #pragma.

CFF wrote:
"kamil" <ka*******@xxxpoczta.onet.pl> wrote in message news:<#5*************@TK2MSFTNGP12.phx.gbl>...
Try this
/////////////////////////// fileB.h ///////////////////////////
//
#include "fileA.h"

class CMyClassB{
public:
CMyClassA m_B1; // no problem
CMyClassA m_B2; // we won't call constructor here

public:
CMyClassB(void);
};
/////////////////////////// fileB.cpp //////////////////////////

#include "fileB.h"

CMyClassB::CMyClassB(void)
: m_B2(this) // we will call it here instead ( hope there is no problem


with passing 'this' pointing to not fully constructed object )
{
CMyClassA B1; // no problem
CMyClassA B2(this); // no problem
}
int main(void){
// ... whatever ...
return 1;
}

Thank you for your help. I've tried it out and it works. But ... I am
getting a warning message saying

"warning C4355: 'this' : used in base member initializer list"

when complied. I am a bit worry about this. Any idea about what's the
problem with it? Does it cause any undersirable effect? Thanks again.

Jul 22 '05 #6

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