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Jun 30th, 2006

Pointer to a data member

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Hi,

I am bit confused with the concept of pointer to a data member of a class.
In the below program what actually will the pointer variable be storing? When i tried printing the value of address I got value 1, Whatt does it mean?

class A{
public:
int d;
};
void main(void)
{
int A:

p = &(A::d);
}

I tried creating an object of class A and assigning the address of the data member to p then also my VC compiler is throwing error 'cannot convert from 'int *' to 'int A:

''. Why this error is thrown when b is of type A. Same with the gcc compiler also.

class A{
public:
int d;
};
void main(void)
{
A bb;
bb.d = 10;
int A:

p = &(A::d);
p = &(bb.d);
}

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dilip.mathews

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Jun 30th, 2006

Re: Pointer to a data member

Where are you getting this from?

C++ Syntax (Toggle Plain Text)
 int A::*p = &(A::d);
 int A::*p = &(A::d);

In C you declare Type <variable name> ;
so to declare a pointer to an integer using the * dereference symbol you do this:

C++ Syntax (Toggle Plain Text)

int * p = &someintegervariable

try this:

C++ Syntax (Toggle Plain Text)
#include <iostream>
 
class A
{
 public:
         int x;
};
 
int main(int argc, char *argv[])
{
  A a;
  a.x = 10;
  int * p = &a.x;
  std::cout << "Memory = " << p << "\n";
  std::cout << "Value pointed to = " << *p << "\n";
  std::cin.get();
  return 0;
}
#include <iostream>

class A
{
 public:
         int x;
};

int main(int argc, char *argv[])
{
  A a;
  a.x = 10;
  int * p = &a.x;
  std::cout << "Memory = " << p << "\n";
  std::cout << "Value pointed to = " << *p << "\n";
  std::cin.get();
  return 0;
}

When we print p we get the value of p itself (which is a memory address where the value of x is being stored) When we print * p we dereference to that address and get the actual bits from that memory location 10 (remember you declare the pointer to be int so the compiler knows to get 4 bytes of memory starting at the address stored in p and it knows the bits stored in those four bytes represent an integer, so cout can diplay them correctly as the decimal integer 10.

Last edited by hollystyles; Jun 30th, 2006 at 8:52 am.

hollystyles

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Jun 30th, 2006

Re: Pointer to a data member

Quote originally posted by hollystyles ...

Where are you getting this from?
C++ Syntax (Toggle Plain Text)
 int A::*p = &(A::d);
int A::*p = &(A::d);
In C you declare Type <variable name> ;
so to declare a pointer to an integer using the * dereference symbol you do this:
C++ Syntax (Toggle Plain Text)
int * p = &someintegervariable
int * p = &someintegervariable
try this:
C++ Syntax (Toggle Plain Text)
#include <iostream>
 
class A
{
 public:
         int x;
};
 
int main(int argc, char *argv[])
{
  A a;
  a.x = 10;
  int * p = &a.x;
  std::cout << "Memory = " << p << "\n";
  std::cout << "Value pointed to = " << *p << "\n";
  std::cin.get();
  return 0;
}
#include <iostream> class A { public: int x; }; int main(int argc, char *argv[]) { A a; a.x = 10; int * p = &a.x; std::cout << "Memory = " << p << "\n"; std::cout << "Value pointed to = " << *p << "\n"; std::cin.get(); return 0; }
When we print p we get the value of p itself (which is a memory address where the value of x is being stored) When we print * p we dereference to that address and get the actual bits from that memory location 10 (remember you declare the pointer to be int so the compiler knows to get 4 bytes of memory starting at the address stored in p and it knows the bits stored in those four bytes represent an integer, so cout can diplay them correctly as the decimal integer 10.

What u said is the common way of doingit and I am aware of that.
But what my doubt is if u do something like what I have shown, the compiler will not throw any error. If that is the case, what is the thing which the pointer is storing?
I have seen in some material also that u can access the data member thru this way. (not sure whether this is a right way)

dilip.mathews

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Jun 30th, 2006

Re: Pointer to a data member

Quote ...

'cannot convert from 'int *' to 'int A:''. Why this error is thrown when b is of type A

Yes b is of type A but you are trying to assign the address of b.d which is an int.

Also A is a class, where as b is an instance of class A. Which are you trying to point to ?

Last edited by hollystyles; Jun 30th, 2006 at 9:20 am.

hollystyles

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Jun 30th, 2006

Re: Pointer to a data member

Quote ...

I have seen in some material also that u can access the data member thru this way

Is this an internet resource? can you point me to the URL I can't find anything that relates to what you are trying to do.

hollystyles

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Jun 30th, 2006

Re: Pointer to a data member

A pointer to a member requires three distinct parts. First, you need to create a pointer (qualified by the class whose member it points to):

C++ Syntax (Toggle Plain Text)
#include <iostream>
 
class test {
public:
  int i;
};
 
int main()
{
  int test::*p;
}
#include <iostream>

class test {
public:
  int i;
};

int main()
{
  int test::*p;
}

Then you need to assign the member you wish to point to by taking its address (also qualified by the class name):

C++ Syntax (Toggle Plain Text)
#include <iostream>
 
class test {
public:
  int i;
};
 
int main()
{
  int test::*p;
  p = &test::i;
}
#include <iostream>

class test {
public:
  int i;
};

int main()
{
  int test::*p;
  p = &test::i;
}

Finally, to actually point to an object, you need an existing object and you need to use a member access operator on it using the dereferenced pointer as the "member":

C++ Syntax (Toggle Plain Text)
#include <iostream>
 
class test {
public:
  int i;
};
 
int main()
{
  int test::*p;
  p = &test::i;
  test a;
  a.*p = 12345;
}
#include <iostream>

class test {
public:
  int i;
};

int main()
{
  int test::*p;
  p = &test::i;
  test a;
  a.*p = 12345;
}

This covers pointers in pretty good detail, and there's a bit of coverage on pointers to members.

Last edited by Narue; Jun 30th, 2006 at 9:53 am.

Narue

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Jun 30th, 2006

Re: Pointer to a data member

Ok I get it now.

C++ Syntax (Toggle Plain Text)
#include <iostream>
 
class A
{
 public:
         int x;
};
 
int main(int argc, char *argv[])
{
  A a, b;
  a.x = 10;
  b.x = 20;
  int * p = &a.x;
  int A::*p2 = &A::x;
 
  std::cout << "p points to a.x ONLY @ " << p << "\n";
  std::cout << "a.x = " << *p << "\n";
  std::cout << "p2 can be used for a.x " << a.*p2 << "\n";
  std::cout << "Or for b.x !! wow. " << b.*p2 << "\n";
  std::cin.get();
  return 0;
}
#include <iostream>

class A
{
 public:
         int x;
};

int main(int argc, char *argv[])
{
  A a, b;
  a.x = 10;
  b.x = 20;
  int * p = &a.x;
  int A::*p2 = &A::x;

  std::cout << "p points to a.x ONLY @ " << p << "\n";
  std::cout << "a.x = " << *p << "\n";
  std::cout << "p2 can be used for a.x " << a.*p2 << "\n";
  std::cout << "Or for b.x !! wow. " << b.*p2 << "\n";
  std::cin.get();
  return 0;
}

hollystyles

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Jul 3rd, 2006

Re: Pointer to a data member

Thanks for ur help guys.

dilip.mathews

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