returning pointers to structs members

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2.00/5 (1 vote)

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Hi,

I want to write a class that returns a pointer to any data in an array:

C++

class CData
{
   private:
      struct
      {
	  void* pVoid;
	  int   iLen;
      }data;

   public:
      void  SetData (void* pVoid);
      void* GetData (int iNum);
};

void CData::SetData (void* pVoid)
{
   data.pVoid = pVoid;
   data.iLen  = iLen;
}

void* CData::GetData (int iNum)
{
   return data.pVoid + iNum*iLen;  //Simplified
}

With normal data arrays this isn't a problem, but for structures like this I need some help:

C++

struct CHILD
{
   TCHAR szString [50];
   int   iInt;
};

struct MASTER
{
   CHILD* pChild;
   TCHAR  szString [50];
   int    iInt;
};

C++

static MASTER master [3];
static CHILD  child;

_tcscpy (child.szString, TEXT ("Child main"));
child.iInt = 12;

master[0].pChild = master[2].pChild = &child;
master[1].pChild = new CHILD;

master[1].pChild->iInt = 11;
_tcscpy (master[1].pChild->szString, TEXT ("Child 1"));
_tcscpy (master[0].szString, TEXT ("Master 0"));
_tcscpy (master[1].szString, TEXT ("Master 1"));
_tcscpy (master[2].szString, TEXT ("Master 2"));

master[0].iInt = 20;
master[1].iInt = 21;
master[2].iInt = 22;

Returning data from the MASTER structure isn't a problem, but returning data from the CHILD structs is a little bit more complicated because the structs are not "aligned" in the memory(?) So i guesse I have to work with pointers to pointers, but I have no idea how to start, so any ideas?

Posted 17-Oct-11 7:51am

TimGalant

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Sergey Alexandrovich Kryukov 17-Oct-11 14:05pm

Why using direct access to memory at all? This is the abuse of technology.
--SA

2 solutions

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Richard MacCutchan · Answer 1 · 2011-10-17T08:01:00

Solution 1

Forget structs and use properly encapsulated classes and you will not need to worry about the issue. Your code should not be using direct copy commands to add or remove data from an object, that is asking for trouble in the long term.

Posted 17-Oct-11 8:01am

Richard MacCutchan

Comments

Sergey Alexandrovich Kryukov 17-Oct-11 14:06pm

My 5. Looks like OP is lost in basic things. Your explanation is correct, but the difference between classes and structures are really irrelevant here. In C++, they are essentially the same (well, default access modifier is different, so what?).
--SA

Richard MacCutchan 17-Oct-11 14:21pm

essentially the same ... you and I know that, but it's better for someone new(ish) to stick with the concept(s) of classes.

TimGalant 17-Oct-11 15:21pm

But when I want to do it as I explained, can it be done and how? Cause the problem with the "alignment" in the memory will be the same, isn't it?

Richard MacCutchan 17-Oct-11 15:27pm

What do you mean "alignment"? This is not something you need to worry about unless you are dealing with structures whose alignment is critical for some external reason.

TimGalant 17-Oct-11 16:07pm

I mean that with a normal array you know "where" in the memory you can find the fourth, tentht, fiftieth etc. variable in the array if you have a pointer to the first variable of the array. With the structures I described above you don't know where in the memory you can find the members of the child structs.

Niklas L 17-Oct-11 16:33pm

Of course you would know where it is. Did you even try
master[index1].pChild[index2]?

Richard MacCutchan 18-Oct-11 4:48am

You need to read up on pointers in C/C++. You index an array of structs exactly the same way you index an array of ints, bytes etc.

TimGalant 18-Oct-11 6:47am

Ok, I'am capable of working with structs and pointers to structs. But the problem is that I have a struct (MASTER) with a member that points to another struct (CHILD). I want to set a pointer to the first member of the array of MASTER structs in the CData class. Then I want the class to return a pointer to a value from for example the second struct (iNum = 1). With the normal data members of MASTER that's not a problem because the "distance" in the memory between the members is the same (the size of the MASTER struct). But the distance between the members of the CHILD structs in the memory is different (for example: the distance between the members of the CHILD structs in master[0] and master [2] is 0 because the structs are the same. The CHILD stuct of master[1] is somewhere else (unknown) in the memory because it was dynamically assigned.

So what I want is to return a pointer to for example master[0].pChild->iInt or master[1].pChild->iInt when I only have a pointer to the first member of the array of MASTER structs.

I hope I made clear what I want to do.

Richard MacCutchan 18-Oct-11 8:40am

Change your pointers in your CData class from void* to struct MASTER* and then you do not need to worry about packing or alignment, because the compiler will do the work for you.

Stefan_Lang · Answer 2 · 2011-10-18T00:38:00

Solution 2

You did it already, in this line:

C++

_tcscpy (master[1].pChild->szString, TEXT ("Child 1"));

Posted 18-Oct-11 0:38am

Stefan_Lang