member access question

[mandrews58]

Thanks for any help on this:

say i define a class which handles a list of structures

class MyList{

public:
/*
constructor creates array of ListItems along the lines of:
pList = new ListItem[nElements];
*/
MyList(UINT nElements);

// subscript operator retruns ListItem reference
ListItem &operator[](UINT pos);

private:
ListItem *pList;
};


struct ListItem {
char strA[32];
char strB[32];
};



i want to use this as follows:

MyList *pList = new MyList(20);
strcpy(pList[3].strA, "Hi There");

the problem is that although operator[] returns a ListItem reference it is still a member of MyList, so pList[3].strA attempts to access a member of MyList and this will not compile. how do i get the functionality where the member access operator looks for a ListItem member, not a MyList member?

[Paul McKenzie]

Why not create a function that assigns to the ListItem instead of having the client do this? Proper C++ class design would hide these details from the client.

class MyList
{
    public:
          void SetStrA(const char *pData,
                               int whichone)
          { 
               strcpy(pList[whichone], pData);
           }
  //...
};

//...
MyList *pList = new MyList(20);
pList[3].SetStrA("Hi There", 3);

There is no check to see if "whichone" is in bounds, but this is the way it should have been coded.

There are a lot of other issues with your class design that I won't comment on, such as the overall safety of the coding that you're doing and non-usage of the C++ standard library, but that is another thread.

Regards,

Paul McKenzie

[jwbarton]

The pList variable is a pointer to a MyList. In order to access the operator[] on this variable, you need an object reference, not a pointer.

So, you can either do this:

MyList *pList = new MyList(20);
strcpy( (*pList)[3].strA, "Hi There"); // dereference pList

or you can declare a list object directly:

MyList myList(20);
strcpy( myList[3].strA, "Hi There" );


If I were coding this, I would instead just use std::vector<ListItem> to hold the array of items.

Best regards,
John

[Alexis Moshinsky]

Direct access to class members is not good (read ugly)
coding practice.
From every reason You want.
The solution suggested by Paul is in any case preferable.

[jwbarton]

I agree that direct access to members is ugly. However, Paul's solution has a minor bug (probably caused by cutting and pasting) - he left in the array access in the statement using the new member routine (accessing past the end of allocated memory).

The correct call (using Paul's method) would be:

pList->SetStrA( "Hi There", 3 );

Best regards,
John

[sandodo]

You have to use your code like the following

MyList *pml = new MyList(20);
strcpy(pml[3][3].strA, "Hi There");
delete pml;

Maybe the variable name (MyList *) pList cause the misconception. pList in your code is a MyList pointer, the operator[] works on the class object not the class pointer of course, as what mentioned by jwbarton.