Copying pointers in structures instead of the structure data?

How is this done? I don't quite understand.

That would depend on what you are actually trying to achieve.

But here's an example:

Code:

---

struct widget
{
  ... some data - fairly large.
};

struct gadget
{
  struct widget *widgetPtr;
...
};


widget a;
widget b;

gadget g;

...

somefunc(int flag)
{
    if (flag)
        g.widgetPtr = &a;
    else
        g.widgetPtr = &b;
...
}

---

The advantage over storing a copy of the struct inside gadget is that we can set it to a or b using a simple pointer move. Copying the whole struct itself would involve a lot more data to be copied (if the struct is large).

I used global variables above to make the code small - it is not a recommended method, but it makes the example short.

--
Mats

I have these structs:

Code:

---

struct Listnode
{       
        char lastname[30];
        char firstname[30];
        int age;
};

struct Listtype
{       
        Listnode* person[30];
        int size;
};

---

and this function:

Code:

---

void doInsert (Listtype &List)
{
        char first[30]; //entry firstname
        char last[30]; //entry lastname
        int years; //entry age
        int number; //slot number to place record at
        int i;

        system("cls");
        cout<<"Enter a record in this format: LASTNAME <SPACE> FIRSTNAME <SPACE> AGE"<<endl;
        cout<<"\n";
        cin>>first>>last>>years;
        cout<<"Insert this record at what position: ";
        cin>>number;


        for (i=List.size+1; i>number; i--) //moving all records down 1 position to make room
        {
                List.person[i]=List.person[List.size];
        }
        List.person[number]->firstname=first;
        List.person[number]->lastname=last;
        List.person[number]->age=years;
        List.size=i;
}

---

Its a function to insert records into the struct... And if you insert somewhere where there already is a record I just want the function to make sure that you move them so that nothing is erased... But I get an error that the "left operand must be an L-value" at these 2 lines:

Quote:

---

List.person[number]->firstname=first;
List.person[number]->lastname=last;

---

I'm a little confused as to whether what I'm attempting here is a copy of the structure data or copying the pointer. And why I'm getting that specific error.

What you're trying to do is assign to an array. Perhaps you mean to strcpy instead.

Quote:

---

Originally Posted by tabstop

What you're trying to do is assign to an array. Perhaps you mean to strcpy instead.

---

Code:

---

        {
                List.person[i]=List.person[i-1];
        }

---

Here, am I copying data or copying a pointer?

Quote:

---

Originally Posted by Sparrowhawk

Code:

---

        {
                List.person[i]=List.person[i-1];
        }

---

Here, am I copying data or copying a pointer?

---

person is an array of pointers, so you are copying pointers.

Quote:

---

Originally Posted by tabstop

person is an array of pointers, so you are copying pointers.

---

Got it. thanks

right now I have:

Code:

---

void doInsert (Listtype &List)
{
        char first[30]; //entry firstname
        char last[30]; //entry lastname
        int years; //entry age
        int number; //slot number to place record at
        int i;

        system("cls");
        cout<<"Enter a record in this format: LASTNAME <SPACE> FIRSTNAME <SPACE> AGE"<<endl;
        cout<<"\n";
        cin>>first>>last>>years;
        cout<<"Insert this record at what position: ";
        cin>>number;


        for (i=List.size; i>number-1; --i) //moving all records down 1 position to make room
        {
                List.person[i]=List.person[i-1];
        }
        strcpy(List.person[number-1]->firstname, first); //placing record in number slot
        strcpy(List.person[number-1]->lastname, last);
        List.person[number-1]->age=years;
        List.size++;
}

---

for some reason this is causing an output where the new record to be inserted appears at List.person[number] and at List.person[number+1]

I don't know why either... I tried outputting the loop verbosely in a separate program which outputs:

4 copies to 5
3 copies to 4
2 copies to 3

and thats the end of the loop. Which is correct if number = 3. In the actual program the record is copied twice though at number 3 and at number 4 for some reason.

[edit] - I figured it out I think, but I don't know why it happens or how I can "fix it"... The problem arises I'm thinking because when I assign a value at List.person[number-1]... that pointer points to 2 positions in the list after the function is done... I'm not sure why though..., shouldn't there just be garbage there if I moved everything else?

[edit2] - Got it... The problem was what I thought it was... I just made a new Listnode object there so it wasn't being pointed too again.