C ++ Struct指针问题

Question

This is probably simple, but im trying to try and understand pointers better. Lets say I have a struct

struct Person{
  char Name[20];
  char ID[15];
  char Address[50];
  char Number[15];
};

and lets say I have a bunch of these stored in memory like one after the other. so now i declare a new pointer to the struct.

struct Person *ptr;

Will this Pointer start at the FIRST entry? (aka the first "Name") and when I iterate it (aka ptr++) will it go to the next STRUCT or the next "entry" aka like name-->ID--->Address

So lets say Im on the first entry for example and the data is:

Jason Adams
111222333
111 Fake Drive
55555551000

and the second entry is

Matt Johns
111555333
555 Derp lane
1000022434

now I iterate the ptr (ptr++) will the ptr point to second struct (the one with matt johns) or will it point to Jason Adams "ID"

I hope this makes sense?

Answer 1

It will point to the second struct. The pointer will be advanced by sizeof(Person) .

Answer 2

As John mentioned, it will be advanced by the size of itself - aka sizeof(Person).

Pointer manipulation is fun however, and you can do things like cast the pointer to a different type. Once you do that, pointer arithmetic will advance by the underlying size of the type. Commonly you'll see pointers casted to (byte *) or (unsigned char *) to allow individual byte access to the underlying data.

Answer 3

First of all, you must allocate memory for your pointer ptr before using it.

auto_ptr<Person> ptr(new Person); //auto_ptr for automatic freeing

After that, you can try below code.

strcpy(ptr->ID, "100"); //For illustraction
int offset = ptr->ID - ptr->Name; //Get offset of ID member
char *pID = ((char *)ptr.get())+ offset;
//Typecast to char pointer and increment by offset to get to ID field
cout<<pID; //Now pId can be used to access ID.

You asked how to access ID attribute by incrementing ptr.

As shown above, you can access ID member of structure.