Export (0) Print
Expand All

Based Pointers (C++)

Microsoft Specific

The __based keyword allows you to declare pointers based on pointers (pointers that are offsets from existing pointers).

type __based( base ) declarator

Pointers based on pointer addresses are the only form of the __based keyword valid in 32-bit or 64-bit compilations. For the Microsoft 32-bit C/C++ compiler, a based pointer is a 32-bit offset from a 32-bit pointer base. A similar restriction holds for 64-bit environments, where a based pointer is a 64-bit offset from the 64-bit base.

One use for pointers based on pointers is for persistent identifiers that contain pointers. A linked list that consists of pointers based on a pointer can be saved to disk, then reloaded to another place in memory, with the pointers remaining valid. For example:

// based_pointers1.cpp
// compile with: /c
void *vpBuffer;
struct llist_t {
   void __based( vpBuffer ) *vpData;
   struct llist_t __based( vpBuffer ) *llNext;
};

The pointer vpBuffer is assigned the address of memory allocated at some later point in the program. The linked list is relocated relative to the value of vpBuffer.

NoteNote:

Persisting identifiers containing pointers can also be accomplished by using memory-mapped files.

When dereferencing a based pointer, the base must be either explicitly specified or implicitly known through the declaration.

For compatibility with previous versions, _based is a synonym for __based.

The following code demonstrates changing a based pointer by changing its base.

// based_pointers2.cpp
// compile with: /EHsc
#include <iostream>

int a1[] = { 1,2,3 };
int a2[] = { 10,11,12 };
int *pBased;

typedef int __based(pBased) * pBasedPtr;

using namespace std;
int main() {
   pBased = &a1[0];
   pBasedPtr pb = 0;

   cout << *pb << endl;
   cout << *(pb+1) << endl;

   pBased = &a2[0];

   cout << *pb << endl;
   cout << *(pb+1) << endl;
}
1 2 10 11

Community Additions

ADD
Show:
© 2014 Microsoft