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Based Pointers (C++)


The latest version of this topic can be found at 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.

System_CAPS_ICON_note.jpg Note

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;