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.
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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