The__based keyword allows you to declare pointers based on pointers (pointers that are offsets from existing pointers). The__based keyword is Microsoft-specific.

Syntax

type __based( base ) declarator

Remarks

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: /cvoid *vpBuffer;struct llist_t {   void __based( vpBuffer ) *vpData;   struct llist_t __based( vpBuffer ) *llNext;};

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

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 unless compiler option/Za (Disable language extensions) is specified.

Example

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;}

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See also

Keywords
alloc_text