Discussion

Emacs Internal #03: Tagged Union, Tagged Pointer, and Poor Man's Inheritance

tialaramex: It's not clear to me (and as an unsafe language it's not called out by your compiler if you do something illegal) what the correct way to spell this kind of trick is in C++I had thought you need the pointer-sized integer types and mustn't do this directly to an actual pointer, but maybe I was wrong (in theory, obviously practice doesn't follow but that's a dangerous game)

thecloudlet: Doing bitwise operations directly on raw pointers is a fast track to Undefined Behavior in standard C/C++. Emacs gets away with it largely due to its age, its heavy reliance on specific GCC behaviors/extensions, and how its build system configures compiler optimizations.In modern C++, the technically "correct" and safe way to spell this trick is exactly as you suggested: using uintptr_t (or intptr_t).

trws: There’s a paper in flight to add a stdlib type to handle pointer tagging as well while preserving pointer provenance and so-forth. It’s currently best to use the intptr types, but the goal is to make it so that an implementation can provide specializations based on what bits of a pointer are insignificant, or even ignored, on a given target without user code having to be specialized. Not sure where it has landed since discussion in SG1 but seemed like a good idea.