This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 114b. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-05-06


800. Safely-derived pointers and object pointers converted from function pointers

Section: 7.6.1.10  [expr.reinterpret.cast]     Status: NAD     Submitter: UK     Date: 3 March, 2009

N2800 comment UK 57

It is not specified under what conditions an object pointer created by converting a function pointer, as described in 7.6.1.10 [expr.reinterpret.cast] paragraph 8, will be safely-derived, particularly in light of the conditionally-supported, implementation-defined nature of such conversions.

Notes from the March, 2009 meeting:

If this is to be addressed, the result should not be as suggested, i.e., a requirement for implementation documentation appearing only in a note. At the least, such a requirement must be in normative text.

Rationale (July, 2009):

The definition of “safely-derived pointer” is clearly and exclusively formulated in terms of pointers to objects. So no implementation is required to maintain safe pointer derivation through conversion to and from a function-pointer type.

On the other hand, any garbage-collecting implementation is free to treat function pointers the same as object pointers for purposes of collection. This would provide the effect of safe pointer derivation through function-pointer types. An implementation is even free to document this behavior, if it so chooses.

However, converting a pointer to a dynamically-allocated object into a function pointer would be a very strange and almost always pointless and unsafe thing to do. There is no need for the standard to encourage this sort of behavior, even to the extent of adding a note mentioning the possibility.