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### 2860. `launder` and base class subobjects

Section: 17.7.5 [ptr.launder] Status: NAD Submitter: Hubert Tong Opened: 2017-01-31 Last modified: 2020-09-06

Priority: 2

View all other issues in [ptr.launder].

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Discussion:

There is an apparent oversight in the wording for `launder` that allows it to return base class subobjects which differ in their polymorphic behaviour between calls to `launder`.

This can be fixed by restricting `launder` from returning pointers to base class subobjects:

1. always, or

2. only for polymorphic class types.

[2017-03-04, Kona]

Set priority to 2. This was discussed in EWG (via a paper). Assign this (and 2859) to Core.

[2017-08-14, CWG telecon note]

Core recommends NAD after discussion in EWG.

Hubert Tong summarizes the outcome of that discussion as follows:

> `launder` does not provide positive confirmation of the dynamic type of the object; it is intended that `launder` can be used to induce a devirtualization barrier even when the static type of the (sub)object to which the returned pointer refers is apparently consistent with prior accesses related to the source pointer.

[2020-02 Status to NAD on Thursday night in Prague.]

Proposed resolution:

This wording is relative to N4618.

1. Option 1:

   1. Modify 17.7.5 [ptr.launder] as indicated:

      >     template <class T> constexpr T* launder(T* p) noexcept;

      > > -1- Requires: `p` represents the address `A` of a byte in memory. An object `X` that is within its lifetime (6.7.3 [basic.life]) and whose type is similar (7.3.6 [conv.qual]) to `T` is located at the address `A`. `X` shall either be a most derived object, or pointer-interconvertible with a most derived object that is within its lifetime. All bytes of storage that would be reachable through the result are reachable through `p` (see below).

2. Option 2:

   1. Modify 17.7.5 [ptr.launder] as indicated:

      >     template <class T> constexpr T* launder(T* p) noexcept;

      > > -1- Requires: `p` represents the address `A` of a byte in memory. An object `X` that is within its lifetime (6.7.3 [basic.life]) and whose type is similar (7.3.6 [conv.qual]) to `T` is located at the address `A`. If `T` is a polymorphic class type, then `X` shall be a most derived object. All bytes of storage that would be reachable through the result are reachable through `p` (see below).