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4636. bit_cast from volatile behaviour unclear

Section: 22.11.3 [bit.cast] Status: New Submitter: Hubert Tong Opened: 2026-10-28 Last modified: 2026-10-31

Priority: Not Prioritized

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Discussion:

From 22.11.3 [bit.cast], it is unclear how std::bit_cast interacts with various expectations around "volatile access". For example, the std::bit_cast wording does not identify what access operation is being performed to inspect the object representation of from. Therefore, it is firstly unclear whether there is any volatile access at all, and secondly unclear whether there may be multiple volatile accesses (e.g., one per byte of the object representation).

This interacts with the undefined behaviour specified by 9.2.9.2 [dcl.type.cv] p5 to occur when an attempt is made to access an object defined with a volatile-qualified type through the use of a non-volatile glvalue. Such undefined behaviour is backed by implementation concerns such as the possible need to use corresponding load instructions in the case of certain memory-mapped registers, where even using another load instruction for the same width may trigger a hardware exception. Therefore, approaches that try to access from in a non-volatile manner or byte-wise are problematic.

This also interacts with the undefined behaviour specified by 7.3.2 [conv.lval] bullet (3.3) for when lvalue-to-rvalue conversion occurs for an object whose value representation is not valid for its type. This raises a problem for approaches that try to access from as a glvalue of its declared type. Such an approach has the additional problem that the load instructions involved for such a volatile access may not preserve the value representation, which may be observable by a program if the object was not actually volatile.

Proposed resolution:

This wording is relative to N5054.

1. Modify 22.11.3 [bit.cast] as indicated:

   template<class To, class From>
     constexpr To bit_cast(const From& from) noexcept;
   

   -1- Constraints:

   […]

   -?- Mandates: is_volatile_v<From> is false.

   -2- Constant When: Neither To nor From has constexpr-unknown representation (7.7.2 [expr.const.core]).