function_record code health enhancements (std::launder, std::is_standard_layout)
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rwgk
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function_record code health enhancements (std::launder, std::is_standard_layout)
…0 20:14:21 -0700 [Browse function_record_std_launder/manuscript tree](https://github.com/rwgk/pybind11/tree/57b9a0af815d19b236b74be06a172bc5c9956618) [Browse function_record_std_launder/manuscript commits](https://github.com/rwgk/pybind11/commits/57b9a0af815d19b236b74be06a172bc5c9956618/)
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henryiii
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Description
This PR refines and extends the use of
std::launderfor callable captures stored infunction_record::data. It generalizes the approach first introduced in PR #2816 by applying it more consistently and defensively.Key Improvements:
🔁 Consistent use of
std::launderAll reinterpretations of the
datafield into the internalcapturetype now go through a centralizedcapture::from_data(void**)helper, which appliesstd::launderwhere supported. This eliminates ad hoc casting and ensures correct behavior under the C++ object model.🛡️ Precondition enforcement
Adds a
static_assertonstd::is_standard_layout<capture>at critical optimization points. This ensures that reinterpret_casts and in-place storage remain well-defined.🧼 Macro centralization
Moves the
PYBIND11_STD_LAUNDERandPYBIND11_HAS_STD_LAUNDERmacro definitions tocommon.h, enabling their reuse across headers (e.g.,functional.hin addition topybind11.h).📉 Reduced reliance on compiler-specific warnings
The change removes special-case
#pragmause around-Wstrict-aliasingby ensuring that laundering is properly applied across the board.Motivation:
The original
std::launderusage in PR #2816 was narrow in scope and only applied during destruction of non-trivially destructible callables. This patch addresses potential undefined behavior more broadly and makes the low-level implementation safer and easier to reason about, while maintaining performance characteristics.ABI Note:
No changes to the ABI are introduced. The layout of
function_recordand the capture storage remains unchanged.Changelog: Not needed.
Upgrade guide: Not needed.
This PR is a by-product of work under PR #5585 (which we will not merge, most likely).
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