Slides for P3969R0 — Fixing std::bit_cast of types with padding bits

Document number:
P4038R0
Date:
2026-03-10
Audience:
LEWG
Project:
ISO/IEC 14882 Programming Languages — C++, ISO/IEC JTC1/SC22/WG21
Reply-to:
Jan Schultke <janschultke@gmail.com>
Source:
github.com/Eisenwave/cpp-proposals/blob/master/src/bit-cast-padding-slides.cow

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Fixing std::bit_cast
of types with padding bits
P3969R0

Jan Schultke  |  Slides for P3969R0 — Fixing std::bit_cast of types with padding bits  |  Telecons 2026  |  Slide 1

The problem

// GCC accepts (x == 0) // Clang rejects constexpr auto x = std::bit_cast<__int128>(0.0L);
Jan Schultke  |  Slides for P3969R0 — Fixing std::bit_cast of types with padding bits  |  Telecons 2026  |  Slide 2

The problem, cont.

struct alignas(4) E { }; // GCC and Clang error: constexpr auto x = std::bit_cast<int>(E{}); // MSVC accepts: static_assert(x == 0);
Jan Schultke  |  Slides for P3969R0 — Fixing std::bit_cast of types with padding bits  |  Telecons 2026  |  Slide 3

Possible solutions

"Mathematically correct" functions eliminate special cases and UB pitfalls:

Single-function Two-function
struct alignas(4) E { }; auto x = bit_cast<int>(E{}); assert(x == 0); // OK struct alignas(4) E { }; auto x = bit_cast<int>(E{}); // error auto y = bit_cast_zero_padding<int>(E{}); assert(x == 0); // OK
✔️ already implemented (accidentally)
✔️ retroactively fixes code
✔️ safety by default
❌ doesn't express intent
❌ not really bit-casting
❌ hidden cost, no-opt out 		✔️ expresses intent
✔️ can opt out of cost
✔️ no semantic change between versions
❌ more complex
❌ unproven
❌ unimplemented
Jan Schultke  |  Slides for P3969R0 — Fixing std::bit_cast of types with padding bits  |  Telecons 2026  |  Slide 4

Gotcha: clearing padding in unions

struct alignas(4) E { }; union U { int x; E e; }; auto x = bit_cast_zero_padding<int>(U{ .e = {} });
Jan Schultke  |  Slides for P3969R0 — Fixing std::bit_cast of types with padding bits  |  Telecons 2026  |  Slide 5

References

[P3969R0] Jan Schultke. Fixing std::bit_cast of types with padding bits 2026-10-03 https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2026/p3969r0.html