Type Punning in C and C++
Type Punning in C and C++ Sep 21, 2026 • Steve SCHNEPP c undefined-behavior c++ About 7 min read I had a bug that took me a while to track down. A pointer cast worked fine at -O0 and silently broke at -O2. The C vs C++ distinction here is genuinely treacherous, and most blog posts on the topic get it wrong.
- ▪Type Punning in C and C++ Sep 21, 2026 • Steve SCHNEPP c undefined-behavior c++ About 7 min read I had a bug that took me a while to track down.
- ▪A pointer cast worked fine at -O0 and silently broke at -O2.
- ▪The C vs C++ distinction here is genuinely treacherous, and most blog posts on the topic get it wrong.
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| Original publisher | Personal Workflow Blog |
| Canonical URL | https://blog.pwkf.org/2026/09/21/correct-type-punning-in-c.html |
| Publication time | Tue, 22 Sep 2026 11:29:47 +0000 |
| Retrieval time | 2026-09-22T11:53:50.953Z |
| Last seen | 2026-09-22T11:53:50.953Z |
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| Summary | WeSearch · cerebras-chat (WeSearch summarizer) |
| Summary source text | contentText |
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| Publisher visit | Yes — open original |
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| Retrieval / RAG | May the content be exposed for third-party retrieval-augmented generation? | Not asserted |
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| Commercial reuse | May the content be reused commercially? | Not permitted |
Basis: Derived from the published RSS/Atom feed. Contact: [email protected]. Reviewed: 2026-07-24.
Opening excerpt (first ~120 words) tap to expand
Type Punning in C and C++ Sep 21, 2026 • Steve SCHNEPP c undefined-behavior c++ About 7 min read I had a bug that took me a while to track down. The problem was type punning. A pointer cast worked fine at -O0 and silently broke at -O2. The C vs C++ distinction here is genuinely treacherous, and most blog posts on the topic get it wrong. Type punning is interpreting memory as different types between reads and writes. It’s essential for serialisation, network protocols, and low-level hardware access. The problem is that “works in practice” and “has defined behaviour” are different things. The Spectrum from Safe to UB In C, the safe ways to type pun are union and memcpy.
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Excerpt limited to ~120 words for fair-use compliance. The full article is at Personal Workflow Blog.
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