Developer knowledge network ยท moderated exchange

UnreliableCode Community

Developer Research, Reverse Engineering & Coding Community

Knowledge indexLive
4Categories
919Threads
2.8KPosts
Guide

Understanding Strict Aliasing Rule and undefined behavior when casting pointer types [StackOverflow Architecture Guide]

sanitizer_sam
UB Hunter
MEMBER
Rep: 119
Join Date: Apr 2021
Posts: 10
Thanks: 53
3 weeks ago ยท Jul 27, 2026 11:50 PM
#1

Why casting float* pFloat = (float*)&intVal; violates the C/C++ Strict Aliasing Rule:

The standard states that two pointers of different types cannot point to the same memory location (with exceptions for char* and std::byte*). If violated, the compiler optimizer assumes writes through pFloat do not affect reads from intVal, causing silent reordering bugs under -O2/-O3.

The Safe Modern Solution in C++20:

CPP
#include <bit>
float f = std::bit_cast<float>(intVal); // 100% standard-compliant zero-copy bit reinterpret!
llvm_compiler_dev
LLVM & Clang Hacker
MEMBER
Rep: 139
Join Date: Jul 2018
Posts: 21
Thanks: 15
3 weeks ago ยท Jul 28, 2026 4:50 AM
#2

std::bit_cast compiles to a single movd or vmov instruction between integer and SSE registers. Zero overhead and 100% type-safe.

memory_model_mook
Low-Level C Veteran
MEMBER
Rep: 163
Join Date: Jan 2019
Posts: 11
Thanks: 33
3 weeks ago ยท Jul 28, 2026 2:53 PM
#3

Compiling with -fno-strict-aliasing hides the bug, but using std::bit_cast fixes the root cause portably.