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Strict aliasing

cppreference Object#Strict aliasing

Accessing an object using an expression of a type other than the type with which it was created is undefined behavior in many cases, see reinterpret_cast for the list of exceptions and examples.

creference Objects and alignment#Strict aliasing

Given an object with effective type T1, using an lvalue expression (typically, dereferencing a pointer) of a different type T2 is undefined behavior

NOTE:

下面是展示“dereferencing a pointer”的一个简单的例子，这个例子源自creference Objects and alignment#Strict aliasing: 

#include <cstdio>

int main()
{
  int i = 7;
  float* pf = (float*) (&i);
  float d = *pf; // UB: float lvalue *pf cannot be used to access int
}

Why?

Alignment requirement

在C-family-language\C-and-C++\From-source-code-to-exec\ABI\Alignment中对alignment进行了深入分析。

如果不遵循alignment，则会导致非常严重的问题，在C-family-language\C-and-C++\From-source-code-to-exec\ABI\Alignment给出了example。

Compiler optimization

对于没有遵循strict aliasing的program，compiler可能会进行错误的optimization，进而导致生成非常诡异的程序，因为它是undefined的。关于此的一个例子是quarkslab Unaligned accesses in C/C++: what, why and solutions to do it properly 。