Why Can a char* Alias Other Pointers, But the Converse is Not True?

Strict Aliasing and Pointer Casting: A Deeper Understanding

Interchanging pointer types without explicitly casting is a common practice, but it raises a significant concern known as the strict aliasing rule. This rule defines limitations on pointer type aliasing and can lead to undefined behavior if violated.

In the context of pointer aliasing, it's often stated that a char* can be used as an alias for any object pointer, while the converse is not true. This statement may seem paradoxical, as pointers of different types ostensibly represent distinct memory locations.

To clarify this apparent contradiction, it's essential to understand the underlying reason behind the restriction. The strict aliasing rule exists to maintain predictable and well-defined memory access behavior for the compiler. By disallowing certain pointer type aliasing, it prevents situations where an object is accessed using an inappropriate representation, leading to unexpected results.

For instance, consider a pointer to a struct something: struct something* points to the base address of the structure, providing access to its members. On the other hand, a char* pointer represents individual bytes of memory.

If a struct something* could be aliased as a char*, it would be possible to access members of the structure directly as if they were individual bytes. However, this operation is not guaranteed to produce valid results because the compiler cannot ensure that the arrangement of members within the structure aligns precisely with the byte boundaries expected by the char*.

Therefore, to maintain memory access predictability, the strict aliasing rule restricts pointer type aliasing to allow only char* to alias other object pointers. This ensures that memory accesses through a char* pointer to any object's constituent bytes are always valid, regardless of the alignment of its members.

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