Strict Aliasing Rule - 2024.1 English - UG1603

The strict aliasing rule dictates that pointers are assumed not to alias if they point to fundamentally different types, except for char* and void* which can alias to any other data type. This is is shown in the following graphic which shows the object universes and the associated pointers.

Pointers are associated with a type universe U(T)

T is the template and in the preceding graphic the various templates are shown, including an int universe and a float universe; there is also a MyClass universe per design. Additionally there is a char universe that includes all universes by default.

Universes do not alias

Pointer p can only point to any address within the int universe whereas pointer q can only point to any address within the float universe. Because of this pointer p and pointer q cannot be aliased.

Derived pointers point to the original universe

Pointers derived from a restrict pointer are considered restrict pointers and point to the same restricted memory region. See Derived Pointers.

char* universe contains all universes

A char pointer can point to any variable in all universes.

For two pointers of the same type, as in the following, where both p and q are int, the compiler is conservative and aliasing is applied, resulting in loss of performance.

For two pointers of different types, as in the following example, where p is an int and q is float, the compiler applies the strict aliasing rule and an undefined behavior occurs if aliasing exists.