Interestingly, g++ gives out a warning (correctly) when compiled with -O2, but not by default (-O0): warning: dereferencing type-punned pointer will break

Interestingly, g++ gives out a warning (correctly) when compiled with -O2, but not by default (-O0): warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing] So, you're right indeed.