Comment by hermitdev

Minor nitpick, the zeroing (or lack thereof) of the padding is not undefined behavior, it's unspecified behavior. Undefined behavior and unspecified behavior often look and perhaps behave the same to the programmer, but have semantic differences. In the face of undefined behavior, the compiler is allowed to do pretty much anything it wants (including formatting your hard drive and/or launching the nukes). With unspecified behavior, the compiler implementer must make a conscious decision on what the behavior will be and document the behavior it will follow.