On my laptop, sizeof(long) is 8; that's implementation defined. It could be different on my phone, or my desktop, or my work laptop.

Undefined means, roughly, "doing X is considered nonsensical, and the compiler does not have to do anything reasonable with code that does X." In "Design and Evolution of C++," Stroustrup says that undefined behavior applies to things that should be errors, but that for some reason the committee doesn't think the compiler will necessarily be able to catch. When he came up with new ideas for the language, he would often have to choose between making a convoluted rule that his compiler could reliably enforce, or a simple rule that his compiler couldn't always give a sensible error message for.

For instance, the original compilers relied on the system's linker. If the compiler could interact with the linker, it could perhaps detect violations of the One Definition Rule ( https://en.cppreference.com/w/cpp/language/definition ), but since the linker might have been written by a completely different company, and it's acceptable for different source files to be compiled by different compilers (and even be written in other languages -- https://en.cppreference.com/w/cpp/language/language_linkage ) and put together by the linker, and it's common for binary libraries to be sold without source, there's no guarantee that the compiler will ever have the information necessary to detect a violation of the One Definition Rule. So the committee says that violations create a nonsense program, which isn't required to behave in any particular way.