Re: [boost] [variant2] never-empty guarantee considered harmful

On 13.04.19 01:47, Frank Mori Hess via Boost wrote:

On Fri, Apr 12, 2019 at 7:26 PM Andrzej Krzemienski via Boost wrote:

...

UB is a useful symptom of a bug and therefore can help detect and remove bugs

It seems like you are using a different definition of undefined behavior than what I understand it to be. UB is not a useful symptom because it can be anything, and can vary by implementation. It doesn't necessarily mean the program crashes, it could for example result in exactly the harmless unnoticed response to a bug you object to.

UB is not a runtime check that detects bugs (although compiling with undefined behavior sanitizer turns it into one). It is a conceptual tool for verifying the correctness of your program. If your program invokes undefined behavior, it is incorrect. Conversely, if your program is correct, then it does not invoke undefined behavior. Undefined behavior is not a defect of the C++ language. It's a deliberate feature. The standards committee could have easily defined the result of reading an uninitialized variable as "whatever arbitrary value happens to occupy that memory location". Instead, they chose to mark it as undefined behavior, because a program that uses uninitialized variables is /wrong/. This bears repeating. Reading from an uninitialized variable is not wrong because it is undefined behavior, but the other way around. Reading from an uninitialized variable is undefined behavior because it is wrong. -- Rainer Deyke (rainerd@eldwood.com)