Question

I understand the reasons for compiler/interpreter language extensions but why is behaviour that has no valid definition allowed to fail silently/do weird things rather then throwing a compiler error? Is it because of the extra difficulty(impossible or simply time consuming) for the compiler to catch them)?

P.S. what languages have undefined behaviour and which don’t?

P.P.S. Are there instances of undefined behaviour which is not impossible/takes too long to catch in compilation and if so are there any good reasons/excuses for those.

Answer 1

The concept of undefined behaviour is required in languages like C and C++, because detecting the conditions that cause it would be impossible or prohibitively expensive. Take for example this code:

int * p = new int(0);
// lots of conditional code, somewhere in which we do
int * q = p;
// lots more conditional code, somewhere in which we do
delete p;
// even more conditional code, somewhere in which we do
delete q;

Here the pointer has been deleted twice, resulting in undefind behaviour. Detecting the error is too hard to do for a language like C or C++.