Yes, this is an example of UB leaving memory in a bad state. If you want an example of something that is not UB leaving memory in a bad state, here is some Go

Yes, this is an example of UB leaving memory in a bad state. If you want an example of something that is not UB leaving memory in a bad state, here is some Go code: global := 7; func main () { go func() { global = 1000000; }() go func() { global = 10 }() fmt.Printf("gloabl is now %d") } The two concurrent writes may partially overlap, and global may have a value that is neither 7 nor 10 nor 1000000. The program's memory is in a bad state, but none of this is UB in the C or C++ sense. In particular, the Go compiler is not free to compile this program into something entirely different. Edit: I should also note that a similar C program using pthreads or win32 threading for concurrent access is also an example of a program which will go into a bad state, but that is not UB per the C standard (since the C standard has no notion of multithreading).