Null

Tony Hoare introduced Null references in ALGOL W back in 1965 “simply because it was so easy to implement”. He talks about that decision considering it “my billion-dollar mistake”. Here is what he said in a talk in 2009 I call it my billion-dollar mistake. At that time, I was designing the first comprehensive type system for references in an object-oriented language. My goal was to ensure that all use of references should be absolutely safe, with checking performed automatically by the compiler. But I couldn’t resist the temptation to put in a null reference, simply because it was so easy to implement. This has led to innumerable errors, vulnerabilities, and system crashes, which have probably caused a billion dollars of pain and damage in the last forty years. Refs Tony Hoare @ Infoq (2009)