The difference between undefined behavior and ill-formed C++ programs

The article discusses the distinction between undefined behavior (UB) and ill-formed programs in C++. Undefined behavior occurs at runtime when a program executes actions that the language standard prohibits, allowing the program to behave unpredictably, including potentially invalidating previous operations. For example, if a function is called with parameters that avoid triggering UB, the program remains safe. In contrast, an ill-formed program violates syntactical or semantic rules, such as attempting to modify a constant variable. There are two types of ill-formed programs: those that require a diagnostic (the compiler must report an error) and those that do not (ill-formed no diagnostic required, IFNDR). The latter can lead to unpredictable behavior without any compiler warnings, as seen in cases where two translation units disagree on inline function definitions. This can result in erratic program behavior, including memory corruption. The article emphasizes the importance of understanding these concepts to avoid potential pitfalls in C++ programming, particularly with IFNDR, which can lead to fundamentally invalid programs. It also mentions tools like Visual Studio's command line options and defensive coding practices to help identify and mitigate these issues.