Lock‐and‐key strategies for handling undefined variables

Abstract

AbstractMost programming languages ignore the problem of undefined variables and permit compilers to allow the leftover contents of the memory cells belonging to such variables to be referenced. Although efficient, this type of semantics does not support software engineering, because defects might arise in subtle ways and might be difficult to locate. Two other types of semantics that better support software engineering are to either initialize all variables to some default, e.g. zero, or to require that all references to undefined variables be treated as errors. However, these types of semantics are obviously more expensive than simply ignoring the undefined variables entirely.In this paper, we propose a simple technique that works equally well for both of these latter two types of semantics, and whose efficiency compares favorably for certain realistic programs with more traditional implementations of these semantics. Furthermore, we provide a mechanism for using this technique through Ada implementations of two abstract data types where undefined variables respectively exhibit these two types of semantics, and whose implementations of these semantics use our technique. These abstract data types allow our technique to be selectively used in strictly those situations where the cost of the technique is justified. We provide practical examples illustrating these situations.