N3963: Centralized Defensive-Programming Support for Narrow Contracts Page 1 of 28 Doc No: N3963 Date: 2014-03-02

Reducing defects in software is a central goal of modern software engineering. Providing essentially defect-free library software can, in large part, be accomplished through thorough unit testing, yet even the best library software—if misused—can lead to defective applications. When invoking a function, not every combination of syntactically valid inputs will (or should) necessarily result in defined behavior. Functions for which certain combinations of inputs and (object) state result in undefined behavior are said to have narrow contracts. Aggressively validating function preconditions at runtime—commonly referred to as defensive programming— can lead to more robust applications by (automatically) detecting out-of-contract use of defensive library software early in the software development life cycle. Most classical approaches to defensive precondition checks, however, necessarily result in suboptimal runtime performance; moreover, when misuse is detected, the action taken is invariably determined by the library, not the application. In this proposal, we describe a centralized facility for supporting defensive runtim