Summary

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From a programmer's perspective, a well-designed, object-oriented architecture manifests itself as an inheritance hierarchy, including a set of abstract data type vertical and horizontal relationships, that exhibits several key characteristics. It is 1) easy to understand, 2) easy to reason about, and 3) easy to extend.

The preconditions of a derived class function should either adopt the same or weaker preconditions as the base class function it is overriding. A derived class function should never strengthen the preconditions specified in a base class version of the function. Derived class functions that strengthen base class function preconditions will render it impossible for programmers to reason about the behavior of subtype objects and lead to broken code should the ill- behaved derived class object be substituted for a base class object.

The open-closed principle (OCP) attempts to optimize object-oriented software architecture design so it can accommodate change. Software modules should be designed so they are closed to modification yet open to extension. The OCP is achieved by depending only upon software abstractions. In C++ this means designing with abstract base classes with the goal of dynamic polymorphic behavior. The OCP relies heavily upon the Liskov substitution principle and design by contract (LSP/DbC).

The OCP and the LSP/DbC, when applied together, result in the realization of a third design principle known as the dependency inversion principle (DIP). The key to the DIP is that high-level software modules should not rely on low-level details, and that software modules at all hierarchy levels should rely only upon abstractions. When a software architecture achieves the goals of the DIP it is easier to extend and maintain (flexible and non-rigid). Software modules that conform to the DIP are easier to reuse in other contexts (mobile).

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