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    <blockquote> <p>Functions that use pointers or references to base classes must be able to use objects of derived classes without knowing it.</p> </blockquote> <p>When I first read about LSP, I assumed that this was meant in a very strict sense, essentially equating it to interface implementation and type-safe casting. Which would mean that LSP is either ensured or not by the language itself. For example, in this strict sense, ThreeDBoard is certainly substitutable for Board, as far as the compiler is concerned.</p> <p>After reading up more on the concept though I found that LSP is generally interpreted more broadly than that.</p> <p>In short, what it means for client code to "know" that the object behind the pointer is of a derived type rather than the pointer type is not restricted to type-safety. Adherence to LSP is also testable through probing the objects actual behavior. That is, examining the impact of an object's state and method arguments on the results of the method calls, or the types of exceptions thrown from the object.</p> <p>Going back to the example again, <strong>in theory</strong> the Board methods can be made to work just fine on ThreeDBoard. In practice however, it will be very difficult to prevent differences in behavior that client may not handle properly, without hobbling the functionality that ThreeDBoard is intended to add.</p> <p>With this knowledge in hand, evaluating LSP adherence can be a great tool in determining when composition is the more appropriate mechanism for extending existing functionality, rather than inheritance.</p>
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    1. POWhat is an example of the Liskov Substitution Principle?
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