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    <p>No - "is a" does not always lead to inheritence. A well cited example is the relationship between a square and a rectangle. A square is a rectangle, but it will be bad to design code that inherits a Square class off a Rectangle class.</p> <p>My suggestion is to enhance your "is a / has a" heuristic with the <a href="http://www.objectmentor.com/resources/articles/lsp.pdf" rel="noreferrer">Liskov Substitution Principle</a>. To check whether an inheritence relationship complies with the Liskov Substitution Principle, ask whether clients of a base class can operate on the sub class without knowing that it is operating on a sub class. Of course, all the properties of the sub class must be preserved.</p> <p>In the square / rectangle example, we must ask whether a client of rectangle can operate on a square without knowing that it is a square. All that the client must know is that it is operating on a rectangle. The following function demonstrates a client that assumes that setting the width of a rectangle leaves the height unchanged. </p> <pre><code>void g(Rectangle&amp; r) { r.SetWidth(5); r.SetHeight(4); assert(r.GetWidth() * r.GetHeight()) == 20); } </code></pre> <p>This assumption is true for a rectangle, but not for a square. So the function cannot operate on a square and therefore the inheritence relationship violates the Liskov Substitution principle. (By the way - the example came from the <a href="http://www.objectmentor.com/resources/articles/lsp.pdf" rel="noreferrer">linked article</a>.)</p>
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    1. POInheritance or composition: Rely on "is-a" and "has-a"?
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    1. COOk, it illustrates the point. But still, why would a square be mutable?
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      1. USJohn Nilsson
    2. COEveryone has heard the square and rectangle problem, but I have never actually come across a real world example of this (I am a junior dev, so that doesn't mean much). Can you provide one?
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      1. USEd S.
    3. CO@Ed - Example: TCP socket uses a file descriptor, so you might say it is-a file. However, most file APIs assume random access is possible (e.g. fseek). A TCP socket is also an IP socket, but no clients should be accessing the IP layer directly (let the TCP stack handle it).
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