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    <p>Good nice question. I really wish that the Standard was a little more explicit about what the intended usage is. Maybe there should be a C++ Rationale document that sits alongside the language standard. In any case, here is the approach that I use:</p> <p>(a) I'm not aware of the existence of any such list. Instead, I use the following list to determine whether a Standard Library type is likely to be designed to be inherited from:</p> <ul> <li>If it doesn't have any <code>virtual</code> methods, then you shouldn't be using it as a base. This rules out <code>std::vector</code> and the like.</li> <li>If it does have <code>virtual</code> methods, then it is a candidate for usage as a base class.</li> <li>If there are lots of <code>friend</code> statements floating around, then steer clear since there is probably an encapsulation problem.</li> <li>If it is a template, then look closer before you inherit from it since you can probably customize it with specializations instead.</li> <li>The presence of policy-based mechanism (e.g., <code>std::char_traits</code>) is a pretty good clue that you shouldn't be using it as a base.</li> </ul> <p>Unfortunately I don't know of a nice comprehensive or <em>black and white</em> list. I usually go by gut feel.</p> <p>(b) I would apply <a href="http://en.wikipedia.org/wiki/Liskov_substitution_principle" rel="noreferrer">LSP</a> here. If someone calls <code>what()</code> on your exception, then it's observable behavior should match that of <code>std::exception</code>. I don't think that it is really a standards conformance issue as much as a correctness issue. The Standard doesn't require that subclasses are substitutable for base classes. It is really just a <em>"best practice"</em>.</p>
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    1. COYou mentioned not inheriting from classes which don't have virtual members – but then you also mentioned policy classes. So (for example) what's wrong with inheriting from an allocator (privately)?
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      1. USKonrad Rudolph
    2. COI would add that some things are meant to have a small degree of extension through inheritance, such as `std::stack` and `std::queue` because they have **protected**, since the only reason for things to be protected is to allow a child class to read the data. Obviously you'd need to be very careful about what you do with this though.
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      1. USEvan Teran
 

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