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POWhat kind of "Traits" are used/defined in the C++0x Standard
 

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  1. POWhat kind of "Traits" are used/defined in the C++0x Standard
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    <p>A trait in C++ encapsulates a family of operations that allow an Algorithm or Data Structure to operator with that type with which it is instantiated. <code>char_traits</code> are an example for grouping <code>string</code>- and file-required functions.</p> <p>But not all <em>traits</em> have "trait" in their name, right? <code>numeric_limits</code> comes to mind. Is this a "Trait", too? Even without the name "trait" in it?</p> <p><strong>So, are there other Templates that could/should be considered a "Trait"?</strong> Besides the examples I found:</p> <ul> <li><code>allocator_traits</code> how to get memory</li> <li><code>pointer_traits</code> how to access an object indirectly</li> <li><code>type_traits</code> meta programming</li> <li><code>char_taits</code> for sequence of symbols</li> <li><code>iterator_traits</code> how to get forward, backward and to the element</li> <li><code>regex_traits</code> for... regexes.</li> </ul> <p>I guess, what I am asking, too, is there a <em>pure definition</em> for traits?</p> <p>Some things I am especially unsure about are:</p> <ul> <li><code>numeric_limits</code> mentioned above</li> <li><code>&lt;chrono&gt;</code>s customization "traits", [20.11.4], i.e. <code>duration_values</code></li> <li>what about Hashing? Can the functor <code>hash&lt;&gt;</code> be considered to be a trait?</li> <li>If thats the case, are not all requirements "traits", like "CopyAssignable", etc?</li> <li>And then, are the abandoned "Concepts" the ultimate "trait"-Definition?</li> </ul> <p><strong>Update:</strong> The question what exactly makes a trait a trait seems a bit controversy in the details. Maybe a another question could be answered: <strong>Is there a comprehensive list which of the trait-like classes are new to C++0x, and which ones have already been in C++03?</strong> Maybe someone knows of a link to somewhere?</p>
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    1. COThis should be CW, since there are many possible "right" answers. I guess that option no longer exists? Anyway, iostreams facets are a form of trait.
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    2. COI do not think `hash<>` is properly considered a trait class, because it does more than just provide compile-time information about the class. How about this definition: "A template class with no non-static members, whose static members depend only on the template argument."
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    3. COI'd say that `numeric_limits` isn't strictly a trait class, because its values aren't all compile-time static constants or `constexpr` -- you have things like `max()` which is a runtime value.
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