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    <p>Constraints of generic programming are best stated in terms of expressions. A more modern rendition of the same constraint on copiability would be that both statements should be valid:</p> <pre><code>T b = a; </code></pre> <p>and</p> <pre><code>T b = ra; </code></pre> <p>where <code>a</code> is an lvalue with type <code>T</code> or <code>const T</code> and <code>ra</code> is an rvalue with type <code>T</code> or <code>const T</code>. (With similar post-conditions.)</p> <p>This formulation is in the spirit of the paper, I believe. Do note that C++03 already makes use of notions like lvalues and rvalues, such that the constraint we've expressed requires that something like <code>T source(); T b = source();</code> be valid -- certainly something that seems sensible.</p> <p>Under those constraints, then not much changes with C++11. Of particular note is that such a (pathological) type is irregular:</p> <pre><code>struct irregular { irregular() = default; irregular(irregular const&amp;) = default; irregular&amp; operator=(irregular const&amp;) = default; irregular(irregular&amp;&amp;) = delete; irregular&amp; operator=(irregular&amp;&amp;) = delete; }; </code></pre> <p>because something like <code>irregular a; irregular b = a;</code> is valid while <code>irregular source(); irregular b = source();</code> isn't. It's a type that is somewhat copyable (resp. copy assignable), but not quite enough. [ This has been considered somewhat of a defect and is slated to be changed for C++1y, where such a type will in fact be copyable. ]</p> <p>Going further, for the post-condition that a copy must be equivalent in some sense to the original (or, for rvalues, to the original before the copy) to hold, a move special member can only ever be an 'optimization' of the respective copy special member. Another way to put it is that copy semantics are a refinement of move semantics. This means that the assertion must hold in the following:</p> <pre><code>T a; T b = a; T c = std::move(a); assert( b == c ); </code></pre> <p>I.e. whether we arrived there via a copy 'request' (that is, an expression involving an lvalue source) or via a move request (an expression involving an rvalue source), we must have the same result regardless of what 'actually' happened (whether a copy special member or move special member was involved, if at all).</p> <p>Of interest is the fact that the traits such as <code>std::is_copy_constructible</code> used to be called <code>std::has_copy_constructor</code>, but were renamed to put the emphasis on expressions rather than intrinsic properties: something like <code>std::is_copy_constructible&lt;int&gt;::value &amp;&amp; std::is_move_assignable&lt;int&gt;::value</code> is true regardless of the fact that <code>int</code> has no constructors or assignment operators.</p> <p>I advise you to really do generic programming by expressing constraints on the expression level because e.g. the presence or absence of a move constructor is neither sufficient nor necessary for a type to be copy constructible.</p>
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    1. POWhat is a "Regular Type" in the context of move semantics?
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    1. USLuc Danton
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