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    <p>If you can live with having to use a different syntax for specifying complete types, you might do the following (warning: untested code):</p> <p>Let's first start with the machinery:</p> <pre><code>// this gives the complete type which needs to be used to create objects // and provides the implementation of clone() template&lt;typename T&gt; class Cloneable: public T { public: template&lt;typename... U&gt; Cloneable(U&amp;&amp;... u): T(std::forward&lt;U&gt;(u) ...) {} T* clone() { return new Cloneable(*this); } private: // this makes the class complete // Note: T:: to make it type dependent, so it can be found despite not yet defined typename T::CloneableBase::CloneableKey unlock() {} }; // this provides the clone function prototype and also makes sure that only // Cloneable&lt;T&gt; can be instantiated class CloneableBase { template&lt;typename T&gt; friend class Cloneable; // this type is only accessible to Clonerable instances struct CloneableKey {}; // this has to be implemented to complete the class; only Cloneable instances can do that virtual CloneableKey unlock() = 0; public: virtual CloneableBase* clone() = 0; virtual ~CloneableBase() {} }; </code></pre> <p>OK, now the actual class hierarchy. That one is pretty standard; no CRTP intermediates or other complications. However no class implements the <code>clone</code> function, but all inherit the declaration (directly or indirectly) from <code>CloneableBase</code>.</p> <pre><code>// Base inherits clone() from CloneableBase class Base: public CloneableBase { // ... }; // Derived can inherit normally from Base, nothing special here class Derived: public Base { // ... }; </code></pre> <p>Here's how you then create objects:</p> <pre><code>// However, to create new instances, we actually need to use Cloneable&lt;Derived&gt; Cloneable&lt;Derived&gt; someObject; Derived* ptr = new Cloneable&lt;Derived&gt;(whatever); // Now we clone the objects Derived* clone1 = someObject.clone(); Derived* clone2 = ptr-&gt;clone(); // we can get rid og the objects the usual way: delete ptr; delete clone1; delete clone2; </code></pre> <p>Note that a <code>Cloneable&lt;Derived&gt;</code> is-a <code>Derived</code> (it is a subclass), therefore you need to use <code>Cloneable</code> only for construction, and can otherwise pretend to work with <code>Derived</code> objects (well, <code>tyepinfo</code> will also identify it as <code>Cloneable&lt;Derived&gt;</code>).</p>
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