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POTrying to wrap std containers to store rvalue references (like unique_ptr, but on the stack)
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  1. POTrying to wrap std containers to store rvalue references (like unique_ptr, but on the stack)
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    copied!<p>I'm trying to do strange things again.</p> <p>Okay, here's the general idea. I want a <code>std::list</code> (and <code>vector</code> and so on) that actually own the objects they contain. I want to <strong>move</strong> the values into it, and access them by reference.</p> <p>Example using a list of unique_ptr:</p> <pre><code>using namespace std; list&lt;unique_ptr&lt;T&gt;&gt; items; // T is whatever type items.push_back(make_unique(...)); items.push_back(make_unique(...)); for ( unique_ptr&lt;T&gt; item : items ) item.dosomething(); </code></pre> <p>With me so far? Good. Now, let's do it with stack semantics and rvalue references. We can't just use a <code>list&lt;T&amp;&amp;&gt;</code> for obvious reasons, so we'd have to make a new class:</p> <pre><code>using namespace std; owninglist&lt;T&gt; items; items.push_back(T()); items.push_back(T()); for ( T&amp; item : items ) item.dosomething(); </code></pre> <p>Of course, I might want an <code>owningstack</code> or <code>owningvector</code> as well, so ideally we want it to be templated:</p> <pre><code>owning&lt;std::list&lt;T&gt;&gt; items; </code></pre> <p>The <code>owning&lt;U&lt;T&gt;&gt;</code> class should inherit whatever <code>push_back()</code> and <code>pop_front()</code> etc functions the underlying collection has. Presumably to achieve that, I'd need to code a generic base class, and derive explicit specialisations for the collections that have unusual functions:</p> <pre><code>template&lt;typename T&gt; owning&lt;std::queue&lt;T&gt;&gt; : owningbase&lt;T&gt; { void push_front() { ... } } </code></pre> <p>I'm getting stuck on the iterators. The <code>begin()</code> and <code>end()</code> functions should return an iterator that works the same as the underlying collection's iterator would, except with an <code>operator*()</code> that returns the item by <strong>lvalue</strong> reference instead of by value.</p> <p>We'd need some way to transfer ownership of an item out of the list again. Perhaps the iterator could have an <code>operator~</code> that returns the item as an <strong>rvalue</strong>, deletes the item from the list, and invalidates the iterator?</p> <p>Of course, all this is assuming the underlying <code>std::list</code> (or whatever) can be convinced to take an rvalue. If <code>push_back()</code> copies the value in as an lvalue, then none of this is going to work. Would I be better off coding a container from scratch? If I did, is there some way to put the majority of the code for <code>list</code>, <code>queue</code>, <code>stack</code> and <code>vector</code> into a single base class, to save rewriting pretty much the same class four times over?</p> <p>Perhaps I could introduce an intermediate class, some kind of wrapper? So <code>owned&lt;list&lt;T&gt;&gt;</code> could inherit from <code>list&lt;refwrapper&lt;T&gt;&gt;</code> or something? I know boost has a <code>reference_wrapper</code>, but I'm not sure it fits this scenario.</p>
 

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