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POHow to manage shared_ptr that points to internal data of already referenced object?
 

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  1. POHow to manage shared_ptr that points to internal data of already referenced object?
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    <p>Suppose I have these classes:</p> <pre><code>struct Engine { int engine_data; }; struct Car { shared_ptr&lt;Engine&gt; engine; int car_data; }; </code></pre> <p>For performance reasons, I want to make them tightly packed in memory (but I don't want to lose the flexibility of the design). So, I can create a "packed" structure, and a factory that will transparently return a new B instance:</p> <pre><code>struct PackedCarAndEngine { Engine engine; Car car; }; shared_ptr&lt;Car&gt; new_Car() { shared_ptr&lt;PackedCarAndEngine&gt; packed = make_shared&lt;PackedCarAndEngine&gt;(); // uses aliasing shared_ptr constructor packed-&gt;car.engine = shared_ptr&lt;Engine&gt;(packed, &amp;packed-&gt;engine); // again shared_ptr&lt;Car&gt; car = shared_ptr&lt;Car&gt;(packed, &amp;packed-&gt;car); return car; } </code></pre> <p>The problem is that this "car" instance will never be destroyed, because it have a reference count of two. When it dies, it will have a reference count of one, forever. Do you know a better way to keep using the internal shared_ptr's (so that I can attribute an "unpacked" reference if I want), and still make this packed structure?</p> <p><strong>UPDATE</strong></p> <p>I could use a no-op deleter, but then it would be very dangerous if I decide to keep the <code>engine</code> but not the <code>car</code>:</p> <pre><code> // ... packed-&gt;car.engine = shared_ptr&lt;Engine&gt;(&amp;packed-&gt;engine, do_nothing_deleter); // ... shared_ptr&lt;Car&gt; my_car = new_Car(); shared_ptr&lt;Engine&gt; my_engine = my_car-&gt;engine; my_car.reset(); // Danger: engine was destroyed here!!! cout &lt;&lt; my_engine-&gt;engine_data; // Crash! </code></pre>
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    1. COWhy don't you just make an Engine a straight member of Car? If you're always creating a Car and Engine as part of the same object it implies they have the same lifetime. Car knows about engine but not the other way around. Unless you're planning on sharing the same Engine instance between different Car instances wouldn't `struct Car { Engine engine; int car_data; };` be a simpler design?
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    2. CO@Charles I've thought about this... in case I can't solve the problem easily with `shared_ptr`'s, I will fall back to this solution. In this example I gave, I have only one composition, but in my real design, I have many classes composed this way, in multiple layers, and I want to have the flexibility of assigning different `engine`s coming from other places.
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