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copied!<blockquote> <p>I was wondering, was this restriction lifted in C++11?</p> </blockquote> <p>How could it be? By returning something by value, you are by definition copying (or moving) it. And while C++ can allow that copy/move to be elided in certain circumstances, it's still copying (or moving) by the specification.</p> <blockquote> <p>I remember it could be useful to allow callers of a function use the returned object, but that they are not able to copy the value and store it somewhere. </p> </blockquote> <p>Yes. You get rid of the copy constructor/assignment, but allow the value to be <em>moved</em>. <code>std::unique_ptr</code> does this.</p> <p>You can return a <code>unique_ptr</code> by value. But in doing so, you are returning an "prvalue": a temporary that is being destroyed. Therefore, if you have a function <code>g</code> as such:</p> <pre><code>std::unique_ptr<SomeType> g() {...} </code></pre> <p>You can do this:</p> <pre><code>std::unique_ptr<SomeType> value = g(); </code></pre> <p>But not <em>this</em>:</p> <pre><code>std::unique_ptr<SomeType> value1 = g(); std::unique_ptr<SomeType> value2 = g(); value1 = value 2; </code></pre> <p>But this <em>is</em> possible:</p> <pre><code>std::unique_ptr<SomeType> value = g(); value = g(); </code></pre> <p>The second line invokes the move assignment operator on <code>value</code>. It will delete the old pointer and <em>move</em> the new pointer into it, leaving the old value empty.</p> <p>In this way, you can ensure that the contents of any <code>unique_ptr</code> is only ever stored in one place. You can't stop them from referencing it in multiple places (via pointers to <code>unique_ptr</code> or whatever), but there will be at most one location in memory where the actual pointer is stored.</p> <p>Removing both the copy and move constructors creates an <em>immobile</em> object. Where it is created is where it's values stay, <em>forever</em>. Movement allows you to have unique ownership, but without being immobile.</p>

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