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copied!<p>Let's say you have two functions on a class, both with the same name and signature. But one of them is declared <code>const</code>:</p> <pre><code>void SomeFunc() const; void SomeFunc(); </code></pre> <p>If a class instance is not <code>const</code>, overload resolution will preferentially select the non-const version. If the instance is <code>const</code>, the user can only call the <code>const</code> version. And the <code>this</code> pointer is a <code>const</code> pointer, so the instance cannot be changed.</p> <p>What "r-value reference for this` does is allow you to add another alternative:</p> <pre><code>void RValueFunc() &&; </code></pre> <p>This allows you to have a function that can <em>only</em> be called if the user calls it through a proper r-value. So if this is in the type <code>Object</code>:</p> <pre><code>Object foo; foo.RValueFunc(); //error: no `RValueFunc` version exists that takes `this` as l-value. Object().RValueFunc(); //calls the non-const, && version. </code></pre> <p>This way, you can specialize behavior based on whether the object is being accessed via an r-value or not.</p> <p>Note that you are not allowed to overload between the r-value reference versions and the non-reference versions. That is, if you have a member function name, all of its versions either use the l/r-value qualifiers on <code>this</code>, or none of them do. You can't do this:</p> <pre><code>void SomeFunc(); void SomeFunc() &&; </code></pre> <p>You must do this:</p> <pre><code>void SomeFunc() &; void SomeFunc() &&; </code></pre> <p>Note that this declaration changes the type of <code>*this</code>. This means that the <code>&&</code> versions all access members as r-value references. So it becomes possible to easily move from within the object. The example given in the first version of the proposal is (note: the following may not be correct with the final version of C++11; it's straight from the initial "r-value from this" proposal):</p> <pre><code>class X { std::vector<char> data_; public: // ... std::vector<char> const & data() const & { return data_; } std::vector<char> && data() && { return data_; } }; X f(); // ... X x; std::vector<char> a = x.data(); // copy std::vector<char> b = f().data(); // move </code></pre>

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