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POHow to copy (or swap) objects of a type that contains members that are references or const?
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copied!<p>The problem I am trying to address arises with making containers such as an <code>std::vector</code> of objects that contain reference and const data members:</p> <pre><code>struct Foo; struct Bar { Bar (Foo & foo, int num) : foo_reference(foo), number(num) {} private: Foo & foo_reference; const int number; // Mutable member data elided }; struct Baz { std::vector<Bar> bar_vector; }; </code></pre> <p>This won't work as-is because the default assignment operator for class <code>Foo</code> can't be built due to the reference member <code>foo_reference</code> and const member <code>number</code>.</p> <p>One solution is to change that <code>foo_reference</code> to a pointer and get rid of the <code>const</code> keyword. This however loses the advantages of references over pointers, and that <code>const</code> member really should be <code>const</code>. They are private members, so the only thing that can do harm is my own code, but I have shot myself in the foot (or higher) with my own code.</p> <p>I've seen solutions to this problem on the web in the form of <code>swap</code> methods that appear to be chock full of undefined behavior based on the wonders of <code>reinterpret_cast</code> and <code>const_cast</code>. It happens that those techniques do appear to work on my computer. Today. With one particular version of one particular compiler. Tomorrow, or with a different compiler? Who knows. I am not going to use a solution that relies on undefined behavior.</p> <p>Related answers on stackoverflow:</p> <ul> <li><a href="https://stackoverflow.com/questions/3658854/c-implement-the-copy-assignment-operator-in-a-class-with-all-const-data-mem">Does it make sense to implement the copy-assignment operator in a class with all const data members?</a><br> The first answer has the amusing line "If that one is immutable, your screwed."</li> <li><a href="https://stackoverflow.com/questions/2210388/swap-method-with-const-members">Swap method with const members</a><br> The first answer doesn't really apply here, and the second is a bit of a kludge.</li> </ul> <p>So is there a way to write a <code>swap</code> method / copy constructor for such a class that does not invoke undefined behavior, or am I just screwed?</p> <p><strong>Edit</strong><br> Just to make it clear, I already am quite aware of this solution:</p> <pre><code>struct Bar { Bar (Foo & foo, int num) : foo_ptr(&foo), number(num) {} private: Foo * foo_ptr; int number; // Mutable member data elided }; </code></pre> <p>This explicitly eliminates the <code>const</code>ness of <code>number</code> and the eliminates the implied <code>const</code>ness of <code>foo_reference</code>. This is not the solution I am after. If this is the only non-UB solution, so be it. I am also quite aware of this solution:</p> <pre><code>void swap (Bar & first, Bar & second) { char temp[sizeof(Bar)]; std::memcpy (temp, &first, sizeof(Bar)); std::memcpy (&first, &second, sizeof(Bar)); std::memcpy (&second, temp, sizeof(Bar)); } </code></pre> <p>and then writing the assignment operator using copy-and-swap. This gets around the reference and const problems, but is it UB? (At least it doesn't use <code>reinterpret_cast</code> and <code>const_cast</code>.) Some of the elided mutable data are objects that contain <code>std::vector</code>s, so I don't know if a shallow copy like this will work here.</p>

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