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copied!<p>None; no copies are done. </p> <p>The address of the caller's Data return value is actually passed as a hidden argument to the function, and the createData function simply writes into the caller's stack frame.</p> <p>This is known as the <a href="http://en.wikipedia.org/wiki/Return_value_optimization" rel="noreferrer">named return value optimisation</a>. Also see the <a href="http://www.parashift.com/c++-faq-lite/ctors.html#faq-10.9" rel="noreferrer">c++ faq on this topic</a>.</p> <blockquote> <p>commercial-grade C++ compilers implement return-by-value in a way that lets them eliminate the overhead, at least in simple cases</p> <p>...</p> <p>When yourCode() calls rbv(), the compiler secretly passes a pointer to the location where rbv() is supposed to construct the "returned" object.</p> </blockquote> <p>You can demonstrate that this has been done by adding a destructor with a printf to your struct. The destructor should only be called once if this return-by-value optimisation is in operation, otherwise twice.</p> <p>Also you can check the assembly to see that this happens:</p> <pre><code>Data createData() { Data data; // initialize data values... data.values[5] = 6; return data; } </code></pre> <p>here's the assembly:</p> <pre><code>__Z10createDatav: LFB2: pushl %ebp LCFI0: movl %esp, %ebp LCFI1: subl $1032, %esp LCFI2: movl 8(%ebp), %eax movl $6, 20(%eax) leave ret $4 LFE2: </code></pre> <p>Curiously, it allocated enough space on the stack for the data item <code>subl $1032, %esp</code>, but note that it takes the first argument on the stack <code>8(%ebp)</code> as the base address of the object, and then initialises element 6 of that item. Since we didn't specify any arguments to createData, this is curious until you realise this is the secret hidden pointer to the parent's version of Data.</p>

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