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POImplementing swap() for objects with internal pointers
 

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    <p>I'm implementing the <a href="https://stackoverflow.com/questions/3279543/what-is-the-copy-and-swap-idiom">copy-and-swap idiom</a> for <code>operator=</code> of a small non-owning-memory-referencing object I've designed. When <code>MemRef</code> is referencing a piece of a buffer whose lifetime I trust, <code>_ptr</code> points into the buffer, as you'd expect.</p> <p>What is unusual about this <code>MemRef</code> is that it consists not only of a <code>_ptr</code> and a <code>_len</code>, but also a <code>_memory</code> <code>std::string</code>: there are certain users (or situations) of this class whom I <em>do not</em> trust to protect their memory; for them, I actually copy their memory into the <code>_memory</code> string during construction, and set <code>_ptr = _memory.c_str()</code>. I can always determine whether I have an "inref" MemRef (referring to its internal memory) or an "exref" MemRef (referring to some external buffer) by asking if <code>_ptr == _memory.c_str()</code>.</p> <p>I'm confused about how to write the swap routine. The following is taken from <a href="https://stackoverflow.com/questions/3279543/what-is-the-copy-and-swap-idiom">copy-and-swap idiom</a>:</p> <p>Here's <code>operator=</code>:</p> <pre><code>MemRef&amp; MemRef::operator=(MemRef other) { swap(*this, other); return *this; } </code></pre> <p>Here's the copy constructor:</p> <pre><code>// Copy ctor MemRef::MemRef(const MemRef&amp; other) : _memory(other._memory), _ptr(other._ptr), _len(other._len) { // Special provision if copying an "inref" MemRef if (other._ptr == other._memory.c_str()) { _ptr = _memory.c_str(); } } </code></pre> <p>And here's my <code>swap(first, second)</code> - which I believe needs more work.</p> <pre><code>void swap(MemRef&amp; first, MemRef&amp; second) { using std::swap; swap(first._memory, second._memory); swap(first._ptr, second._ptr); swap(first._len, second._len); } </code></pre> <p>So if I have:</p> <pre><code>MemRef mr_a("foo"); // creates an "inref" memref MemRef mr_b(buffer_ptr, length); // creates an "exref" memref -&gt; "blarch" mr_a = mr_b; </code></pre> <p><code>operator=()</code> gets called with a temporary MemRef built by copy-constructing mr_b; it calls <code>swap(mr_a, mr_b_copy);</code> <code>swap()</code> exchanges the pointer, length, and string (so that mr_a's former contents will be destructed along with mr_b_copy). </p> <p>What I don't understand is whether the pointers in mr_a and mr_b_copy are correct at this point, or if they're tangled up with each other.</p> <p><strong>UPDATE 1</strong>: The above example doesn't illustrate the problem. Consider this one:</p> <pre><code>MemRef mr_a; // creates a memref with _ptr(NULL), _len(0) mr_a = "woof"; // </code></pre> <p>For passing by value to operator=(), a temporary <strong>inref</strong> is constructed for "woof" and bound to the parameter <code>other</code>. Then, references to <code>mr_a</code> and to <code>other</code> are passed to swap() and bound as <code>first</code> and <code>second</code> respectively. After the swap, <code>first._ptr</code> was ... well, <em>wrong</em>. Pointing to garbage. Here's what I had to do:</p> <pre><code>void swap(MemRef&amp; first, MemRef&amp; second) { using std::swap; // second is an exref if (second._ptr != second._memory.c_str()) { swap(first._memory, second._memory); swap(first._len, second._len); swap(first._ptr, second._ptr); } // second is an inref else { swap(first._memory, second._memory); swap(first._len, second._len); first._ptr = first._memory.c_str(); } } </code></pre> <p>All I can conclude is that std::swap(string, string) is doing something strange.</p>
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    1. CO@Mat: I believe it does, because the RHS is passed to operator=() *by value*. That's part of the trick to the "copy and swap idiom", I gather. See the link.
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    2. COI'm not really sure why they wouldn't be - you seem to have the copyswap idiom pretty down if I'm reading you correctly. `mr_a` changes from an inref to an exref and I'm not sure if that's what you intended - what exactly do you feel like you're doing wrong?
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    3. CO@CommanderCorianderSalamander: I added UPDATE 1 to illustrate where I was actually having the problem. For some reason, a simple swapping of the three components, when `second` is an inref, seems to leave first._ptr pointing to garbage. The same address, but now there's garbage. It's as if swapping strings is moving more than just the "handle" around.
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