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PODo I need to make a type a POD to persist it with a memory-mapped file?
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copied!<p>Pointers cannot be persisted directly to file, because they point to absolute addresses. To address this issue I wrote a <code>relative_ptr</code> template that holds an offset instead of an absolute address.</p> <p>Based on the fact that only trivially copyable types can be safely copied bit-by-bit, I made the assumption that this type needed to be trivially copyable to be safely persisted in a memory-mapped file and retrieved later on.</p> <p>This restriction turned out to be a bit problematic, because the compiler generated copy constructor does not behave in a meaningful way. I found nothing that forbid me from defaulting the copy constructor and making it private, so I made it private to avoid accidental copies that would lead to undefined behaviour.</p> <p>Later on, I found <a href="http://www.boost.org/doc/libs/1_47_0/doc/html/interprocess/offset_ptr.html"><code>boost::interprocess::offset_ptr</code></a> whose creation was driven by the same needs. However, it turns out that <code>offset_ptr</code> is not trivially copyable because it implements its own custom copy constructor.</p> <p>Is my assumption that the smart pointer needs to be trivially copyable to be persisted safely wrong?</p> <p>If there's no such restriction, I wonder if I can safely do the following as well. If not, exactly what are the requirements a type must fulfill to be usable in the scenario I described above?</p> <pre><code>struct base { int x; virtual void f() = 0; virtual ~base() {} // virtual members! }; struct derived : virtual base { int x; void f() { std::cout << x; } }; using namespace boost::interprocess; void persist() { file_mapping file("blah"); mapped_region region(file, read_write, 128, sizeof(derived)); // create object on a memory-mapped file derived* d = new (region.get_address()) derived(); d.x = 42; d->f(); region.flush(); } void retrieve() { file_mapping file("blah"); mapped_region region(file, read_write, 128, sizeof(derived)); derived* d = region.get_address(); d->f(); } int main() { persist(); retrieve(); } </code></pre> <hr> <p>Thanks to all those that provided alternatives. It's unlikely that I will be using something else any time soon, because as I explained, I already have a working solution. And as you can see from the use of question marks above, I'm really interested in knowing why Boost can get away without a trivially copyable type, and how far can you go with it: it's quite obvious that classes with virtual members will not work, but <strong>where do you draw the line?</strong></p>

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