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POPointer-to-member, type descriptors and references
 

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  1. POPointer-to-member, type descriptors and references
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    <p>I'm working on a type descriptor project in C++11. The type descriptor's job is to know the types of every member in a class, it's size and it's offset from the base of an object. I don't support multiple inheritance as well as objects with virtual methods so I am making this a lot simpler for now. The goal is to be able to serialize and unserialize objects using the descriptor.</p> <p><em>Note that this is a pet project to mess around with features like variadic templates, pointer to members and other features of C++ I am not familiar with, so there's no need to point me toward something like boost::archiving. :)</em></p> <p>The way I actually register members is very similar to boost::python::class_'s way.</p> <pre><code>ClassDescriptor fooDesc( "Foo" ); fooDesc.addMember( "a", &amp;Foo:: a ); fooDesc.addMember( "b", &amp;Foo:: b ); // (abridged for clarity) : template&lt; typename ClassType, typename MemberType &gt; ClassDescriptor&amp; ClassDescriptor::addMember( const char* name, MemberType ClassType::* member ) { return addMember&lt; MemberType &gt;( name, reinterpret_cast&lt; size_t &gt;( &amp;(((ClassType*)0)-&gt;*member)) ); } </code></pre> <p>Unfortunately, the pointer-to-member feature of C++ can't be used with references in C++, as I've learned earlier this week : <a href="https://stackoverflow.com/a/8336479/1074536">https://stackoverflow.com/a/8336479/1074536</a>, so I am not able to use &amp;Foo::refToAndInt for example.</p> <p>As far as how I am computing the offset of the member, I am not using the offset of macro since my classes will not always be PODs.</p> <p>So since I can't use pointer-to-members for computing the offset of references, I thought I'd try :</p> <pre><code>&amp;(((Foo*)nullptr)-&gt;refToAnInt) </code></pre> <p>but has it has been pointed out in another stack overflow thread, this is undefined behavior and on obviously on LLVM it crashes. :(</p> <p>I'd like to avoid doing stuff like taking the previous member's offset, adding it's size and then somehow computing the padding required to align my next member, since it would look messy and error prone</p> <p>So, I can't use both of those tricks and offsetof is only for PODs. Any suggestion on what I could try next, apart from my other horrible suggestion?</p> <p>Thanks!</p>
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    1. COWhy would you serialize/unserialize references? They must be initialized in the constructor and cannot be changed later anyway.
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    2. CO@7vies : If you treat an object as a simple array of bytes, it becomes very easy to simply consider references as blocks of 8 bytes (on 64-bit) to fill. The idea here is to simply allocate an array of bytes big enough to hold the object in memory and then punch in the values using the class descriptor. I'm not dealing with vtables, so I don't need the constructor for initialization. Obviously, you have to make sure there is no side effect during regular construction so that loading an asset brings the system in the same state as it was before being persisted.
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    3. COas you already have limitations by not supporting vtables I would say just don't support references neither - this is kind of minor limitation with respect to not supporting vtables. By the way, how are you going to unserialize pointers or references? (obviously, you can't just restore them to the same value they had during serialization, as memory location might be different)
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