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POWhat is the Performance, Safety, and Alignment of a Data member hidden in an embedded char array in a C++ Class?
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copied!<p>I have seen a codebase recently that I fear is violating alignment constraints. I've scrubbed it to produce a minimal example, given below. Briefly, the players are:</p> <ul> <li><p><em>Pool</em>. This is a class which allocates memory efficiently, for some definition of 'efficient'. <em>Pool</em> is guaranteed to return a chunk of memory that is aligned for the requested size.</p></li> <li><p><em>Obj_list</em>. This class stores homogeneous collections of objects. Once the number of objects exceeds a certain threshold, it changes its internal representation from a list to a tree. The size of <em>Obj_list</em> is one pointer (8 bytes on a 64-bit platform). Its populated store will of course exceed that.</p></li> <li><p><em>Aggregate</em>. This class represents a very common object in the system. Its history goes back to the early 32-bit workstation era, and it was 'optimized' (in that same 32-bit era) to use as little space as possible as a result. <em>Aggregate</em>s can be empty, or manage an arbitrary number of objects.</p></li> </ul> <p>In this example, <em>Aggregate</em> items are always allocated from <em>Pool</em>s, so they are always aligned. The only occurrences of <em>Obj_list</em> in this example are the 'hidden' members in <em>Aggregate</em> objects, and therefore they are always allocated using <em>placement new</em>. Here are the support classes:</p> <pre><code>class Pool { public: Pool(); virtual ~Pool(); void *allocate(size_t size); static Pool *default_pool(); // returns a global pool }; class Obj_list { public: inline void *operator new(size_t s, void * p) { return p; } Obj_list(const Args *args); // when constructed, Obj_list will allocate representation_p, which // can take up much more space. ~Obj_list(); private: Obj_list_store *representation_p; }; </code></pre> <p>And here is Aggregate. Note that member declaration <em>member_list_store_d</em>:</p> <pre><code>// Aggregate is derived from Lesser, which is twelve bytes in size class Aggregate : public Lesser { public: inline void *operator new(size_t s) { return Pool::default_pool->allocate(s); } inline void *operator new(size_t s, Pool *h) { return h->allocate(s); } public: Aggregate(const Args *args = NULL); virtual ~Aggregate() {}; inline const Obj_list *member_list_store_p() const; protected: char member_list_store_d[sizeof(Obj_list)]; }; </code></pre> <p>It is that data member that I'm most concerned about. Here is the pseudocode for initialization and access:</p> <pre><code>Aggregate::Aggregate(const Args *args) { if (args) { new (static_cast<void *>(member_list_store_d)) Obj_list(args); } else { zero_out(member_list_store_d); } } inline const Obj_list *Aggregate::member_list_store_p() const { return initialized(member_list_store_d) ? (Obj_list *) &member_list_store_d : 0; } </code></pre> <p>You may be tempted to suggest that we replace the char array with a pointer to the <em>Obj_list</em> type, initialized to NULL or an instance of the class. This gives the proper semantics, but just shifts the memory cost around. If memory were still at a premium (and it might be, this is an EDA database representation), replacing the char array with a pointer to an <em>Obj_list</em> would cost one more pointer in the case when <em>Aggregate</em> objects <em>do</em> have members.</p> <p>Besides that, I don't really want to get distracted from the main question here, which is alignment. I <em>think</em> the above construct is problematic, but can't really find more in the standard than some vague discussion of the alignment behavior of the 'system/library' <em>new</em>.</p> <p>So, does the above construct do anything more than cause an occasional pipe stall?</p> <p><strong>Edit</strong>: I realize that there are ways to <em>replace</em> the approach using the embedded char array. So did the original architects. They discarded them because memory was at a premium. Now, if I have a reason to touch that code, I'll probably change it.</p> <p>However, my question, about the alignment issues inherent in this approach, is what I hope people will address. Thanks!</p>

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