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POInlining an array of non-default constructible objects in a C++ class
 

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  1. POInlining an array of non-default constructible objects in a C++ class
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    <p>C++ doesn't allow a class containing an array of items that are not default constructible:</p> <pre><code>class Gordian { public: int member; Gordian(int must_have_variable) : member(must_have_variable) {} }; class Knot { Gordian* pointer_array[8]; // Sure, this works. Gordian inlined_array[8]; // Won't compile. Can't be initialized. }; </code></pre> <p>As even beginning C++ users know, the language guarantees that all non-POD members are initialized when constructing a class. And it doesn't trust the user to initialize everything in the constructor - one has to provide valid arguments to the constructors of all members before the body of the constructor even starts.</p> <p>Generally, that's a great idea as far as I'm concerned, but I've come across a situation where it would be a lot easier if I could actually have an array of non-default constructible objects.</p> <p>The obvious solution: Have an array of pointers to the objects. This is not optimal in my case, as I am using shared memory. It would force me to do extra allocation from an already contended resource (that is, the shared memory). The entire reason I want to have the array inlined in the object is to reduce the number of allocations.</p> <p>This is a situation where I would be willing to use a hack, even an ugly one, provided it works. One possible hack I am thinking about would be:</p> <pre><code>class Knot { public: struct dummy { char padding[sizeof(Gordian)]; }; dummy inlined_array[8]; Gordian* get(int index) { return reinterpret_cast&lt;Gordian*&gt;(&amp;inlined_array[index]); } Knot() { for (int x = 0; x != 8; x++) { new (get(x)) Gordian(x*x); } } }; </code></pre> <p>Sure, it compiles, but I'm not exactly an experienced C++ programmer. That is, I couldn't possibly trust my hacks less. So, the questions:</p> <p>1) Does the hack I came up with seem workable? What are the issues? (I'm mainly concerned with C++0x on newer versions of GCC).</p> <p>2) Is there a better way to inline an array of non-default constructible objects in a class?</p>
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    1. COThe code is legal. However, I think placing non-POD C++ objects in shared memory is a bad design.
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    2. COI think your current solution is OK. You might want to use a `vector` with a custom allocator that would allocate memory from the buffer you prepare, but this approach afaik doesn't work if you don't want to preallocate any extra memory in this buffer.
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    3. COThe statement, "the language guarantees that all members are initialized when constructing a class," is incorrect. C++ doesn't guarantee initialisation of POD types (ints, C-style structs, etc.).
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