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POC++ / C# ABC design pattern with static member
 

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  1. POC++ / C# ABC design pattern with static member
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    <p>I have an ABC class I want to make generic:</p> <pre><code>struct BinaryVariable { static const int N = 2; static std::string outcome_names[N] = { "heads", "tails" }; }; struct EvidenceBinaryVariable : public BinaryVariable { double likelihoods[ BinaryVariable::N ]; }; struct LatentBinaryVariable : public BinaryVariable { }; </code></pre> <p>The important thing is that every instance of these three nodes shares the same <code>::N</code>, and uses the same memory for the <code>outcome_names</code> variable. <br> <br></p> <p>Let's say I want to make these three node types to also allow for a variable with 6 possible outcomes (i.e. I want to support dice in addition to coins). In C++ I can do this with templates:</p> <pre><code>#include&lt;cstddef&gt; #include&lt;string&gt; template&lt;std::size_t N&gt; struct DiscreteVariable { static std::string outcome_names[N]; }; template&lt;std::size_t N&gt; struct EvidenceVariable : public DiscreteVariable&lt;N&gt; { double likelihoods[ DiscreteVariable::N ]; }; template&lt;std::size_t N&gt; struct LatentVariable : public DiscreteVariable&lt;N&gt; { }; int main() { LatentVariable&lt;2&gt; schrodingers_coin; LatentVariable&lt;6&gt; schrodingers_die; } </code></pre> <p>I can't figure out a way to get the three types to have a static <code>outcome_names</code> member (and where <code>shrodingers_coin.outcome_names</code> and <code>schrodingers_dice.outcome_names</code> do not refer to the same static piece of memory) without using templates in this way.</p> <p>C# only supports type-based templates (i.e. you could provide <code>&lt;int&gt;</code> but not <code>&lt;6&gt;</code>.</p> <p>First question: is there a good design pattern for this in C++? (e.g. should I use these triplets as internal classes within a master class?)</p> <p>Second question: does this good design pattern work in C# (or is C++'s template magic just a good fit here)?</p> <p>Just for posterity, the reason I want these static is because I have tons of these objects, and am writing something where efficiency is paramount (think big data). </p> <p>Thanks a lot for your help.</p>
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    1. COOP: I edited your C++ code to at least make it valid code.
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      1. USTony The Lion
    2. COFew questions. First, should `LatentVariable<2> foo;` and `LatentVaraible<2> bar;` point to the same `DiscreteVariable` string array? If not, then you don't want `outcome_names[]` to be static. Second, this is an example of when composition makes more sense than inheritance; `DiscreteVariable`, or rather `outcome_names[]` should be a (probably nonstatic depending on the answer to the first question) member of `LatentVariable`.
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    3. COWhy not use an enum?
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      1. USNeil Kirk
 

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