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copied!<p>It's a very common problem indeed, last time I saw it was with matrices (dimensions as template parameters and how to deal with runtime supplied value).</p> <p>It's unfortunately an intractable problem.</p> <p>The issue is not specific to C++ per se, it's specific to strong typing coupled with compile-time checking. For example Haskell could exhibit a similar behavior.</p> <p>There are 2 ways to deal with this:</p> <ul> <li>You use a <code>switch</code> not to create the type but actually to launch the full computation, ie <code>main</code> is almost empty and only serve to read the input value</li> <li>You use boxing: you put the actual type in a generic container (either by hand-crafted class or <code>boost::any</code> or <code>boost::variant</code>) and then, when necessary, unbox the value for specific treatment.</li> </ul> <p>I personally prefer the second approach.</p> <p>The easier way to do this is to use a base class (interface):</p> <pre><code>struct UnsignedBase: boost::noncopyable { virtual ~UnsignedBase() {} virtual UnsignedBase* clone() const = 0; virtual size_t bytes() const = 0; virtual void add(UnsignedBase const& rhs) = 0; virtual void substract(UnsignedBase const& rhs) = 0; }; </code></pre> <p>Then you wrap this class in a simple manager to ease memory management for clients (you hide the fact that you rely on heap allocation + <code>unique_ptr</code>):</p> <pre><code>class UnsignedBox { public: explicit UnsignedBox(std::string const& integer); template <size_t N> explicit UnsignedBox(Unsigned<N> const& integer); size_t bytes() const { return mData->bytes(); } void add(UnsignedBox const& rhs) { mData->add(rhs.mData); } void substract(UnsignedBox const& rhs) { mData->substract(rhs.mData); } private: std::unique_ptr<UnsignedBase> mData; }; </code></pre> <p>Here, the virtual dispatch takes care of unboxing (somewhat), you can also unbox manually using a <code>dynamic_cast</code> (or static_cast if you know the number of digits):</p> <pre><code>void func(UnsignedBase* i) { if (Unsigned<2>* ptr = dynamic_cast< Unsigned<2> >(i)) { } else if (Unsigned<4>* ptr = dynamic_cast< Unsigned<4> >(i)) { } // ... else { throw UnableToProceed(i); } } </code></pre>

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