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    <p>If i had to define a variant object, i'd probably start with the following :</p> <pre><code>template&lt;typename Type1, typename Type2&gt; class VariantVisitor; template&lt;typename Type1, typename Type2&gt; class Variant { public: friend class VariantVisitor&lt;Type1, Type2&gt;; Variant(); Variant(Type1); Variant(Type2); // + appropriate operators = ~Variant(); // deal with memory management private: int type; // 0 for invalid data, 1 for Type1, 2 for Type2 void* data; }; template&lt;typename Visitor, typename Type1, typename Type2&gt; class VariantVisitor { private: Visitor _customVisitor; public: void doVisit(Variant&lt;Type1, Type2&gt;&amp; v) { if( v.type == 1 ) { _customVisitor( *(Type1*)(v.data)); } else if( v.type == 2 ) { _customVisitor( *(Type2*)(v.data)); } else { // deal with empty variant } } }; template&lt;typename Visitor, typename Type1, typename Type2&gt; void visit( Visitor visitor, Variant&lt;Type1, Type2&gt; v ) { VariantVisitor&lt;Visitor, Type1, Type2&gt;(visitor).doVisit(v); } </code></pre> <p>then use <a href="http://www.boost.org/doc/libs/release/libs/mpl/doc/refmanual/vector.html" rel="nofollow noreferrer">MPL vectors</a> to make the approach work for more than just two different types.</p> <p>In the end, you could write something like this :</p> <pre><code>Variant&lt;Type1, Type2&gt; v; class MyVisitor { public: operator()(Type1); operator()(Type2); }; MyVisitor visitor; v = Type1(); visit(visitor, v); v = Type2(); visit(visitor, v); </code></pre> <p>NB : there is no chance this code compiles, but this describes the ideas i'd use.</p>
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    1. POHow to implement a basic Variant (& a visitor on the Variant) template in C++?
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    1. COI doubt that it is anything like Boost.Variant, though. For example, isn't sizeof(variant<X, Y>) dependent on the largest of the types - which indicates that the values are not stored as a `void*`? - Unfortunately, the source is too long and complicated for me to read (one probably needs to fight the compiler for every little step towards the goal, so I'm grateful I can just use other peoples' work if I need to :))
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    2. COI completely agree. I would never use a personal version of variants. Boost.Variant works fine and i don't really need to know how. I was simply explaining how it *could* be done.
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