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PO
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19611856
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0
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0
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2
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2013-10-26T21:02:16.143
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0
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2017-12-03T13:15:23.630
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2017-12-03T13:15:23.630
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2712726
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2712726
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9122028
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2
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2
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<p>So far all solutions have drawbacks, unnecessary dependencies, unnecessary helpers and all are restricted to the Cartesian power of two. The following solution has no such drawbacks and supports:</p> <ol> <li>Any cartesian power including 0.</li> <li>Returning the empty set if any of the factors is an empty set.</li> <li>The code is self contained and does not depend on any include files.</li> <li>The inputs of the function can be of any template type.</li> <li>The type of the output list can be specified via the first template parameter.</li> </ol> <p>It was actually to harder to implement (but good as homework) then I thought. I am actually thinking about creating a little generator which allows me an extended template syntax which makes these things really easy. </p> <p>Simplified the code works as follows: <code>product</code> converts an input list <code>tuple<A...>,tuple<B...>,tuple<C...></code> into <code>tuple<tuple<A>...>, tuple<B...>, tuple<C...></code>. This second list is then passed to <code>product_helper</code> which does the recursive Cartesian product computation.</p> <pre><code>template <typename... T> struct cat2; template <template<typename...> class R, typename... As, typename... Bs> struct cat2 <R<As...>, R<Bs...> > { using type = R <As..., Bs...>; }; template <typename... Ts> struct product_helper; template <template<typename...> class R, typename... Ts> struct product_helper < R<Ts...> > { // stop condition using type = R< Ts...>; }; template <template<typename...> class R, typename... Ts> struct product_helper < R<R<> >, Ts... > { // catches first empty tuple using type = R<>; }; template <template<typename...> class R, typename... Ts, typename... Rests> struct product_helper < R<Ts...>, R<>, Rests... > { // catches any empty tuple except first using type = R<>; }; template <template<typename...> class R, typename... X, typename H, typename... Rests> struct product_helper < R<X...>, R<H>, Rests... > { using type1 = R <typename cat2<X,R<H> >::type...>; using type = typename product_helper<type1, Rests...>::type; }; template <template<typename...> class R, typename... X, template<typename...> class Head, typename T, typename... Ts, typename... Rests> struct product_helper < R<X...>, Head<T, Ts...>, Rests... > { using type1 = R <typename cat2<X,R<T> >::type...>; using type2 = typename product_helper<R<X...> , R<Ts...> >::type; using type3 = typename cat2<type1,type2>::type; using type = typename product_helper<type3, Rests...>::type; }; template <template<typename...> class R, typename... Ts> struct product; template <template<typename...> class R> struct product < R > { // no input, R specifies the return type using type = R<>; }; template <template<typename...> class R, template<typename...> class Head, typename... Ts, typename... Tail> struct product <R, Head<Ts...>, Tail... > { // R is the return type, Head<A...> is the first input list using type = typename product_helper< R<R<Ts>...>, Tail... >::type; }; </code></pre> <p>Here is a <a href="https://stackoverflow.com/a/19922797/2712726">compilable example</a> of how the code can be used.</p>
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1. POHow to create the Cartesian product of a type list?
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1. USPatrick Fromberg
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1. USPatrick Fromberg
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1. COIt seems to me as if you could write `cat2` as `template<class A, class B> struct cat2; template< template<class...> class R, class... A, class... B> struct<R<A...>, R<B...>> { .. };`
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2. CO@DyP, you are absolutely right. I will do right away!
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  USPatrick Fromberg

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