diff options
Diffstat (limited to 'articles/2014-07-11_mapping_arrays_using_tuples_in_cpp11.md')
-rw-r--r-- | articles/2014-07-11_mapping_arrays_using_tuples_in_cpp11.md | 63 |
1 files changed, 0 insertions, 63 deletions
diff --git a/articles/2014-07-11_mapping_arrays_using_tuples_in_cpp11.md b/articles/2014-07-11_mapping_arrays_using_tuples_in_cpp11.md deleted file mode 100644 index 99a8dce..0000000 --- a/articles/2014-07-11_mapping_arrays_using_tuples_in_cpp11.md +++ /dev/null @@ -1,63 +0,0 @@ -# Mapping arrays using tuples in C++11 - -During my proof-of-concept implementation of external functions enabling [XSLT based static site generation](https://github.com/KnairdA/InputXSLT) I came upon the problem of calling a template method specialized on the Nth type of a `std::tuple` specialization using the Nth element of a array-like type instance as input. This was needed to implement a generic template-based interface for implementing [Apache Xalan](http://xalan.apache.org/xalan-c/index.html) external functions. This article aims to explain the particular approach taken to solve this problem. - -While it is possible to unpack a `std::tuple` instance into individual predefined objects using `std::tie` the standard library offers no such helper template for `unpacking` an array into individual objects and calling appropriate casting methods defined by a `std::tuple` mapping type. Sadly exactly this functionality is needed so that we are able to call a `constructDocument` member method of a class derived from the [`FunctionBase`](https://github.com/KnairdA/InputXSLT/blob/master/src/function/base.h) external function interface template class using static polymorphism provided through the [curiously recurring template pattern](https://en.wikipedia.org/wiki/Curiously_Recurring_Template_Pattern). This interface template accepts the desired external function arguments as variadic template types and aims to provide the required validation and conversion boilerplate implementation. While we could recursively generate a `std::tuple` specialization instance from an array-like type using a approach simmilar to the one detailed in my article on [mapping binary structures as tuples using template metaprogramming](/article/mapping_binary_structures_as_tuples_using_template_metaprogramming) this wouldn't solve the problem of passing on the resulting values as individual objects. This is why I had to take the new approach of directly calling a template method on individual array elements using a `std::tuple` specialization as a kind of blueprint and passing the result values of this method to the `constructDocument` method as separate arguments. - -Extracting array elements obviously requires some way of defining the appropriate indexes and mapping the elements using a tuple blueprint additionally requires this way to be statically resolvable as one can not pass a dynamic index value to `std::tuple_element`. So the first step to fullfilling the defined requirements involved the implementation of a template based index or sequence type. - -~~~ -template <std::size_t...> -struct Sequence { - typedef Sequence type; -}; - -template < - std::size_t Size, - std::size_t Index = 0, - std::size_t... Current -> -struct IndexSequence { - typedef typename std::conditional< - Index < Size, - IndexSequence<Size, Index + 1, Current..., Index>, - Sequence<Current...> - >::type::type type; -}; -~~~ -{: .language-cpp} - -This is achieved by the [`IndexSequence` template](https://github.com/KnairdA/InputXSLT/blob/master/src/support/type/sequence.h) above by recursively specializing the `Sequence` template using static recursion controlled by the standard libraries template metaprogramming utility template `std::conditional`. This means that e.g. the type `Sequence<0, 1, 2, 3>` can also be defined as `IndexSequence<4>::type`. - -Now all that is required to accomplish the goal is instantiating the sequence type and passing it to a variadic member template as [follows](https://github.com/KnairdA/InputXSLT/blob/master/src/function/base.h): - -~~~ -[...] -this->callConstructDocument( - parameters, - locator, - typename IndexSequence<parameter_count>::type() -) -[...] -template <std::size_t... Index> -inline xalan::XalanDocument* callConstructDocument( - const XObjectArgVectorType& parameters, - const xalan::Locator* locator, - Sequence<Index...> -) const { - [...] - return this->document_cache_->create( - static_cast<const Implementation*>(this)->constructDocument( - valueGetter.get<typename std::tuple_element< - Index, - std::tuple<Types...> - >::type>(parameters[Index])... - ) - ); -} -~~~ -{: .language-cpp} - -As we can see a `IndexSequence` template specialization instance is passed to the variadic `callConstructDocument` method to expose the actual sequence values as `Index`. This method then resolves the `Index` parameter pack as both the array and `std::tuple` index inside the calls to the `valueGetter.get` template method which is called for every sequence element because of this. What this means is that we are now able to implement non-template `constructDocument` methods inside XSLT external function implementations such as [FunctionTransform](https://github.com/KnairdA/InputXSLT/blob/master/src/function/transform.h). The values passed to these methods are automatically extracted from the argument array and converted into their respective types as required. - -While this article only provided a short overview of mapping arrays using tuples in C++11 one may view the full implementation on [Github](https://github.com/KnairdA/InputXSLT/blob/master/src/function/base.h) or [cgit](http://code.kummerlaender.eu/InputXSLT/tree/src/function/base.h). |