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C++17 implementation of Dispatch_(or_drop)?_output_iterator #8854

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C++17 implementation of Dispatch_(or_drop)?_output_iterator
lrineau Apr 21, 2025
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suggestion by Copilot review
lrineau Apr 21, 2025
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finish the work
lrineau Apr 22, 2025
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improvements after review
lrineau Apr 22, 2025
2f92809
Dispatch_output_iterator: fix the implementation of `assign`
lrineau May 13, 2025
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179 changes: 85 additions & 94 deletions STL_Extension/include/CGAL/iterator.h
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Original file line number Diff line number Diff line change
Expand Up @@ -21,20 +21,21 @@

#include <CGAL/disable_warnings.h>

#include <boost/config.hpp>

#include <CGAL/assertions.h>
#include <CGAL/circulator.h>
#include <CGAL/Iterator_range.h>
#include <CGAL/tuple.h>
#include <CGAL/type_traits.h>
#include <CGAL/use.h>

#include <variant>
#include <optional>
#include <boost/config.hpp>

#include <vector>
#include <map>
#include <optional>
#include <tuple>
#include <utility>
#include <variant>
#include <vector>

namespace CGAL {

Expand Down Expand Up @@ -1282,124 +1283,118 @@ struct Output_visitor {

} // internal

namespace tuple_internal {
template <typename ...Args, std::size_t ...Is>
auto to_tuple(std::tuple<Args...> &t, std::index_sequence<Is...>)
{
return std::tuple<Args&...>(std::get<Is>(t)...);
}

}//end namespace tuple_internal

namespace internal {
// OutputIterator which accepts several types in *o++= and dispatches,
// wraps several other output iterators, and dispatches accordingly.
template < typename V, typename O, bool or_drop = false >
class Dispatch_output_iterator;

template < typename D, bool with_drop, typename V = std::tuple<>, typename O = std::tuple<> >
struct Derivator
{
typedef Derivator<D, with_drop, V, O> Self;
Derivator() = default;
Derivator(const Self&) = default;
Self& operator=(const Self&) = delete;
template <class Tuple>
void tuple_dispatch(const Tuple&)
{}
template <std::size_t N, typename V, typename O>
struct Output_to_nth_of_tuple_of_iterators {
template <typename Tuple>
static void assign(Tuple& tuple, const V& v) {
*std::get<N>(tuple)++ = v;
}
};

template < typename D>
struct Derivator<D, true, std::tuple<>, std::tuple<>>
template <bool drop_value>
struct Drop_output_iterator
{
typedef Derivator<D, true, std::tuple<>, std::tuple<>> Self;
Derivator() = default;
Derivator(const Self&) = default;
Self& operator=(const Self&) = delete;
template <class Tuple>
void tuple_dispatch(const Tuple&){}
template <class T>
Self& operator=(const T&) { return *this; } // dropping value
template <typename Tuple, typename T>
static void assign(Tuple&, const T&) {
// Drop the value... Only if `drop_value==true`.
}
};

template < typename D, bool with_drop, typename V1, typename O1, typename... V, typename... O>
struct Derivator<D, with_drop, std::tuple<V1, V...>, std::tuple<O1, O...> >
: public Derivator<D, with_drop, std::tuple<V...>, std::tuple<O...> >
template <>
struct Drop_output_iterator<false>
{
typedef Derivator<D, with_drop, std::tuple<V1, V...>, std::tuple<O1, O...> > Self;
typedef Derivator<D, with_drop, std::tuple<V...>, std::tuple<O...> > Base;

Derivator() = default;
Derivator(const Self&) = default;
Self& operator=(const Self&) = delete;

using Base::operator=;

D& operator=(const V1& v)
{
* std::get< D::size - sizeof...(V) - 1 >(static_cast<typename D::Iterator_tuple&>(static_cast<D&>(*this))) ++ = v;
return static_cast<D&>(*this);
}

template <class Tuple>
void tuple_dispatch(const Tuple& t)
{
* std::get< D::size - sizeof...(V) - 1 >(static_cast<typename D::Iterator_tuple&>(static_cast<D&>(*this))) ++ =
std::get< D::size - sizeof...(V) - 1 >(t);
static_cast<Base&>(*this).tuple_dispatch(t);
}
// fake assign, declared but deleted
template <typename Tuple>
static void assign(Tuple& tuple, Null_tag) = delete;
};

} // internal
template <typename Values, typename OutputIterators, typename Indices, bool>
struct Dispatch_output_iterator_aux;

namespace tuple_internal {
template <typename ...Args, std::size_t ...Is>
auto to_tuple(std::tuple<Args...> &t, std::index_sequence<Is...>)
template <typename... V, typename... O, std::size_t... Indices, bool drop_unknown_value_types>
struct Dispatch_output_iterator_aux<std::tuple<V...>,
std::tuple<O...>,
std::index_sequence<Indices...>,
drop_unknown_value_types>
: public Output_to_nth_of_tuple_of_iterators<Indices, V, O>...,
public Drop_output_iterator<drop_unknown_value_types>
{
return std::tuple<Args&...>(std::get<Is>(t)...);
}

}//end namespace tuple_internal
using Output_to_nth_of_tuple_of_iterators<Indices, V, O>::assign...;
using Drop_output_iterator<drop_unknown_value_types>::assign;

// OutputIterator which accepts several types in *o++= and dispatches,
// wraps several other output iterators, and dispatches accordingly.
template < typename V, typename O, bool drop_unknown_value_types>
class Dispatch_output_iterator_impl;
template <typename This, typename Tuple>
void tuple_dispatch(This& self, const Tuple& t) {
// Assign each element of the tuple to the corresponding output iterator,
// using a fold expression on "operator," to expand the parameter pack.
( static_cast<Output_to_nth_of_tuple_of_iterators<Indices, V, O>&>(*this).
assign(self, std::get<Indices>(t)), ...);
}
};

template < typename... V, typename... O, bool drop_unknown_value_types>
class Dispatch_output_iterator_impl < std::tuple<V...>, std::tuple<O...>, drop_unknown_value_types>
: private internal::Derivator<Dispatch_output_iterator_impl< std::tuple<V...>, std::tuple<O...>, drop_unknown_value_types >, drop_unknown_value_types, std::tuple<V...>, std::tuple<O...> >
, public std::tuple<O...>
template < typename... V, typename... O, bool drop_unknown_value_types >
class Dispatch_output_iterator < std::tuple<V...>, std::tuple<O...>, drop_unknown_value_types >
: public Dispatch_output_iterator_aux< std::tuple<V...>, std::tuple<O...>,
std::make_index_sequence<sizeof...(O)>, drop_unknown_value_types >
, public std::tuple<O...>
{
static_assert(sizeof...(V) == sizeof...(O),
"The number of explicit template parameters has to match the number of arguments");
static_assert(
sizeof...(V) == sizeof...(O),
"Dispatch_output_iterator: The number of value types (V...) must match the number of output iterators (O...).");

static const int size = sizeof...(V);

template <typename D, bool with_drop, typename V_, typename O_>
friend struct internal::Derivator;

public:
using Iterator_tuple = std::tuple<O...>;
using Value_type_tuple = std::tuple<V...>;

typedef std::tuple<O...> Iterator_tuple;
typedef std::tuple<V...> Value_type_tuple;

typedef std::output_iterator_tag iterator_category;
typedef void value_type;
typedef void difference_type;
typedef void pointer;
typedef void reference;
using iterator_category = std::output_iterator_tag;
using value_type = void;
using difference_type = void;
using pointer = void;
using reference = void;

private:

typedef Dispatch_output_iterator_impl Self;
typedef internal::Derivator<Self, drop_unknown_value_types, Value_type_tuple, Iterator_tuple > Base;
using Self = Dispatch_output_iterator;
using Base = Dispatch_output_iterator_aux<std::tuple<V...>,
std::tuple<O...>,
std::index_sequence_for<O...>,
drop_unknown_value_types>;

public:

using Base::operator=;
using Base::tuple_dispatch;

Dispatch_output_iterator_impl(O... o) : std::tuple<O...>(o...) {}
Dispatch_output_iterator(O... o) : std::tuple<O...>(o...) {}


Dispatch_output_iterator_impl(const Dispatch_output_iterator_impl&)=default;
Dispatch_output_iterator(const Dispatch_output_iterator&)=default;

Self& operator=(const Self& s)
{
static_cast<Iterator_tuple&>(*this) = static_cast<const Iterator_tuple&>(s);
return *this;
}

template <typename T, typename = std::enable_if_t<!std::is_convertible_v<T, Self>>>
Self& operator=(const T& t) {
this->assign(*this, t);
return *this;
}

template<typename ... T>
Self& operator=(const std::variant< T ... >& t) {
internal::Output_visitor<Self> visitor(this);
Expand All @@ -1422,8 +1417,8 @@ class Dispatch_output_iterator_impl < std::tuple<V...>, std::tuple<O...>, drop_u

Self& operator=(const std::tuple<V...>& t)
{
tuple_dispatch(t);
return *this;
this->tuple_dispatch(*this, t);
return *this;
}

operator std::tuple<O&...>()
Expand All @@ -1437,10 +1432,6 @@ class Dispatch_output_iterator_impl < std::tuple<V...>, std::tuple<O...>, drop_u
}
};


template<class V, class O>
using Dispatch_output_iterator = Dispatch_output_iterator_impl<V,O,false>;

template < typename... V, typename... O>
Dispatch_output_iterator<std::tuple<V...>, std::tuple<O...> >
dispatch_output(O... o)
Expand All @@ -1452,8 +1443,8 @@ dispatch_output(O... o)
// Same as Dispatch_output_iterator, but has a dummy *o++= for all other types
// that drops the data (same as Emptyset_iterator).

template<class V, class O>
using Dispatch_or_drop_output_iterator = Dispatch_output_iterator_impl<V,O,true>;
template < typename Tuple_of_v, typename Tuple_of_output_it >
using Dispatch_or_drop_output_iterator = Dispatch_output_iterator< Tuple_of_v, Tuple_of_output_it, true >;

template < typename... V, typename... O>
inline
Expand Down