Extracted and unified default template parameter handling helpers.

The new eval_if_default helper replaces both ia_dflt_help and
ia_eval_if_default helpers. Additionally, added a new if_default
helper, which expands to the type or the default parameter directly.

The new helpers are placed in separate headers which can be used
without the iterator_adaptor template.

include/boost/iterator/counting_iterator.hpp

@@ -2,8 +2,8 @@
 // Distributed under the Boost Software License, Version 1.0. (See
 // accompanying file LICENSE_1_0.txt or copy at
 // http://www.boost.org/LICENSE_1_0.txt)
-#ifndef COUNTING_ITERATOR_DWA200348_HPP
-#define COUNTING_ITERATOR_DWA200348_HPP
+#ifndef BOOST_ITERATOR_COUNTING_ITERATOR_DWA200348_HPP
+#define BOOST_ITERATOR_COUNTING_ITERATOR_DWA200348_HPP
 
 #include <limits>
 #include <type_traits>
@@ -13,15 +13,17 @@
 #include <boost/detail/numeric_traits.hpp>
 #include <boost/iterator/iterator_adaptor.hpp>
 #include <boost/iterator/iterator_categories.hpp>
+#include <boost/iterator/detail/if_default.hpp>
+#include <boost/iterator/detail/eval_if_default.hpp>
 #include <boost/iterator/detail/type_traits/type_identity.hpp>
 
 namespace boost {
 namespace iterators {
 
 template<
-    class Incrementable,
-    class CategoryOrTraversal,
-    class Difference
+    typename Incrementable,
+    typename CategoryOrTraversal,
+    typename Difference
 >
 class counting_iterator;
 
@@ -29,7 +31,7 @@ namespace detail {
 
 // Try to detect numeric types at compile time in ways compatible
 // with the limitations of the compiler and library.
-template <class T>
+template< typename T >
 struct is_numeric :
     public std::integral_constant< bool, std::numeric_limits< T >::is_specialized >
 {};
@@ -62,7 +64,7 @@ struct is_numeric<wchar_t> :
     public std::true_type
 {};
 
-template <class T>
+template< typename T >
 struct numeric_difference
 {
     using type = typename boost::detail::numeric_traits< T >::difference_type;
@@ -83,26 +85,38 @@ struct numeric_difference<boost::uint128_type>
 };
 #endif
 
-template <class Incrementable, class CategoryOrTraversal, class Difference>
+template< typename Incrementable, typename CategoryOrTraversal, typename Difference, bool IsNumeric = is_numeric< Incrementable >::value >
+struct counting_iterator_types
+{
+    using traversal = detail::eval_if_default_t<
+        CategoryOrTraversal,
+        iterator_traversal< Incrementable >
+    >;
+
+    using difference = detail::eval_if_default_t<
+        Difference,
+        iterator_difference< Incrementable >
+    >;
+};
+
+template< typename Incrementable, typename CategoryOrTraversal, typename Difference >
+struct counting_iterator_types< Incrementable, CategoryOrTraversal, Difference, true >
+{
+    using traversal = detail::if_default_t<
+        CategoryOrTraversal,
+        random_access_traversal_tag
+    >;
+
+    using difference = detail::eval_if_default_t<
+        Difference,
+        numeric_difference< Incrementable >
+    >;
+};
+
+template< typename Incrementable, typename CategoryOrTraversal, typename Difference >
 struct counting_iterator_base
 {
-    using traversal = typename detail::ia_dflt_help<
-        CategoryOrTraversal,
-        typename std::conditional<
-            is_numeric<Incrementable>::value,
-            iterators::detail::type_identity<random_access_traversal_tag>,
-            iterator_traversal<Incrementable>
-        >::type
-    >::type;
-
-    using difference = typename detail::ia_dflt_help<
-        Difference,
-        typename std::conditional<
-            is_numeric<Incrementable>::value,
-            numeric_difference<Incrementable>,
-            iterator_difference<Incrementable>
-        >::type
-    >::type;
+    using iterator_types = counting_iterator_types< Incrementable, CategoryOrTraversal, Difference >;
 
     using type = iterator_adaptor<
         counting_iterator< Incrementable, CategoryOrTraversal, Difference >, // self
@@ -112,14 +126,14 @@ struct counting_iterator_base
                // criterion (see boost/iterator/detail/facade_iterator_category.hpp)
 #endif
         Incrementable,                                           // Value
-        traversal,
+        typename iterator_types::traversal,
         Incrementable const&,                                    // reference
-        difference
+        typename iterator_types::difference
     >;
 };
 
 // A distance calculation policy for wrapped iterators
-template <class Difference, class Incrementable1, class Incrementable2>
+template< typename Difference, typename Incrementable1, typename Incrementable2 >
 struct iterator_distance
 {
     static Difference distance(Incrementable1 x, Incrementable2 y)
@@ -129,7 +143,7 @@ struct iterator_distance
 };
 
 // A distance calculation policy for wrapped numbers
-template <class Difference, class Incrementable1, class Incrementable2>
+template< typename Difference, typename Incrementable1, typename Incrementable2 >
 struct number_distance
 {
     static Difference distance(Incrementable1 x, Incrementable2 y)
@@ -141,19 +155,20 @@ struct number_distance
 } // namespace detail
 
 template<
-    class Incrementable,
-    class CategoryOrTraversal = use_default,
-    class Difference = use_default
+    typename Incrementable,
+    typename CategoryOrTraversal = use_default,
+    typename Difference = use_default
 >
 class counting_iterator :
     public detail::counting_iterator_base< Incrementable, CategoryOrTraversal, Difference >::type
 {
+    friend class iterator_core_access;
+
+private:
     using super_t = typename detail::counting_iterator_base<
         Incrementable, CategoryOrTraversal, Difference
     >::type;
 
-    friend class iterator_core_access;
-
 public:
     using reference = typename super_t::reference;
     using difference_type = typename super_t::difference_type;
@@ -174,7 +189,7 @@ private:
         return this->base_reference();
     }
 
-    template <class OtherIncrementable>
+    template< typename OtherIncrementable >
     difference_type
     distance_to(counting_iterator< OtherIncrementable, CategoryOrTraversal, Difference > const& y) const
     {
@@ -189,7 +204,7 @@ private:
 };
 
 // Manufacture a counting iterator for an arbitrary incrementable type
-template <class Incrementable>
+template< typename Incrementable >
 inline counting_iterator< Incrementable > make_counting_iterator(Incrementable x)
 {
     return counting_iterator< Incrementable >(x);
@@ -202,4 +217,4 @@ using iterators::make_counting_iterator;
 
 } // namespace boost
 
-#endif // COUNTING_ITERATOR_DWA200348_HPP
+#endif // BOOST_ITERATOR_COUNTING_ITERATOR_DWA200348_HPP

include/boost/iterator/detail/eval_if_default.hpp

@@ -0,0 +1,44 @@
+/*
+ * Distributed under the Boost Software License, Version 1.0.
+ * (See accompanying file LICENSE_1_0.txt or copy at
+ * https://www.boost.org/LICENSE_1_0.txt)
+ *
+ * Copyright (c) 2025 Andrey Semashev
+ */
+
+#ifndef BOOST_ITERATOR_DETAIL_EVAL_IF_DEFAULT_HPP_INCLUDED_
+#define BOOST_ITERATOR_DETAIL_EVAL_IF_DEFAULT_HPP_INCLUDED_
+
+#include <boost/core/use_default.hpp>
+#include <boost/iterator/detail/type_traits/type_identity.hpp>
+
+namespace boost {
+namespace iterators {
+namespace detail {
+
+// If T is use_default, return the result of invoking
+// DefaultNullaryFn, otherwise - of NondefaultNullaryFn.
+// By default, NondefaultNullaryFn returns T, which means
+// the metafunction can be called with just two parameters
+// and in that case will return either T or the result of
+// invoking DefaultNullaryFn.
+template< typename T, typename DefaultNullaryFn, typename NondefaultNullaryFn = detail::type_identity< T > >
+struct eval_if_default
+{
+    using type = typename NondefaultNullaryFn::type;
+};
+
+template< typename DefaultNullaryFn, typename NondefaultNullaryFn >
+struct eval_if_default< use_default, DefaultNullaryFn, NondefaultNullaryFn >
+{
+    using type = typename DefaultNullaryFn::type;
+};
+
+template< typename T, typename DefaultNullaryFn, typename NondefaultNullaryFn = detail::type_identity< T > >
+using eval_if_default_t = typename eval_if_default< T, DefaultNullaryFn, NondefaultNullaryFn >::type;
+
+} // namespace detail
+} // namespace iterators
+} // namespace boost
+
+#endif // BOOST_ITERATOR_DETAIL_EVAL_IF_DEFAULT_HPP_INCLUDED_

include/boost/iterator/detail/if_default.hpp

@@ -0,0 +1,41 @@
+/*
+ * Distributed under the Boost Software License, Version 1.0.
+ * (See accompanying file LICENSE_1_0.txt or copy at
+ * https://www.boost.org/LICENSE_1_0.txt)
+ *
+ * Copyright (c) 2025 Andrey Semashev
+ */
+
+#ifndef BOOST_ITERATOR_DETAIL_IF_DEFAULT_HPP_INCLUDED_
+#define BOOST_ITERATOR_DETAIL_IF_DEFAULT_HPP_INCLUDED_
+
+#include <boost/core/use_default.hpp>
+
+namespace boost {
+namespace iterators {
+namespace detail {
+
+// If T is use_default, return Default, otherwise - Nondefault.
+// By default, Nondefault is T, which means
+// the metafunction can be called with just two parameters
+// and in that case will return either T or Default.
+template< typename T, typename Default, typename Nondefault = T >
+struct if_default
+{
+    using type = Nondefault;
+};
+
+template< typename Default, typename Nondefault >
+struct if_default< use_default, Default, Nondefault >
+{
+    using type = Default;
+};
+
+template< typename T, typename Default, typename Nondefault = T >
+using if_default_t = typename if_default< T, Default, Nondefault >::type;
+
+} // namespace detail
+} // namespace iterators
+} // namespace boost
+
+#endif // BOOST_ITERATOR_DETAIL_IF_DEFAULT_HPP_INCLUDED_

include/boost/iterator/indirect_iterator.hpp

@@ -12,6 +12,7 @@
 
 #include <boost/iterator/iterator_adaptor.hpp>
 #include <boost/iterator/enable_if_convertible.hpp>
+#include <boost/iterator/detail/eval_if_default.hpp>
 
 #include <boost/pointee.hpp>
 #include <boost/indirect_reference.hpp>
@@ -34,14 +35,14 @@ struct indirect_base
     using type = iterator_adaptor<
         indirect_iterator< Iter, Value, Category, Reference, Difference >,
         Iter,
-        detail::ia_dflt_help_t<
+        detail::eval_if_default_t<
             Value,
             pointee< dereferenceable >
         >,
         Category,
-        detail::ia_dflt_help_t<
+        detail::eval_if_default_t<
             Reference,
-            detail::ia_eval_if_default<
+            detail::eval_if_default<
                 Value,
                 indirect_reference< dereferenceable >,
                 std::add_lvalue_reference< Value >

include/boost/iterator/iterator_adaptor.hpp

@@ -15,6 +15,7 @@
 #include <boost/iterator/iterator_facade.hpp>
 #include <boost/iterator/iterator_traits.hpp>
 #include <boost/iterator/enable_if_convertible.hpp> // for backward compatibility; remove once downstream users are updated
+#include <boost/iterator/detail/eval_if_default.hpp>
 
 #include <boost/iterator/detail/config_def.hpp>
 
@@ -26,45 +27,8 @@ namespace iterators {
 // explicitly in order to specify that the default should be used.
 using boost::use_default;
 
-//
-// Default template argument handling for iterator_adaptor
-//
 namespace detail {
 
-// If T is use_default, return the result of invoking
-// DefaultNullaryFn, otherwise return T.
-template< typename T, typename DefaultNullaryFn >
-struct ia_dflt_help
-{
-    using type = T;
-};
-
-template< typename DefaultNullaryFn >
-struct ia_dflt_help< use_default, DefaultNullaryFn >
-{
-    using type = typename DefaultNullaryFn::type;
-};
-
-template< typename T, typename DefaultNullaryFn >
-using ia_dflt_help_t = typename ia_dflt_help< T, DefaultNullaryFn >::type;
-
-// If T is use_default, return the result of invoking
-// DefaultNullaryFn, otherwise - of NondefaultNullaryFn.
-template< typename T, typename DefaultNullaryFn, typename NondefaultNullaryFn >
-struct ia_eval_if_default
-{
-    using type = typename NondefaultNullaryFn::type;
-};
-
-template< typename DefaultNullaryFn, typename NondefaultNullaryFn >
-struct ia_eval_if_default< use_default, DefaultNullaryFn, NondefaultNullaryFn >
-{
-    using type = typename DefaultNullaryFn::type;
-};
-
-template< typename T, typename DefaultNullaryFn, typename NondefaultNullaryFn >
-using ia_eval_if_default_t = typename ia_eval_if_default< T, DefaultNullaryFn, NondefaultNullaryFn >::type;
-
 // A metafunction which computes an iterator_adaptor's base class,
 // a specialization of iterator_facade.
 template<
@@ -79,36 +43,36 @@ using iterator_adaptor_base_t = iterator_facade<
     Derived,
 
 #ifdef BOOST_ITERATOR_REF_CONSTNESS_KILLS_WRITABILITY
-    detail::ia_dflt_help_t<
+    detail::eval_if_default_t<
         Value,
-        detail::ia_eval_if_default<
+        detail::eval_if_default<
             Reference,
             iterator_value< Base >,
             std::remove_reference< Reference >
         >
     >,
 #else
-    detail::ia_dflt_help_t<
+    detail::eval_if_default_t<
         Value,
         iterator_value< Base >
     >,
 #endif
 
-    detail::ia_dflt_help_t<
+    detail::eval_if_default_t<
         Traversal,
         iterator_traversal< Base >
     >,
 
-    detail::ia_dflt_help_t<
+    detail::eval_if_default_t<
         Reference,
-        detail::ia_eval_if_default<
+        detail::eval_if_default<
             Value,
             iterator_reference< Base >,
             std::add_lvalue_reference< Value >
         >
     >,
 
-    detail::ia_dflt_help_t<
+    detail::eval_if_default_t<
         Difference,
         iterator_difference< Base >
     >

include/boost/iterator/transform_iterator.hpp

@@ -13,6 +13,7 @@
 #include <boost/iterator/iterator_adaptor.hpp>
 #include <boost/iterator/iterator_categories.hpp>
 #include <boost/iterator/enable_if_convertible.hpp>
+#include <boost/iterator/detail/eval_if_default.hpp>
 
 #include <boost/utility/result_of.hpp>
 
@@ -35,33 +36,33 @@ namespace iterators {
      private:
         // By default, dereferencing the iterator yields the same as
         // the function.
-        typedef typename ia_dflt_help<
-            Reference
+        using reference = detail::eval_if_default_t<
+            Reference,
 #ifdef BOOST_RESULT_OF_USE_TR1
-          , result_of<const UnaryFunc(typename std::iterator_traits<Iterator>::reference)>
+            result_of<const UnaryFunc(typename std::iterator_traits<Iterator>::reference)>
 #else
-          , result_of<const UnaryFunc&(typename std::iterator_traits<Iterator>::reference)>
+            result_of<const UnaryFunc&(typename std::iterator_traits<Iterator>::reference)>
 #endif
-        >::type reference;
+        >;
 
         // To get the default for Value: remove any reference on the
         // result type, but retain any constness to signal
         // non-writability.  Note that if we adopt Thomas' suggestion
         // to key non-writability *only* on the Reference argument,
         // we'd need to strip constness here as well.
-        typedef typename ia_dflt_help<
-            Value
-          , remove_reference<reference>
-        >::type cv_value_type;
+        using cv_value_type = detail::eval_if_default_t<
+            Value,
+            remove_reference<reference>
+        >;
 
      public:
-        typedef iterator_adaptor<
-            transform_iterator<UnaryFunc, Iterator, Reference, Value>
-          , Iterator
-          , cv_value_type
-          , use_default    // Leave the traversal category alone
-          , reference
-        > type;
+        using type = iterator_adaptor<
+            transform_iterator<UnaryFunc, Iterator, Reference, Value>,
+            Iterator,
+            cv_value_type,
+            use_default,    // Leave the traversal category alone
+            reference
+        >;
     };
   }