Constrain distance function using is_iterator type trait (#96)

* Constrain distance function using is_iterator type trait

* Add enable_if include

* Fix template syntax in distance function

* Fix syntax issue in distance enable_if

* Test which overload of distance is called

Added a test for custom distance function using Foo struct.

* Fix wrong spelling of std::ptrdiff_t

* Move Foo struct to global scope

* Andrey magic

* Fix headers, add copyright

* Constrain advance to iterators

* Test contraint of advance function with int overload

Added overload for advance function to handle integers.

* Fix enable_if condition for advance function

* Fix template syntax in advance constraint

* Fix advance test

* Remove constexpr specifier

* Make advance overload require conversion in parameter

* Explain choice of type long for n parameter

include/boost/iterator/advance.hpp

@@ -8,8 +8,11 @@
 #define BOOST_ITERATOR_ADVANCE_HPP
 
 #include <boost/config.hpp>
+#include <boost/iterator/is_iterator.hpp>
 #include <boost/iterator/iterator_categories.hpp>
 
+#include <type_traits>
+
 namespace boost {
 namespace iterators {
 namespace detail {
@@ -56,7 +59,9 @@ inline BOOST_CXX14_CONSTEXPR void advance_impl(RandomAccessIterator& it, Distanc
 namespace advance_adl_barrier {
 
 template< typename InputIterator, typename Distance >
-inline BOOST_CXX14_CONSTEXPR void advance(InputIterator& it, Distance n)
+inline BOOST_CXX14_CONSTEXPR
+typename std::enable_if< boost::is_iterator< InputIterator >::value, void >::type
+advance(InputIterator& it, Distance n)
 {
     detail::advance_impl(it, n, typename iterator_traversal< InputIterator >::type());
 }

include/boost/iterator/distance.hpp

@@ -1,4 +1,5 @@
 // Copyright (C) 2017 Michel Morin.
+// Copyright (C) 2026 Jeremy W. Murphy
 //
 // Distributed under the Boost Software License, Version 1.0.
 // (See accompanying file LICENSE_1_0.txt or copy at
@@ -8,9 +9,12 @@
 #define BOOST_ITERATOR_DISTANCE_HPP
 
 #include <boost/config.hpp>
+#include <boost/iterator/is_iterator.hpp>
 #include <boost/iterator/iterator_categories.hpp>
 #include <boost/iterator/iterator_traits.hpp>
 
+#include <type_traits>
+
 namespace boost {
 namespace iterators {
 namespace detail {
@@ -40,7 +44,11 @@ distance_impl(RandomAccessIterator first, RandomAccessIterator last, random_acce
 namespace distance_adl_barrier {
 
 template< typename SinglePassIterator >
-inline BOOST_CXX14_CONSTEXPR typename iterator_difference< SinglePassIterator >::type
+inline BOOST_CXX14_CONSTEXPR
+typename std::enable_if<
+    boost::is_iterator< SinglePassIterator >::value,
+    iterator_difference< SinglePassIterator >
+>::type::type
 distance(SinglePassIterator first, SinglePassIterator last)
 {
     return detail::distance_impl(first, last, typename iterator_traversal< SinglePassIterator >::type());

test/advance_test.cpp

@@ -20,6 +20,20 @@ void test_advance(Iterator it_from, Iterator it_to, int n)
     BOOST_TEST(it_from == it_to);
 }
 
+// Definitely not an iterator
+struct Foo
+{
+    int x = 0;
+
+    // Don't use type "int" for "n", otherwise it matches literal int exactly
+    // and doesn't demonstrate the effect of enable_if.
+    friend
+    void advance(Foo &value, long n)
+    {
+        value.x += 10 * n;
+    }
+};
+
 int main()
 {
     int array[3] = {1, 2, 3};
@@ -87,5 +101,12 @@ int main()
         );
     }
 
+    {
+        using boost::advance;
+        Foo bar;
+        advance(bar, 3);
+        BOOST_TEST(bar.x == 30);
+    }
+    
     return boost::report_errors();
 }

test/distance_test.cpp

@@ -4,6 +4,7 @@
 // (See accompanying file LICENSE_1_0.txt or copy at
 // http://www.boost.org/LICENSE_1_0.txt)
 
+#include <cstddef>
 #include <vector>
 #include <list>
 #include <boost/container/slist.hpp>
@@ -19,6 +20,13 @@ void test_distance(Iterator it_from, Iterator it_to, int n)
     BOOST_TEST(boost::distance(it_from, it_to) == n);
 }
 
+// Definitely not an iterator.
+struct Foo
+{
+    constexpr friend
+    std::ptrdiff_t distance(Foo const &, Foo const &) { return -1; }
+};
+
 int main()
 {
     int array[3] = {1, 2, 3};
@@ -80,5 +88,11 @@ int main()
         );
     }
 
+    {
+        // Make boost::distance visible since we're not actually in the boost namespace here.
+        using boost::distance;
+        auto result = distance(Foo{}, Foo{});
+        BOOST_TEST(result == -1);
+    }
     return boost::report_errors();
 }