Implement detection of nested container

include/boost/gil/extension/numeric/kernel.hpp

@@ -158,7 +158,30 @@ inline Kernel reverse_kernel(Kernel const& kernel)
 
 namespace detail {
 
-template <typename Core>
+template <typename Container>
+struct is_nested_container {
+private:
+    // use decltype and skip expression with coma operator
+    // note that decltype argument must be an expression
+    template <typename Inner>
+    static constexpr decltype(std::declval<typename Inner::value_type>(), true) is_nested_impl(int)
+    {
+        return true;
+    }
+
+    // C style variable size arguments have least priority
+    // in overload resolution, thus previous definition
+    // will be selected if possible
+    template <typename U>
+    static constexpr bool is_nested_impl(...)
+    {
+        return false;
+    }
+public:
+    constexpr static bool value = is_nested_impl<typename Container::value_type>(0);
+};
+
+template <typename Core, bool is_nested = is_nested_container<Core>::value>
 class kernel_2d_adaptor : public Core
 {
 public:
@@ -256,7 +279,7 @@ class kernel_2d_adaptor : public Core
         {
             throw std::out_of_range("Index out of range");
         }
-        
+
         return this->begin()[y * this->size() + x];
     }
 
@@ -267,6 +290,11 @@ class kernel_2d_adaptor : public Core
 protected:
     std::size_t square_size{ 0 };
 };
+
+template <typename Core>
+class kernel_2d_adaptor<Core, true> {
+    // Miral, over here! :-)
+};
 } //namespace detail
 
 /// \brief variable-size kernel

test/extension/numeric/kernel.cpp

@@ -9,6 +9,8 @@
 #define BOOST_DISABLE_ASSERTS 1 // kernel_1d_adaptor assertions are too strict
 #include <boost/gil/extension/numeric/kernel.hpp>
 #include <vector>
+#include <array>
+#include <type_traits>
 
 #define BOOST_TEST_MODULE test_ext_numeric_kernel
 #include "unit_test.hpp"
@@ -32,7 +34,7 @@ BOOST_AUTO_TEST_CASE(kernel_2d_default_constructor)
     BOOST_TEST(k.center_horizontal() == 0);
 
     //BOOST_TEST(k.left_size() == 0);
-    //BOOST_TEST(k.right_size() == -1); 
+    //BOOST_TEST(k.right_size() == -1);
     BOOST_TEST(k.upper_size() == 0);
     BOOST_TEST(k.lower_size() == -1);
     // std::vector interface
@@ -304,4 +306,17 @@ BOOST_AUTO_TEST_CASE(kernel_1d_fixed_reverse_Kernel)
     BOOST_TEST(k == values_rev);
 }
 
+template <typename Kernel2d>
+struct is_detected_as_nested: std::false_type {};
 
+template <typename T>
+struct is_detected_as_nested<gil::detail::kernel_2d_adaptor<T, true>>: std::true_type {};
+
+static_assert(is_detected_as_nested<gil::detail::kernel_2d_adaptor<std::vector<std::vector<double>>>>::value,
+    "did not detect nested container properly for kernel_2d_adaptor");
+static_assert(is_detected_as_nested<gil::detail::kernel_2d_adaptor<std::array<std::array<double, 3>, 3>>>::value,
+    "did not detect nested container properly for kernel_2d_adaptor");
+static_assert(!is_detected_as_nested<gil::detail::kernel_2d_adaptor<std::vector<double>>>::value,
+    "false positive for nested container detection for kernel_2d_adaptor");
+static_assert(!is_detected_as_nested<gil::detail::kernel_2d_adaptor<std::array<double, 3>>>::value,
+    "false positive for nested container detection for kernel_2d_adaptor");