Add use_default to Core

doc/core.qbk

@@ -58,3 +58,4 @@ criteria for inclusion is that the utility component be:
 [include swap.qbk]
 [include typeinfo.qbk]
 [include uncaught_exceptions.qbk]
+[include use_default.qbk]

doc/use_default.qbk

@@ -0,0 +1,47 @@
+[/
+Copyright 2019 Glen Joseph Fernandes
+(glenjofe@gmail.com)
+
+Distributed under the Boost Software License, Version 1.0.
+(http://www.boost.org/LICENSE_1_0.txt)
+]
+
+[section:use_default use_default]
+
+[section Overview]
+
+The header <boost/core/use_default.hpp> provides the type `boost::use_default`
+which is used by other Boost libraries as a sentinel type in a templates to
+indicate defaults.
+
+[endsect]
+
+[section Example]
+
+```
+template<class Derived, class Base,
+   class Value = boost::use_default,
+   class CategoryOrTraversal = boost::use_default,
+   class Reference = boost::use_default,
+   class Difference = boost::use_default>
+class iterator_adaptor;
+
+template<class Value>
+class node_iterator
+    : public iterator_adaptor<node_iterator<Value>, Value*,
+        boost::use_default, boost::forward_traversal_tag>;
+```
+
+[endsect]
+
+[section Reference]
+
+```
+namespace boost {
+  struct use_default { };
+}
+```
+
+[endsect]
+
+[endsect]

include/boost/core/use_default.hpp

@@ -0,0 +1,17 @@
+/*
+Copyright 2019 Glen Joseph Fernandes
+(glenjofe@gmail.com)
+
+Distributed under the Boost Software License, Version 1.0.
+(http://www.boost.org/LICENSE_1_0.txt)
+*/
+#ifndef BOOST_CORE_USE_DEFAULT_HPP
+#define BOOST_CORE_USE_DEFAULT_HPP
+
+namespace boost {
+
+struct use_default { };
+
+} /* boost */
+
+#endif

test/Jamfile.v2

@@ -128,10 +128,13 @@ run exchange_move_test.cpp ;
 
 run empty_value_test.cpp ;
 run empty_value_size_test.cpp ;
+run empty_value_final_test.cpp ;
 
 run quick_exit_test.cpp ;
 run-fail quick_exit_fail.cpp ;
 
+compile use_default_test.cpp ;
+
 lib lib_typeid : lib_typeid.cpp : <link>shared:<define>LIB_TYPEID_DYN_LINK=1 ;
 
 run test_lib_typeid.cpp lib_typeid : : : <link>shared : test_lib_typeid_shared ;

test/empty_value_final_test.cpp

@@ -0,0 +1,62 @@
+/*
+Copyright 2018 Glen Joseph Fernandes
+(glenjofe@gmail.com)
+
+Distributed under the Boost Software License, Version 1.0.
+(http://www.boost.org/LICENSE_1_0.txt)
+*/
+#include <boost/core/empty_value.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+#if !defined(BOOST_NO_CXX11_FINAL)
+class type final {
+public:
+    explicit type(int count)
+        : value_(count) { }
+    int value() const {
+        return value_ + 1;
+    }
+    int value() {
+        return value_ + 2;
+    }
+private:
+    int value_;
+};
+
+struct empty final {
+    int value() const {
+        return 1;
+    }
+    int value() {
+        return 2;
+    }
+};
+
+void test_type()
+{
+    const boost::empty_value<type> v1(boost::empty_init_t(), 3);
+    BOOST_TEST(v1.get().value() == 4);
+    boost::empty_value<type> v2(boost::empty_init_t(), 3);
+    BOOST_TEST(v2.get().value() == 5);
+}
+
+void test_empty()
+{
+    const boost::empty_value<empty> v1 = boost::empty_init_t();
+    BOOST_TEST(v1.get().value() == 1);
+    boost::empty_value<empty> v2;
+    BOOST_TEST(v2.get().value() == 2);
+}
+
+int main()
+{
+    test_type();
+    test_empty();
+    return boost::report_errors();
+}
+#else
+int main()
+{
+    return 0;
+}
+#endif

test/empty_value_test.cpp

@@ -9,67 +9,63 @@ Distributed under the Boost Software License, Version 1.0.
 #include <boost/core/lightweight_test.hpp>
 
 struct empty {
-    operator bool() const {
-        return false;
+    int value() const {
+        return 1;
     }
-    operator bool() {
-        return true;
+    int value() {
+        return 2;
     }
 };
 
 class type {
 public:
-    type()
-        : value_(false) { }
-    explicit type(bool value)
-        : value_(value) { }
-    operator bool() const {
-        return value_;
+    explicit type(int count)
+        : value_(count) { }
+    int value() const {
+        return value_ + 1;
+    }
+    int value() {
+        return value_ + 2;
     }
 private:
-    bool value_;
+    int value_;
 };
 
-void test_bool()
+void test_int()
 {
-    const boost::empty_value<bool> v1(boost::empty_init_t(), true);
-    BOOST_TEST(v1.get());
-    boost::empty_value<bool> v2 = boost::empty_init_t();
-    BOOST_TEST(!v2.get());
+    const boost::empty_value<int> v1(boost::empty_init_t(), 7);
+    BOOST_TEST(v1.get() == 7);
+    boost::empty_value<int> v2 = boost::empty_init_t();
+    BOOST_TEST(v2.get() == 0);
     v2 = v1;
-    BOOST_TEST(v2.get());
-    v2.get() = false;
-    BOOST_TEST(!v2.get());
+    BOOST_TEST(v2.get() == 7);
+    v2.get() = 8;
+    BOOST_TEST(v2.get() == 8);
 }
 
 void test_empty()
 {
-    empty e;
-    const boost::empty_value<empty> v1(boost::empty_init_t(), e);
-    BOOST_TEST(!v1.get());
+    const boost::empty_value<empty> v1 = boost::empty_init_t();
+    BOOST_TEST(v1.get().value() == 1);
     boost::empty_value<empty> v2;
-    BOOST_TEST(v2.get());
-    v2 = v1;
-    BOOST_TEST(v2.get());
-    v2.get() = empty();
-    BOOST_TEST(v2.get());
+    BOOST_TEST(v2.get().value() == 2);
 }
 
 void test_type()
 {
-    const boost::empty_value<type> v1(boost::empty_init_t(), true);
-    BOOST_TEST(v1.get());
-    boost::empty_value<type> v2;
-    BOOST_TEST(!v2.get());
+    const boost::empty_value<type> v1(boost::empty_init_t(), 2);
+    BOOST_TEST(v1.get().value() == 3);
+    boost::empty_value<type> v2(boost::empty_init_t(), 3);
+    BOOST_TEST(v2.get().value() == 5);
     v2 = v1;
-    BOOST_TEST(v2.get());
-    v2.get() = type();
-    BOOST_TEST(!v2.get());
+    BOOST_TEST(v2.get().value() == 4);
+    v2.get() = type(4);
+    BOOST_TEST(v2.get().value() == 6);
 }
 
 int main()
 {
-    test_bool();
+    test_int();
     test_empty();
     test_type();
     return boost::report_errors();

test/use_default_test.cpp

@@ -0,0 +1,15 @@
+/*
+Copyright 2019 Glen Joseph Fernandes
+(glenjofe@gmail.com)
+
+Distributed under the Boost Software License, Version 1.0.
+(http://www.boost.org/LICENSE_1_0.txt)
+*/
+
+#include <boost/core/use_default.hpp>
+
+template<class, class = boost::use_default>
+struct type { };
+
+template class type<int>;
+template class type<void, boost::use_default>;