Move identity from functional to core

test/identity_test.cpp

@@ -0,0 +1,66 @@
+/*
+Copyright 2021-2023 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/identity.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <algorithm>
+#include <iterator>
+#include <string>
+#include <vector>
+
+bool test(std::string&)
+{
+    return true;
+}
+
+bool test(const std::string&)
+{
+    return true;
+}
+
+template<class T>
+bool test(T&)
+{
+    return false;
+}
+
+template<class T>
+bool test(const T&)
+{
+    return false;
+}
+
+void simple_test()
+{
+    std::string s1("a");
+    BOOST_TEST(boost::identity()(s1) == s1);
+    BOOST_TEST(&boost::identity()(s1) == &s1);
+    BOOST_TEST(test(boost::identity()(s1)));
+    const std::string s2("a");
+    BOOST_TEST(boost::identity()(s2) == s2);
+    BOOST_TEST(&boost::identity()(s2) == &s2);
+    BOOST_TEST(test(boost::identity()(s2)));
+}
+
+void algorithm_test()
+{
+    std::vector<std::string> v1;
+    v1.push_back(std::string("a"));
+    v1.push_back(std::string("b"));
+    v1.push_back(std::string("c"));
+    std::vector<std::string> v2;
+    std::transform(v1.begin(), v1.end(), std::back_inserter(v2),
+         boost::identity());
+    BOOST_TEST(v2 == v1);
+}
+
+int main()
+{
+    simple_test();
+    algorithm_test();
+    return boost::report_errors();
+}