add constrained perfect forwarding sample

1-common-tasks/templates/constrained-perfect-forwarding.cpp

@@ -0,0 +1,51 @@
+// Constrained Perfect forwarding
+// C++11 C++14
+
+#include <string>
+#include <utility>
+
+class foo
+{
+	public:
+		template <
+			typename T,
+			typename = typename std::enable_if<
+				std::is_constructible<std::string, T>::value
+			>::type
+		>
+		foo(T&& t) : s(std::forward<T>(t)...) {}
+	private:
+		std::string s;
+};
+
+// Constraining
+// [perfect forwarding](/common-tasks/templates/perfect-forwarding.html)
+// to legal types only improves error messages when needed, without hampering
+// versatility or performance.
+//
+// In this example, we want `foo` to be constructible with a string,
+// be it a [`std::string`](cpp/string/basic_string), `char*` or or
+// string literal, and initialize the member string in the most efficient
+// way possible.
+//
+// Lines [12-14] is a variant of the
+// [function template SFINAE](/common-tasks/function-template-sfinae.html)
+// ensuring that the constructor [16] only exists for some types `T`.
+// In this case line [13],
+// [`std::is_constructible`](cpp/types/is_constructible) provides the
+// enabling constraint that `T` must be a type from which a
+// [`std::string`](cpp/string/basic_string) can be constructed.
+//
+// Removing lines [12-14] gives the same functionality for a correct program,
+// but the constraint makes the compiler issue much better error messages
+// in the case of type errors, referring to the non-existence of matching
+// constructor for `foo` instead of mismatching constructors in
+// [`std::string`](cpp/string/basic_string).
+
+int main(int argc, char *argv[])
+{
+	foo obj1(std::string("a"));
+	foo obj2("a");
+	foo obj3(argv[0]);
+	//  foo obj4(1);
+}