typename_keyword.cpp

Author: keineahnung2345

typename_keyword.cpp

@@ -0,0 +1,37 @@
+#include <iostream>
+#include <memory> //std::shared_ptr
+
+/**
+this program demos the usage of "typename" keyword
+"typename" is needed to be added for qualified name when template argument is unsure
+**/
+
+//[C++ Cast Template](https://stackoverflow.com/questions/37983562/c-cast-template)
+//[Where and why do I have to put the “template” and “typename” keywords?](https://stackoverflow.com/questions/610245/where-and-why-do-i-have-to-put-the-template-and-typename-keywords)
+//[Qualified name lookup](https://en.cppreference.com/w/cpp/language/qualified_lookup)
+
+template<typename T>
+class Animal{
+public:
+    using Ptr = std::shared_ptr< Animal< T > >;
+};
+
+template<typename T>
+void f(){
+    Animal<T> a;
+
+    //error: need ‘typename’ before ‘Animal<T>::Ptr’ because ‘Animal<T>’ is a dependent scope
+    //error: expected ‘;’ before ‘ap’
+    //Animal<T>::Ptr ap;
+
+    //correct
+    typename Animal<T>::Ptr ap;
+}
+
+int main(){
+    //ok
+    Animal<int> a;
+    //when template argument is concrete, we don't need to add "typename"
+    Animal<int>::Ptr ap;
+    return 0;
+}