Nested templates with dependent scope

Question

What is dependent scope and what is the meaning of typename in the context of the following error?

$ make g++ -std=gnu++0x main.cpp main.cpp:18:10: error: need 'typename' before 'ptrModel<std::vector<Data> >::Type' because 'ptrModel<std::vector<Data> >' is a dependent scope make: *** [all] Error 1   /*  * main.cpp  */  #include <vector> #include <memory>  template<typename T> struct ptrModel {  typedef std::unique_ptr<T> Type; };   template<typename Data> struct ptrType {  typedef ptrModel< std::vector<Data> >::Type Type; };  int main() {  return 0; } 

Answer 1

The compiler told you exactly what to do. Write typename before ptrModel<std::vector<Data> >::Type, like so:

 typedef typename ptrModel<std::vector<Data> >::Type Type; 

The reason for this requirement is that the compiler doesn't at this point know whether ptrModel<std::vector<Data> >::Type describes a member variable or a nested type. It can't even figure that out by looking at the definition of ptrModel because there might be a specialization of ptrModel for std::vector<Data> somewhere else in the program that it hasn't gotten to yet which changes which of these things ::Type refers to. So you need to tell it explicitly.

The name ptrModel<std::vector<Data> >::Type has a "dependent scope" because it is in a scope that depends on the instantiation of a template.