template declaration of `typedef typename Foo<T>::Bar Bar'

Question

I am encountering great difficulty in declaring a templated type as shown below.

#include <cstdlib> #include <iostream>  using namespace std;   template <class T> class Foo {  typedef T Bar; };  template <class T> typedef typename Foo<T>::Bar Bar;     int main(int argc, char *argv[]) {      Bar bar;      Foo<int> foo;       system("PAUSE");     return EXIT_SUCCESS; } 

I get error

template declaration of `typedef typename Foo<T>::Bar Bar'  

about line

template <class T> typedef typename Foo<T>::Bar Bar; 

I am doing this because I want avoid writing typename Foo::Bar throught my code.

What am I doing wrong?

Answer 1

The typedef declaration in C++ cannot be a template. However, C++11 added an alternative syntax using the using declaration to allow parametrized type aliases:

template <typename T> using Bar = typename Foo<T>::Bar; 

Now you can use:

Bar<int> x;   // is a Foo<int>::Bar 

Answer 2

typedef's cannot be templates. This is exactly the reason C++11 invented alias templates. Try

template <class T> using Bar = typename Foo<T>::Bar;