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

c++ templates typedef typename
geraldCelente picture geraldCelente · Oct 5, 2013 · Viewed 19.7k times · Source

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

Kerrek SB picture Kerrek SB · Oct 5, 2013

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

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