How to send a template to another template without declaring the type?

Question

I'm trying to compile following code :

#include <iostream>

template<class T> struct Container1;//forward declaration
template<class T> struct Container2;//forward declaration

template<class Container,class Type>
using _Make_Container=typename Container<Type>;

template<class T>
struct Container1
{};

template<class T>
struct Container2
{};

int main()
{
    _Make_Container<Container1,int> a;
}

Which give me some errors:

 expected nested-name-specifier before 'Container'
  using _Make_Container=typename Container<Type>;
                                ^
 error: 'Container' is not a template

It seems OK to me because we send Container1 and int to using Then it becomes :

using _Make_Container=typename Container1<int>;
//then 
Container1<int> a;

I can't understand the reason for that errors !! Any idea?

Is there any way for making that works ?

score 3 · Accepted Answer · edited May 23 '17 at 12:27

Those error messages are pretty clear.

expected nested-name-specifier before 'Container'
 using _Make_Container=typename Container<Type>;
                          ^

You're not referring to a nested dependent type so there's no need for the typename keyword here. Read this for a detailed explanation on where typename is required.

error: 'Container' is not a template

Just like it says, your alias template doesn't indicate Container is a template, but then it proceeds to use it as one. You need to use a template template parameter for Container

template<template<typename> class Container,class Type>
using _Make_Container = Container<Type>;

And borrowing Yakk's excellent suggestion, you should change the alias template definition to

template<template <typename...> class T1, typename... T2>
using Make_Container = T1<T2...>;

This lets you do the following:

template<class T, class T1>
struct Container2
{};

Make_Container<Container2,int,long> b;

Finally, names using a leading underscore followed by an upper case letter are reserved by the implementation. So you should rename _Make_Container to Make_Container.

score 2 · Answer 2 · answered Jun 05 '14 at 14:39

2

You have to write the alias template like this

template<template <typename> class T1,class T2>
using _Make_Container= T1<T2>;

answered Jun 05 '14 at 14:39

Nikos Athanasiou

29,616
15
87
153

I prefer the alternative with more `class` : `templateclass T1`. Also sometimes `templateclass T1` is wise, as the way `template`s match is sort of backwards. – Yakk - Adam Nevraumont Jun 05 '14 at 14:47
1

@Yakk `class`ical Yakk :P – Nikos Athanasiou Jun 05 '14 at 14:51

score 1 · Answer 3 · answered Jun 05 '14 at 14:57

1

template< template <class> class Container,class Type>
using _Make_Container = Container<Type>;

answered Jun 05 '14 at 14:57

LukeCodeBaker

151
5

How to send a template to another template without declaring the type?

3 Answers3