template template return type from member function

Question

How can I return any container of ints from a member function? In the following code I've tried maany permutations between the two test classes but nothing seems to compile using g++-4.8.2:

#include "vector"

struct test {
    template<class Container> Container<int> ret() {
        return Container<int>();
    }
};

struct test {
    template<
        template<class Int> class Container,
        class Int
    > typename Container<Int> ret() {
        return Container<Int>();
    }
};

int main() {
    std::vector<int> v = test().ret<std::vector<int> >();
    return 0;
}

Ideally the program would be c++03 and compile only if int is the contained type. Printing a readable error to the user in other cases would also be nice but I guess that would require either boost or std static_assert(). Thanks!

EDIT 1

Unfortunately the 2 template parameter version only works for a few std containers, the commented out ones cause a compilation error as they require different template parameters:

struct test {
    template<
        template<class, class> class Container
    > Container<int, std::allocator<int> > ret() {
        return Container<int, std::allocator<int> >();
    }
};

int main() {
    std::vector<int> v = test().ret<std::vector>();
    std::list<int> l = test().ret<std::list>();
    //std::set<int> se = test().ret<std::set>();
    std::deque<int> d = test().ret<std::deque>();
    //std::stack<int> st = test().ret<std::stack>();
    //std::queue<int> q = test().ret<std::queue>();
    //std::priority_queue<int> p = test().ret<std::priority_queue>();
    return 0;
}

but the following c++11 version seems to work for each container:

struct test {
    template<
        template<class, class...> class Container,
        class... Container_Params
    > Container<int, Container_Params... > ret() {
        return Container<int, Container_Params... >();
    }
};

int main() {
    auto v = test().ret<std::vector>();
    auto l = test().ret<std::list>();
    auto se = test().ret<std::set>();
    auto d = test().ret<std::deque>();
    auto st = test().ret<std::stack>();
    auto q = test().ret<std::queue>();
    auto p = test().ret<std::priority_queue>();
    auto us = test().ret<boost::unordered_set>();
    return 0;
}

Answer 1

test().ret<std::vector, int>();

I think it is correct

and.. your test structure is two??

Answer 2

template<bool _Test, class _Ty = void>
struct enable_if
{};

template<class _Ty>
struct enable_if<true, _Ty>
{ typedef _Ty type; };

template < typename _Type >
struct is_int_container { static const bool value = false; };

template < typename _Alloc >
struct is_int_container < std::vector < int, _Alloc > > { static const bool value = true; };

template < typename _Alloc >
struct is_int_container < std::list < int, _Alloc > > { static const bool value = true; };

template < typename _Equal, typename _Alloc >
struct is_int_container < std::set < int, _Equal, _Alloc > > { static const bool value = true; };

// Add any required container support

struct test 
{
    template< typename _Type> 
    typename enable_if < is_int_container <_Type> ::value, _Type > ::type
    ret() 
    {
        return _Type();
    }
};

int main(int agrc, char *argv[])
{
    test test_;

    test_.ret<std::vector<int>>();
    test_.ret<std::set<int>>();
    test_.ret<std::vector<double>>();  // <- compilator error

    return 0;
}

Answer 3

One of your problem is that std::vector takes 2 templates parameters ( T and an Allocator)

So the following may help you, if you want to take template classes:

struct test {
    template<template<typename, typename> class Container>
    Container<int, std::allocator<int>> ret() {
        return Container<int, std::allocator<int>>();
    }
};

int main(int argc, char *argv[])
{
    std::vector<int> v = test().ret<std::vector>();
    return 0;
}

An alternative if you use class is to use SFINAE as:

template <typename Container, typename T>
struct is_container_of :
    std::conditional<std::is_same<
            typename std::decay<decltype(*std::begin(std::declval<Container>()))>::type,
            T>::value,
            std::true_type, std::false_type>::type {};

struct test {
    template<class Container>
    typename std::enable_if<is_container_of<Container, int>::value, Container>::type
    ret() {
        return Container();
    }
};

int main(int argc, char *argv[])
{
    auto v = test().ret<std::vector<int>>();
    return 0;
}