Returning templated result

Question

I have the following code:

#include <array>
#include <iostream>
#include <typeinfo>
#include <type_traits>
#include <utility>

namespace impl
{
    template <typename T>
    struct Matrix_traits {

    };
}

template <size_t M, size_t N, typename T>
class Matrix
{

};

template <size_t M, size_t N, typename T>
struct impl::Matrix_traits<Matrix<M, N, T>> {
    template <typename U>
    struct scalar_mult_type
    {
        // just for testing
        using type = std::pair<std::array<T, M>, std::array<T, N>>;
    };
};

int main()
{
    Matrix<3, 4, char> m;

    mult(3, m);

    return 0;
}

When I use the following function implementation, where I'm explicitly specifying the return type:

template <typename T, typename U>
std::pair<std::array<char, 3>, std::array<char, 4>> mult(const T& lambda, const U& m)
{
    typename impl::Matrix_traits<U>::scalar_mult_type<T>::type result;
    std::cout << typeid(result).name() << "\tEUREKA!\n";
    return result;
}

It works... but this is obviously not what I want... but when I'm trying to be more flexible:

template <typename T, typename U>
typename impl::Matrix_traits<U>::scalar_mult_type<T>::type mult(const T& lambda, const U& m)
{
    typename impl::Matrix_traits<U>::scalar_mult_type<T>::type result;
    std::cout << typeid(result).name() << "\tEUREKA!\n";
    return result;
}

I have "unrecognizable template declaration/definition" error. It's a real puzzle to me. Why the same declaration works for local variable 'result' but fails as the return type?

What is that `struct impl::Matrix_traits>` supposed to mean? — SergeyA, Nov 02 '15 at 16:39
Your `Matrix_Traits` take one template parameter in one case and 3 in the other case. Could you try to explain what you want to use the traits for? — Simon Kraemer, Nov 02 '15 at 16:51
@SimonKraemer: the second case is a (partial) specialization. — Jarod42, Nov 02 '15 at 17:07
see [where-and-why-do-i-have-to-put-the-template-and-typename-keywords](http://stackoverflow.com/questions/610245/where-and-why-do-i-have-to-put-the-template-and-typename-keywords) — Jarod42, Nov 02 '15 at 17:14

score 0 · Accepted Answer · answered Nov 02 '15 at 17:06

0

template are missing, it should be

template <typename T, typename U>
typename impl::Matrix_traits<U>::template scalar_mult_type<T>::type
mult(const T& , const U& )
{
    typename impl::Matrix_traits<U>::template scalar_mult_type<T>::type result;
    std::cout << typeid(result).name() << "\tEUREKA!\n";
    return result;
}

Demo

answered Nov 02 '15 at 17:06

Jarod42

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@Mirek: The way to thank is to accept/upvote answers. Comment is mostly to ask or add complement. – Jarod42 Nov 03 '15 at 16:13

Returning templated result

1 Answers1