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Editorial Team
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Asked: 2026-05-27T23:30:37+00:00

The following code does not compiles using clang 3.0, is this because I have

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The following code does not compiles using clang 3.0, is this because I have done it wrongly? Because it is not allowed in c++11 or because it is not supported in clang?

template<int OFFSET>
struct A {
    enum O { offset = OFFSET };
};

template < template <int T> class Head, typename... Tail>
struct C : public Head<1>, private C<Tail> { };

int main()
{
    C< A, A > c1;

    return 0;
}

Compiler error:

test3.cxx:99:42: error: template argument for template template parameter must be a class template or type alias template
    struct C : public Head<1>, private C<Tail> { };
                                         ^
test3.cxx:103:15: error: use of class template A requires template arguments
        C< A, A > c1;
              ^
test3.cxx:94:12: note: template is declared here
    struct A {
           ^
2 errors generated.
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1 Answer

  1. Editorial Team

    Three issues:

    Tail is to be a variadic list of templates, not of types. Hence it should be 

    template<int> class... Tail

    instead of

    typename... Tail

    and you need to explicitly expand the parameter pack with private C<Tail...> instead of private C<Tail>.

    And you’ll need to implement the base case, for when Tail... is empty:

    // base case
template < template <int> class Head>
struct C<Head> : public Head<1> { };

    (This is compiling for with Clang 3.0)

    The entire piece of code now:

    template<int OFFSET>
struct A {
    enum O { offset = OFFSET };
};

template < template <int> class Head, template<int> class... Tail>
struct C : public Head<1>, private C<Tail...> { };
template < template <int> class Head>
struct C<Head> : public Head<1> { };

int main()
{
    C< A, A > c1;
    return 0;
}
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