可变参数模板继承中的类型不匹配

Question

I have a template which applies the template given in argument list to all of the types in a parameter pack and inherits all of them (called ApplyOnPack):

template<template<int, typename> class Template, typename Seq, typename... Args>
struct _Map {};

template<template<int, typename> class Template,
         int firstIndex, int... indexes,
         typename First, typename... Args>
struct _Map<Template, Seq<firstIndex, indexes...>, First, Args...>
    : Template<firstIndex, First>,
      _Map<Template, Seq<indexes...>, Args...>
{};

template<template<int, typename> class Template, typename... Args>
struct ApplyOnPack : _Map<Template, Sequence<sizeof...(Args)>, Args... > 
{
    template <int I>
    struct Base {
        typedef Template<I, GetNthParameter<I, Args...> > Type;
    };

    template <int I>
    typename Base<I>::Type& base() { return *this; }
};

The problem is, that this last base() method won't compile with gcc (4.9.2) claiming invalid reference initialization. The return type should be one of the base classes of the type of *this, so what could be the problem? Or how can I modify the code to be compileable? The code compiles and works under msvc (2013).

I've tested it with the following example:

template <int i, typename T>
struct Part {
    void foo() {}
};

template <typename ... T>
struct Foo : ApplyOnPack<Part, T...>
{
    void bar() { this->template base<0>().foo();  }
};

typedef Foo<int, bool> MyFoo;

int main() {
    MyFoo myFoo;
    myFoo.bar();
}

Which gcc failed upon with:

a.cpp: In instantiation of ‘typename ApplyOnPack<Template, Args>::Base<I>::Type& ApplyOnPack<Template, Args>::base() [with int I = 0; Template = Part; Args = {int, bool}; typename ApplyOnPack<Template, Args>::Base<I>::Type = Part<0, int>]’:

a.cpp:62:15:   required from ‘void Foo<T>::bar() [with T = {int, bool}]’
a.cpp:69:12:   required from here
a.cpp:51:43: error: invalid initialization of reference of type ‘ApplyOnPack<Part, int, bool>::Base<0>::Type& {aka Part<0, int>&}’ from expression of type ‘ApplyOnPack<Part, int, bool>’
  typename Base<I>::Type& base() { return *this; }

Following additional templates used above: A template (GetNthParameter) for extracting n-th parameter from a parameter pack:

template <int I, class... T>
struct _GetNthParameter;

template <int I, class Head, class... Tail>
struct _GetNthParameter<I, Head, Tail...>
    : _GetNthParameter<I-1, Tail...>{};

template <class Head, class... Tail>
struct _GetNthParameter<0, Head, Tail...> {
    typedef Head Type;
};

template<int index, typename... Types>
using GetNthParameter = typename _GetNthParameter<index, Types...>::Type;

and one for building integer sequence (Sequence):

template<unsigned...>
struct Seq { typedef int value_type; };

template<unsigned max, unsigned... numbers>
struct _ExpandSeq : _ExpandSeq<max-1, max-1, numbers...> {};

template<unsigned... numbers>
struct _ExpandSeq<0, numbers...> {
  typedef Seq<numbers...> type;
};

template<unsigned max>
using Sequence = typename _ExpandSeq<max>::type;

Answer 1

You have a type mismatch. Seq takes a bunch of unsigned s:

template <unsigned...>
struct Seq { typedef int value_type; };

But you're specializing on it taking a bunch of int s:

template<template<int, typename> class Template,
         int firstIndex, int... indexes, // <==
         typename First, typename... Args>
struct _Map<Template, Seq<firstIndex, indexes...>, First, Args...>

Those need to line up. Clang accepts the code, but this is a clang bug ( #28010 ).

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As I mentioned in the comments, you are using many identifiers that are reserved by the standard. Any word that starts with an underscore and is followed by a capital letter is reserved: don't use them!