c++ - Variadic 模板和 "expected a type"错误

Question

我正在(主要是出于学习目的)自己实现 tuple，我刚刚遇到了一个问题。我有以下代码:

namespace Rose
{
    template<typename T>
    struct RemoveReference
    {
        typedef T Type;
    };

    template<typename T>
    struct RemoveReference<T &>
    {
        typedef T Type;
    };

    template<typename... Elems>
    class Tuple;

    template<typename First, typename... Elems>
    class Tuple<First, Elems...>
    {
    public:
        Tuple(First a, Elems... more)
            : More(more...), Element(a)
        {
        }

        Tuple<First, Elems...> & operator=(const Tuple<RemoveReference<First>::Type,
                                           RemoveReference<Elems>::Type...> & rhs)
        {
            this->Element = rhs.Element;
            this->More = rhs.More;

            return *this;
        }

    private:
        Tuple<Elems...> More;
        First Element;
    };

    template<typename Only>
    class Tuple<Only>
    {
    public:
        Tuple(Only a) : Element(a)
        {
        }

        Tuple<Only> & operator=(const Tuple<RemoveReference<Only>::Type> & rhs)
        {
            this->Element = rhs.Element;

            return *this;
        }

    private:
        Only Element;
    };
}

int main()
{
    Rose::Tuple<int, float, int> t(1, 1.f, 2);
}

导致如下错误(错误较多，但这个是必不可少的):

error: type/value mismatch at argument 1 in template parameter list for 'template struct Rose::Tuple' error: expected a type, got 'Rose::RemoveReference::Type'

我不太明白这是怎么回事。 RemoveReference 特性在单独使用时有效。

这里有两个测试用例:

• not working code I pasted above
• proof that RemoveReference works when used alone

我已经用 G++ 4.6.1、4.5.1 和 Clang++ 2.9 试过这段代码。

出现这些错误的原因是什么？

Answer 1

RemoveReference<T>::Type是依赖类型，所以需要加上typename这里:

        Tuple<First, Elems...> & operator=(const Tuple<typename RemoveReference<First>::Type,
                                                       typename RemoveReference<Elems>::Type...> & rhs)

可能还有其他地方。