c++14可变参数模板问题 'is ambiguous'

Question

我有这样的片段:

    template<typename Last>
    bool apply_impl(data_t * d) const
    {
        return this->Last::apply(*this, vs);
    }

    template<typename Head, typename ...Tail>
    bool apply_impl(data_t * d) const
    {
        return this->Head::apply(*this, d) && this->template apply_impl<Tail...>(d);
    }

编译器错误是:

error: call to member function 'apply_impl' is ambiguous
        return this->Head::apply(*this, vs) && this->template apply_impl<Tail...>(vs);

如何解决？

Answer 1

您应该能够使用标记分派(dispatch)来标记结束条件来解决此问题:

template <class...>
struct TypeList {};

template<typename Head, typename ...Tail>
bool apply_impl(data_t * d, TypeList<Head, Tail...> = {}) const
{
    return this->Head::apply(*this, d) && this->apply_impl(d, TypeList<Tail...>{});
}

bool apply_impl(data_t * d, TypleList<>) const
{ return true; }

这样，模板版本将处理所有模板参数，而非模板将只提供一个终止符。