While compiling the code below with Qt 5.12 using Microsoft Visual C++ Compiler 15.9.28307.1300 (amd64) and c++17 standard I get the following error:
error C3520: ‘Args’: parameter pack must be expanded in this context
note: see reference to class template instantiation
‘Helper<Args…>’ being compiled
template<typename T>
struct Base {
void operator()(const T& arg){}
};
template <typename... Args>
class Helper : Base<Args>... {
public:
using Base<Args>::operator()...;
};
Is this a bug with msvc? Is there any known workaround? The same code compiles on macOS using clang.
>Solution :
As workaround to variadic using (C++17), you might use the recursive way:
template <typename... Args>
class Helper;
template <>
class Helper<>
{
};
template <typename T>
class Helper<T> : Base<T>
{
public:
using Base<T>::operator();
};
template <typename T, typename... Ts>
class Helper<T, Ts...> : Base<T>, Helper<Ts...>
{
public:
using Base<T>::operator();
using Helper<Ts...>::operator();
};