Iterating over a parameter pack??

[JUAN DENT]

Hi,

I have this:

template <typename... T_values>
class Thing {

    void something_else(T_values... values) {
	for (auto&& x : values... ) {
	     std::cout << x << std::endl;
	}
     }
}

The compiler (MSVC 2015) is giving me the following error:

error C3520: 'values': parameter pack must be expanded in this context

I thought this could work... Is it possible somehow?

Regards,
Juan

[naraku9333]

for (auto&& x : { values... }) {
		std::cout << x << std::endl;
	}

[JUAN DENT]

this does not compile on Visual Studio 2015 Update 2...

any workarounds?

[Peter87]

auto is used to automatically deduce the type but here you are asking it to be multiple different types which could make sense in a scripting language but not in a statically typed language like C++.

The only way I know of to iterate through a parameter pack is to use recursion.

template <typename T>
void print(T value) {
	std::cout << value << std::endl;
}

template <typename First, typename... Rest>
void print(First firstValue, Rest... rest) {
	print(firstValue);
	print(rest...);
}

template <typename... T_values>
class Thing {

	void something_else(T_values... values) {
		print(values...);
	}
};

[prestokeys]

#include <iostream>

template <typename... T_values>
struct Thing {
    void something_else(T_values... values) {
		const int a[] = {(std::cout << values << ' ', 0)...};
		static_cast<void>(a);
    }
};

int main() {
	Thing<int, char, std::string> thing;
	thing.something_else(2, 'a', "hello");  // 2 a hello
}

When C++17 comes out, you can write as a binary right fold:

template<typename ...Args>
void print(Args&&... args) {
    (std::cout << ... << args) << '\n';
}