Apply function to all elements of parameter pack from a variadic function [duplicate]

Question

Consider the following (not working!) example:

#include <iostream>

template <typename type> void print(const type & item)
{
    std :: cout << item << std :: endl;
}

template <typename... types> void printall(const types & ... items)
{
    print(items)...;
}

int main()
{
    printall(1, 2, "hello");
}

Here I have a function print that simply prints out its argument, and a variadic function printall that accepts a pack of arguments. Now, what I would like to do is to simply have printall apply print to each element of the pack items. How can I get that done?

Note: I am not asking how to print a pack of values. I am aware of the existence of fold expressions and I know I could just throw the whole items in std::cout using them. Here print is just an example, could be any function.

How can I get that done? This sounds like something that should remarkably simple to do and yet I couldn't find any (reasonable) syntax to do it.

Answer 1

What I would like to do is to simply have printall apply print to each element of the pack items. How can I get that done?

Option - 1

As user @liliscent and user @max66 suggested in the comments, in C++11/ C++14 you can use the following hacky-way which act act like fold expressions in C++17.

SEE HERE

#include <iostream>

template <typename type> void print(const type& item)
{
    std::cout << item << '\n';
}

template <typename... types> 
void printall (const types&... items)
{
    using dummy = int[];
    (void)dummy { 0, (print(items), 0)... };
}

---

Option - 2

If the above does not look like good enough, provide a classical variadic templates overload as a wrapper/ helper between your printall() and print() functions, so that each template function arguments can be accessed in print().

SEE HERE

#include <iostream>

template <typename Type> void print(const Type& item)
{
    std::cout << item << '\n';  // print each argument
}

namespace helper 
{
    void printall() {}          // nontemplate overload for last call(i.e, no arguments call)

    template<typename FirstArg, typename... Types>
    void printall(const FirstArg& firstItem, Types&&... items)  
    {
        ::print(firstItem);                             // call print() for each argument
        helper::printall(std::forward<Types>(items)...);// calls the same wrapper::printalll()
    }
}

template <typename... Types> void printall(const Types& ... items)
{
    helper::printall(items...); // calls the wrapper::printall()
}

---

Option - 3

However, if you have access to the C++17, just use fold expressions. That provides a clean(non-hacky) and less amount of code.

SEE HERE

template <typename type> void print(const type& item)
{
    std::cout << item << '\n';
}

template <typename... types> void printall(const types&... items)
{
    (print(items),...);
}