read arguments from variadic template

Question

Hi all,

I am a little confused about how can I read each argument from the tuple by using variadic templates.

Consider this function:

template<class...A> int func(A...args){
int size = sizeof...(A);
.... }

I call it from the main file like:

func(1,10,100,1000);

Now, I don't know how I have to extend the body of func to be able to read each argument separately so that I can, for example, store the arguments in an array.

Answer 1

You have to provide overrides for the functions for consuming the first N (usually one) arguments.

void foo() {
   // end condition argument pack is empty
}

template <class First, class... Rest> 
void foo(First first, Rest... rest) {
    // Do something with first
    cout << first << endl; 

    foo(rest...); // Unpack the arguments for further treatment
}

When you unpack the variadic parameter it finds the next overload.

Example:

foo(42, true, 'a', "hello");
// Calls foo with First = int, and Rest = { bool, char, char* }
// foo(42, Rest = {true, 'a', "hello"}); // not the real syntax

Then next level down we expand the previous Rest and get:

foo(true, Rest = { 'a', "hello"}); // First = bool

And so on until Rest contains no members in which case unpacking it calls foo() (the overload with no arguments).

---

Storing the pack if different types

If you want to store the entire argument pack you can use an std::tuple

template <class... Pack>
void store_pack(Pack... p) {
    std::tuple<Pack...> store( p... );
    // do something with store
}

However this seems less useful.

Storing the pack if it's homogeneous

If all the values in the pack are the same type you can store them all like this:

vector<int> reverse(int i) {
    vector<int> ret;
    ret.push_back(i);
    return ret;
}

template <class... R>
vector<int> reverse(int i, R... r) {
    vector<int> ret = reverse(r...);
    ret.push_back(i);
    return ret; 
}

int main() {
    auto v = reverse(1, 2, 3, 4);
    for_each(v.cbegin(), v.cend(), 
        [](int i ) { 
            std::cout << i << std::endl; 
        }
    );
}

However this seems even less useful.

Answer 2

If you need to store arguments in the array you could use array of boost::any as follows:

template<typename... A> int func(const A&... args)
{
  boost::any arr[sizeof...(A)] = { args... };
  return 0;
}

Answer 3

If the arguments are all of the same type, you could store the arguments in an array like this (using the type of the first argument for the array):

template <class T, class ...Args>
void foo(const T& first, const Args&... args)
{
    T arr[sizeof...(args) + 1] = { first, args...};
}

int main()
{
    foo(1);
    foo(1, 10, 100, 1000);
}

If the types are different, I suppose you could use boost::any but then I don't see how you are going to find out outside of the given template, which item is of which type (how you are going to use the stored values).

---

Edit: If the arguments are all of the same type and you want to store them into a STL container, you could rather use the std::initializer_list<T>. For example, Motti's example of storing values in reverse:

#include <vector>
#include <iostream>
#include <iterator>

template <class Iter>
std::reverse_iterator<Iter> make_reverse_iterator(Iter it)
{
    return std::reverse_iterator<Iter>(it);
}

template <class T>
std::vector<T> reverse(std::initializer_list<T> const & init)
{

    return std::vector<T>(make_reverse_iterator(init.end()), make_reverse_iterator(init.begin()));
}

int main() {
    auto v = reverse({1, 2, 3, 4});
    for (auto it = v.begin(); it != v.end(); ++it) {
        std::cout << *it << std::endl;
    }
} 

Answer 4

For sticking into an array if the arguments have different types, you can use also std::common_type<>

template<class ...A> void func(A ...args){
   typedef typename common_type<A...>::type common;
   std::array<common, sizeof...(A)> a = {{ args... }};
}

So for example, func(std::string("Hello"), "folks") creates an array of std::string.