I want strip argument from variable args by using template meta programming - c++

Question

I am new to template meta programming. I want to strip args from variable argument in c++. I am making a function which will push_back() element to any type of container. Its very easy to do in C++ 17 but i want to provide support for C++ 11 . Please find the code below push_back() function implementation i am looking for. Please avoid va_start(), va_end() c style solution.
#include <iostream>
#include <vector>
template<class Container, class T, class... Args>
void push_back(Container& con, T tail, Args... args);
template<class T>
T get_tail(T data) {
return data;
}
template<class T, class ...Args>
T get_tail(T& tail, Args&... args) {
return tail;
}
template<class Container , class T,class... Args>
void push_back(Container& con, T tail,Args... args ) {
//C++ 17 ((con.push_back(args), ...);
con.push_back(tail);
std::cout << (tail) << std::endl;
T newTail = get_tail(args...);
push_back(con,newTail,args...);
}
template<typename T, typename... Args>
bool pair_comparer(T a, T b, Args... args) {
return a == b && pair_comparer(args...);
}
int main()
{
std::vector<int> v_int;
push_back(v_int,1,2,3,4 );
std::cout << "Hello World!\n";
for (auto iter = v_int.begin(); iter != v_int.begin(); iter++) {
std::cout << "=== " << *iter << " ===" << std::endl;
}
}

Answer 1

You've already stripped the first argument away when you did
void push_back(Container& con, T tail, Args... args ) {
tail is the first argument and args is the rest, so your recursive call at the end simply needs to say
pus_back(con, args...)
No need for any of this get_tail hackery. Then, simply have another overload of push_back that looks like this.
template <class Container>
void push_back(Container& con) {
// A whole lot of nothing happens here...
}
That handles the base case, and the function you already wrote (with the one minor modification) handles the recursive case.

Answer 2

Not sure to understand what do you exactly want but...
Are you sure you need recursion?
You can develop something similar the C++17 way unpacking the variadic args..., using the initialization of an unused C-style array, as follows
template <typename Container, typename ... Args>
void push_back (Container & con, Args ... args ) {
using unused = int[];
(void)unused { 0, ((void)con.push_back(args), std::cout << args << std::endl, 0)... };
}
Or simply as follows
template <typename Container, typename ... Args>
void push_back (Container & con, Args ... args ) {
using unused = int[];
(void)unused { 0, ((void)con.push_back(args), 0)... };
}
if the std::cout part is just for debugging purposes.

Answer 3

The problem in your code is that you call push_back inside of itself with the same number of arguments. This makes the recursion infinite. Instead you need to "strip" one argument each time. And certainly provide the recursion base. Like this:
template<class Container>
void push_back(Container& con) {
}
template<class Container , class T,class... Args>
void push_back(Container& con, T tail, Args... args ) {
con.push_back(tail);
std::cout << (tail) << std::endl;
push_back(con, args...);
}
int main()
{
std::vector<int> v_int;
push_back(v_int,1,2,3,4 );
std::cout << "Hello World!\n";
for (auto x : v_int)
std::cout << "=== " << x << " ===" << std::endl;
}