filename: everyother.h

#pragma once
#include <iostream>
#include <vector>
#include <sstream>
#include <initializer_list>
using std::initializer_list;
using std::vector;
using std::ostream;
using std::cout;
template<typename T>
class EveryOther;

template<typename T>
ostream & operator<<(ostream & o, const EveryOther<T>& e);

template <class T>
class EveryOther{
public:
vector<T>vec_;
EveryOther()=default;
EveryOther(initializer_list<T> c): vec_(c){};
void push_back(T var){
vec_.push_back(var);
}

int size(){
vector<T>vec1;
for(size_t i = 0; i < vec_.size(); i+=2){
vec1.push_back(vec_[i]);
}
return vec1.size();
}
friend ostream & operator<< <T>(ostream &, const EveryOther&);

};
template<typename T>
ostream & operator<<(ostream & o,const EveryOther<T>& e){
o << "EveryOther(";
for(size_t i = 0; i < e.vec_.size(); i+=2){
o << e.vec_[i] << ", ";
}
o << ')';
return o;
}

filename: main.cpp

#include <iostream>
#include "everyother.h"
int main() 
{
EveryOther<char> eo_1;
EveryOther<std::string> eo_2 {"include", "dont", "keep", "leave"};

std::cout << eo_2 << std::endl;
}

Output EveryOther(include, keep, )