feat: implement join-string functor

Functor/main.cpp

@@ -1,8 +1,39 @@
 #include <iostream>
+#include <numeric>
+#include <vector>
+
+class join_strings_functor {
+private:
+  std::string separator_ = " ";
+
+public:
+  join_strings_functor() = default;
+  join_strings_functor(std::string_view separator) : separator_(separator) {}
+
+  std::string operator()(const std::string &left,
+                         const std::string &right) const {
+    return left + separator_ + right;
+  }
+};
 
 int main(int argc, char *argv[]) {
 
   std::cout << "Functor" << std::endl;
 
+  std::vector<std::string> names = {"Albert", "Bernd", "Cora",
+                                    "Dave",   "Emil",  "Frank"};
+
+  auto space_joined_string =
+      std::accumulate(std::next(names.begin()), names.end(), names.front(),
+                      join_strings_functor());
+
+  std::cout << "Space-joined string: " << space_joined_string << std::endl;
+
+  auto comma_joined_string =
+      std::accumulate(std::next(names.begin()), names.end(), names.front(),
+                      join_strings_functor(", "));
+
+  std::cout << "Comma-joined string: " << comma_joined_string << std::endl;
+
   return EXIT_SUCCESS;
 }