callback_without_using_std_function.cpp

#include <iostream>
#include <memory>
#include <tuple>

class Allocated {};

class Callback : public Allocated {
public:
  virtual ~Callback() {}
  virtual void run() = 0;
};

template <class F, class... Args>
class LambdaCallback : public Callback {
public:
  LambdaCallback(F&& func, std::tuple<Args...>&& args)
      : func_(std::move(func)) 
      , args_(std::move(args)) {}

  void run() { 
    std::apply(func_, args_);  // C++17 convenience function, but can be done in C++11
  }

private:
  F func_;
  std::tuple<Args...> args_;
};

template <class... Args>
void do_callback_internal(std::unique_ptr<Callback> callback) {
  callback->run();
}

template <class F, class... Args>
void do_callback(F&& func, Args&&... args) {
  do_callback_internal(std::make_unique<LambdaCallback<F, Args...>>(std::forward<F>(func), std::forward_as_tuple(std::move(args)...)));
}

int main() {
  do_callback([](std::string s) {
              std::cout << s << "\n";
              }, "hello!");
}