on_exit - useful with C Apis

on_exit.cpp

#define _CRT_SECURE_NO_WARNINGS
#include <cstdio>
#include <memory>
#include <type_traits>

namespace
{
  template<typename TAction>
  struct scope_guard
  {
    scope_guard (TAction a)
      : action        (std::move (a))
      , invoke_action (true)
    {
    }

    ~scope_guard () noexcept
    {
      if (invoke_action)
      {
        invoke_action = false;
        action ();
      }
    }

    scope_guard (scope_guard const &)             = delete;
    scope_guard & operator= (scope_guard const &) = delete;
    scope_guard & operator= (scope_guard &&)      = delete;

    scope_guard (scope_guard && sg)
      : action        (std::move (sg.action))
      , invoke_action (sg.invoke_action)
    {
      sg.invoke_action = false;
    }

    bool    invoke_action ;

  private:
    TAction action        ;
  };

  template<typename TAction>
  auto on_exit (TAction && action)
  {
    return scope_guard<std::decay_t<TAction>> (std::forward<TAction> (action));
  }
}

int main()
{
  auto file = fopen ("hello.txt", "r");
  if (file)
  {
    // on_exit allows us to create cleanup functions that cleanups C resources when scope closes.
    // We can create "smart" pointers for all these resources but it adds a fair bit of overhead.
    // I prefer on_exit for quick and not so dirty stuff
    auto on_exit__close_file = on_exit ([file] { fclose (file); });
  }
}