scope_guard.hpp

/*
 *  Created on: 13/02/2018
 *      Author: ricab
 *
 * See README.md for documentation of this header's public interface.
 */

#ifndef SCOPE_GUARD_HPP_
#define SCOPE_GUARD_HPP_

#include <type_traits>
#include <utility>

#if __cplusplus >= 201703L && defined(SG_REQUIRE_NOEXCEPT_IN_CPP17)
#define SG_REQUIRE_NOEXCEPT
#endif

namespace sg
{
  namespace detail
  {
    /* --- Some custom type traits --- */

    // Type trait determining whether a type is callable with no arguments
    template<typename T, typename = void>
    struct is_noarg_callable_t
      : public std::false_type
    {}; // in general, false

    template<typename T>
    struct is_noarg_callable_t<T, decltype(std::declval<T&&>()())>
      : public std::true_type
    {}; // only true when call expression valid

    // Type trait determining whether a no-argument callable returns void
    template<typename T>
    struct returns_void_t
      : public std::is_same<void, decltype(std::declval<T&&>()())>
    {};

    /* Type trait determining whether a no-arg callable is nothrow invocable if
    required. This is where SG_REQUIRE_NOEXCEPT logic is encapsulated. */
    template<typename T>
    struct is_nothrow_invocable_if_required_t
      : public
#ifdef SG_REQUIRE_NOEXCEPT
          std::is_nothrow_invocable<T> /* Note: _r variants not enough to
                                          confirm void return: any return can be
                                          discarded so all returns are
                                          compatible with void */
#else
          std::true_type
#endif
    {};

    // logic AND of two or more type traits
    template<typename A, typename B, typename... C>
    struct and_t : public and_t<A, and_t<B, C...>>
    {}; // for more than two arguments

    template<typename A, typename B>
    struct and_t<A, B> : public std::conditional<A::value, B, A>::type
    {}; // for two arguments

    // Type trait determining whether a type is a proper scope_guard callback.
    template<typename T>
    struct is_proper_sg_callback_t
      : public and_t<is_noarg_callable_t<T>,
                     returns_void_t<T>,
                     is_nothrow_invocable_if_required_t<T>,
                     std::is_nothrow_destructible<T>>
    {};


    /* --- The actual scope_guard template --- */

    template<typename Callback,
             typename = typename std::enable_if<
               is_proper_sg_callback_t<Callback>::value>::type>
    class scope_guard;


    /* --- Now the friend maker --- */

    template<typename Callback>
    detail::scope_guard<Callback> make_scope_guard(Callback&& callback)
    noexcept(std::is_nothrow_constructible<Callback, Callback&&>::value); /*
    we need this in the inner namespace due to MSVC bugs preventing
    sg::detail::scope_guard from befriending a sg::make_scope_guard
    template instance in the parent namespace (see https://is.gd/xFfFhE). */


    /* --- The template specialization that actually defines the class --- */

    template<typename Callback>
    class scope_guard<Callback> final
    {
    public:
      typedef Callback callback_type;

      scope_guard(scope_guard&& other)
      noexcept(std::is_nothrow_constructible<Callback, Callback&&>::value);

      ~scope_guard() noexcept; // highlight noexcept dtor

      void dismiss() noexcept;

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

    private:
      explicit scope_guard(Callback&& callback)
      noexcept(std::is_nothrow_constructible<Callback, Callback&&>::value); /*
                                                      meant for friends only */

      friend scope_guard<Callback> make_scope_guard<Callback>(Callback&&)
      noexcept(std::is_nothrow_constructible<Callback, Callback&&>::value); /*
      only make_scope_guard can create scope_guards from scratch (i.e. non-move)
      */

    private:
      Callback m_callback;
      bool m_active;

    };

  } // namespace detail


  /* --- Now the single public maker function --- */

  using detail::make_scope_guard; // see comment on declaration above

} // namespace sg

////////////////////////////////////////////////////////////////////////////////
template<typename Callback>
sg::detail::scope_guard<Callback>::scope_guard(Callback&& callback)
noexcept(std::is_nothrow_constructible<Callback, Callback&&>::value)
  : m_callback(std::forward<Callback>(callback)) /* use () instead of {} because
    of DR 1467 (https://is.gd/WHmWuo), which still impacts older compilers
    (e.g. GCC 4.x and clang <=3.6, see https://godbolt.org/g/TE9tPJ and
    https://is.gd/Tsmh8G) */
  , m_active{true}
{}

////////////////////////////////////////////////////////////////////////////////
template<typename Callback>
sg::detail::scope_guard<Callback>::~scope_guard() noexcept
{
  if(m_active)
    m_callback();
}

////////////////////////////////////////////////////////////////////////////////
template<typename Callback>
sg::detail::scope_guard<Callback>::scope_guard(scope_guard&& other)
noexcept(std::is_nothrow_constructible<Callback, Callback&&>::value)
  : m_callback(std::forward<Callback>(other.m_callback)) // idem
  , m_active{std::move(other.m_active)}
{
  other.m_active = false;
}

////////////////////////////////////////////////////////////////////////////////
template<typename Callback>
inline void sg::detail::scope_guard<Callback>::dismiss() noexcept
{
  m_active = false;
}

////////////////////////////////////////////////////////////////////////////////
template<typename Callback>
inline auto sg::detail::make_scope_guard(Callback&& callback)
noexcept(std::is_nothrow_constructible<Callback, Callback&&>::value)
-> detail::scope_guard<Callback>
{
  return detail::scope_guard<Callback>{std::forward<Callback>(callback)};
}

#endif /* SCOPE_GUARD_HPP_ */