ResultExtensions.h

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
 * vim: set ts=8 sts=2 et sw=2 tw=80:
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

/* Extensions to the Result type to enable simpler handling of XPCOM/NSPR
 * results. */

#ifndef mozilla_ResultExtensions_h
#define mozilla_ResultExtensions_h

#include "mozilla/Assertions.h"
#include "nscore.h"
#include "prtypes.h"

namespace mozilla {

// Allow nsresult errors to automatically convert to nsresult values, so MOZ_TRY
// can be used in XPCOM methods with Result<T, nserror> results.
template <>
class MOZ_MUST_USE_TYPE GenericErrorResult<nsresult> {
  nsresult mErrorValue;

  template <typename V, typename E2>
  friend class Result;

 public:
  explicit GenericErrorResult(nsresult aErrorValue) : mErrorValue(aErrorValue) {
    MOZ_ASSERT(NS_FAILED(aErrorValue));
  }

  operator nsresult() { return mErrorValue; }
};

// Allow MOZ_TRY to handle `PRStatus` values.
inline Result<Ok, nsresult> ToResult(PRStatus aValue);

}  // namespace mozilla

#include "mozilla/Result.h"

namespace mozilla {

inline Result<Ok, nsresult> ToResult(nsresult aValue) {
  if (NS_FAILED(aValue)) {
    return Err(aValue);
  }
  return Ok();
}

inline Result<Ok, nsresult> ToResult(PRStatus aValue) {
  if (aValue == PR_SUCCESS) {
    return Ok();
  }
  return Err(NS_ERROR_FAILURE);
}

namespace detail {
template <typename R>
auto ResultRefAsParam(R& aResult) {
  return &aResult;
}

template <typename R, typename RArgMapper, typename Func, typename... Args>
Result<R, nsresult> ToResultInvokeInternal(const Func& aFunc,
                                           const RArgMapper& aRArgMapper,
                                           Args&&... aArgs) {
  R res;
  nsresult rv = aFunc(std::forward<Args>(aArgs)..., aRArgMapper(res));
  if (NS_FAILED(rv)) {
    return Err(rv);
  }
  return res;
}

template <typename T>
struct outparam_as_pointer;

template <typename T>
struct outparam_as_pointer<T*> {
  using type = T*;
};

template <typename T>
struct outparam_as_reference;

template <typename T>
struct outparam_as_reference<T*> {
  using type = T&;
};

template <typename R, template <typename> typename RArg, typename Func,
          typename... Args>
using to_result_retval_t = decltype(
    std::declval<Func&>()(std::declval<Args&&>()...,
                          std::declval<typename RArg<decltype(
                              ResultRefAsParam(std::declval<R&>()))>::type>()),
    Result<R, nsresult>(NS_OK));

// There are two ToResultInvokeSelector overloads, which cover the cases of a) a
// pointer-typed output parameter, and b) a reference-typed output parameter,
// using to_result_retval_t in connection with outparam_as_pointer and
// outparam_as_reference type traits. These type traits may be specialized for
// types other than raw pointers to allow calling functions with argument types
// that implicitly convert/bind to a raw pointer/reference. The overload that is
// used is selected by expression SFINAE: the decltype expression in
// to_result_retval_t is only valid in either case.
template <typename R, typename Func, typename... Args>
auto ToResultInvokeSelector(const Func& aFunc, Args&&... aArgs)
    -> to_result_retval_t<R, outparam_as_pointer, Func, Args...> {
  return ToResultInvokeInternal<R>(
      aFunc, [](R& res) -> decltype(auto) { return ResultRefAsParam(res); },
      std::forward<Args>(aArgs)...);
}

template <typename R, typename Func, typename... Args>
auto ToResultInvokeSelector(const Func& aFunc, Args&&... aArgs)
    -> to_result_retval_t<R, outparam_as_reference, Func, Args...> {
  return ToResultInvokeInternal<R>(
      aFunc, [](R& res) -> decltype(auto) { return *ResultRefAsParam(res); },
      std::forward<Args>(aArgs)...);
}

}  // namespace detail

/**
 * Adapts a function with a nsresult error type and an R* output parameter as
 * the last parameter to a function returning a mozilla::Result<R, nsresult>
 * object.
 *
 * This can also be used with member functions together with std::men_fn, e.g.
 *
 *    nsCOMPtr<nsIFile> file = ...;
 *    auto existsOrErr = ToResultInvoke<bool>(std::mem_fn(&nsIFile::Exists),
 *                                            *file);
 *
 * but it is more convenient to use the member function overload, which
 * has the additional benefit of enabling the deduction of the success result
 * type:
 *
 *    nsCOMPtr<nsIFile> file = ...;
 *    auto existsOrErr = ToResultInvoke(*file, &nsIFile::Exists);
 */
template <typename R, typename Func, typename... Args>
Result<R, nsresult> ToResultInvoke(const Func& aFunc, Args&&... aArgs) {
  return detail::ToResultInvokeSelector<R, Func, Args&&...>(
      aFunc, std::forward<Args>(aArgs)...);
}

namespace detail {
template <typename T>
struct tag {
  using type = T;
};

template <typename... Ts>
struct select_last {
  using type = typename decltype((tag<Ts>{}, ...))::type;
};

template <typename... Ts>
using select_last_t = typename select_last<Ts...>::type;

template <>
struct select_last<> {
  using type = void;
};

template <typename RArg, typename T, typename Func, typename... Args>
auto ToResultInvokeMemberFunction(T& aObj, const Func& aFunc, Args&&... aArgs) {
  if constexpr (std::is_pointer_v<RArg> ||
                (std::is_lvalue_reference_v<RArg> &&
                 !std::is_const_v<std::remove_reference_t<RArg>>)) {
    auto lambda = [&](RArg res) {
      return (aObj.*aFunc)(std::forward<Args>(aArgs)..., res);
    };
    return detail::ToResultInvokeSelector<
        std::remove_reference_t<std::remove_pointer_t<RArg>>, decltype(lambda)>(
        lambda);
  } else {
    // No output parameter present, return a Result<Ok, nsresult>
    return mozilla::ToResult((aObj.*aFunc)(std::forward<Args>(aArgs)...));
  }
}
}  // namespace detail

template <typename T, typename U, typename... XArgs, typename... Args>
auto ToResultInvoke(T& aObj, nsresult (U::*aFunc)(XArgs...), Args&&... aArgs) {
  return detail::ToResultInvokeMemberFunction<detail::select_last_t<XArgs...>>(
      aObj, aFunc, std::forward<Args>(aArgs)...);
}

template <typename T, typename U, typename... XArgs, typename... Args>
auto ToResultInvoke(const T& aObj, nsresult (U::*aFunc)(XArgs...) const,
                    Args&&... aArgs) {
  return detail::ToResultInvokeMemberFunction<detail::select_last_t<XArgs...>>(
      aObj, aFunc, std::forward<Args>(aArgs)...);
}

template <typename T, typename U, typename... XArgs, typename... Args>
auto ToResultInvoke(T* const aObj, nsresult (U::*aFunc)(XArgs...),
                    Args&&... aArgs) {
  return ToResultInvoke(*aObj, aFunc, std::forward<Args>(aArgs)...);
}

template <typename T, typename U, typename... XArgs, typename... Args>
auto ToResultInvoke(const T* const aObj, nsresult (U::*aFunc)(XArgs...) const,
                    Args&&... aArgs) {
  return ToResultInvoke(*aObj, aFunc, std::forward<Args>(aArgs)...);
}

#if defined(XP_WIN) && !defined(_WIN64)
template <typename T, typename U, typename... XArgs, typename... Args>
auto ToResultInvoke(T& aObj, nsresult (__stdcall U::*aFunc)(XArgs...),
                    Args&&... aArgs) {
  return detail::ToResultInvokeMemberFunction<detail::select_last_t<XArgs...>>(
      aObj, aFunc, std::forward<Args>(aArgs)...);
}

template <typename T, typename U, typename... XArgs, typename... Args>
auto ToResultInvoke(const T& aObj,
                    nsresult (__stdcall U::*aFunc)(XArgs...) const,
                    Args&&... aArgs) {
  return detail::ToResultInvokeMemberFunction<detail::select_last_t<XArgs...>>(
      aObj, aFunc, std::forward<Args>(aArgs)...);
}

template <typename T, typename U, typename... XArgs, typename... Args>
auto ToResultInvoke(T* const aObj, nsresult (__stdcall U::*aFunc)(XArgs...),
                    Args&&... aArgs) {
  return ToResultInvoke(*aObj, aFunc, std::forward<Args>(aArgs)...);
}

template <typename T, typename U, typename... XArgs, typename... Args>
auto ToResultInvoke(const T* const aObj,
                    nsresult (__stdcall U::*aFunc)(XArgs...) const,
                    Args&&... aArgs) {
  return ToResultInvoke(*aObj, aFunc, std::forward<Args>(aArgs)...);
}
#endif

}  // namespace mozilla

#endif  // mozilla_ResultExtensions_h