mfbt/FunctionTypeTraits.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 /* Helpers to manipulate function types that don't fit in TypeTraits.h */
   8
   9 #ifndef mozilla_FunctionTypeTraits_h
  10 #define mozilla_FunctionTypeTraits_h
  11
  12 #include <cstddef> /* for size_t */
  13 #include <tuple>
  14
  15 namespace mozilla {
  16
  17 // Main FunctionTypeTraits declaration, taking one template argument.
  18 //
  19 // Given a function type, FunctionTypeTraits will expose the following members:
  20 // - ReturnType: Return type.
  21 // - arity: Number of parameters (size_t).
  22 // - ParameterType<N>: Type of the Nth** parameter, 0-indexed.
  23 //
  24 // ** `ParameterType<N>` with `N` >= `arity` is allowed and gives `void`.
  25 // This prevents compilation errors when trying to access a type outside of the
  26 // function's parameters, which is useful for parameters checks, e.g.:
  27 //   template<typename F>
  28 //   auto foo(F&&)
  29 //    -> enable_if(FunctionTypeTraits<F>::arity == 1 &&
  30 //                 is_same<FunctionTypeTraits<F>::template ParameterType<0>,
  31 //                         int>::value,
  32 //                 void)
  33 //   {
  34 //     // This function will only be enabled if `F` takes one `int`.
  35 //     // Without the permissive ParameterType<any N>, it wouldn't even compile.
  36 //
  37 // Note: FunctionTypeTraits does not work with generic lambdas `[](auto&) {}`,
  38 // because parameter types cannot be known until an actual invocation when types
  39 // are inferred from the given arguments.
  40 template <typename T>
  41 struct FunctionTypeTraits;
  42
  43 // Remove reference and pointer wrappers, if any.
  44 template <typename T>
  45 struct FunctionTypeTraits<T&> : public FunctionTypeTraits<T> {};
  46 template <typename T>
  47 struct FunctionTypeTraits<T&&> : public FunctionTypeTraits<T> {};
  48 template <typename T>
  49 struct FunctionTypeTraits<T*> : public FunctionTypeTraits<T> {};
  50
  51 // Extract `operator()` function from callables (e.g. lambdas, std::function).
  52 template <typename T>
  53 struct FunctionTypeTraits
  54     : public FunctionTypeTraits<decltype(&T::operator())> {};
  55
  56 namespace detail {
  57
  58 // If `safe`, retrieve the `N`th type from `As`, otherwise `void`.
  59 // See top description for reason.
  60 template <bool safe, size_t N, typename... As>
  61 struct TupleElementSafe;
  62 template <size_t N, typename... As>
  63 struct TupleElementSafe<true, N, As...> {
  64   using Type = typename std::tuple_element<N, std::tuple<As...>>::type;
  65 };
  66 template <size_t N, typename... As>
  67 struct TupleElementSafe<false, N, As...> {
  68   using Type = void;
  69 };
  70
  71 template <typename R, typename... As>
  72 struct FunctionTypeTraitsHelper {
  73   using ReturnType = R;
  74   static constexpr size_t arity = sizeof...(As);
  75   template <size_t N>
  76   using ParameterType =
  77       typename TupleElementSafe<(N < sizeof...(As)), N, As...>::Type;
  78 };
  79
  80 }  // namespace detail
  81
  82 // Specialization for free functions.
  83 template <typename R, typename... As>
  84 struct FunctionTypeTraits<R(As...)>
  85     : detail::FunctionTypeTraitsHelper<R, As...> {};
  86
  87 // Specialization for non-const member functions.
  88 template <typename C, typename R, typename... As>
  89 struct FunctionTypeTraits<R (C::*)(As...)>
  90     : detail::FunctionTypeTraitsHelper<R, As...> {};
  91
  92 // Specialization for const member functions.
  93 template <typename C, typename R, typename... As>
  94 struct FunctionTypeTraits<R (C::*)(As...) const>
  95     : detail::FunctionTypeTraitsHelper<R, As...> {};
  96
  97 #ifdef NS_HAVE_STDCALL
  98 // Specialization for __stdcall free functions.
  99 template <typename R, typename... As>
 100 struct FunctionTypeTraits<R NS_STDCALL(As...)>
 101     : detail::FunctionTypeTraitsHelper<R, As...> {};
 102
 103 // Specialization for __stdcall non-const member functions.
 104 template <typename C, typename R, typename... As>
 105 struct FunctionTypeTraits<R (NS_STDCALL C::*)(As...)>
 106     : detail::FunctionTypeTraitsHelper<R, As...> {};
 107
 108 // Specialization for __stdcall const member functions.
 109 template <typename C, typename R, typename... As>
 110 struct FunctionTypeTraits<R (NS_STDCALL C::*)(As...) const>
 111     : detail::FunctionTypeTraitsHelper<R, As...> {};
 112 #endif  // NS_HAVE_STDCALL
 113
 114 }  // namespace mozilla
 115
 116 #endif  // mozilla_FunctionTypeTraits_h