1 //
2 // Copyright 2019 The Abseil Authors.
3 //
4 // Licensed under the Apache License, Version 2.0 (the "License");
5 // you may not use this file except in compliance with the License.
6 // You may obtain a copy of the License at
7 //
8 //      https://www.apache.org/licenses/LICENSE-2.0
9 //
10 // Unless required by applicable law or agreed to in writing, software
11 // distributed under the License is distributed on an "AS IS" BASIS,
12 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 // See the License for the specific language governing permissions and
14 // limitations under the License.
15 
16 #ifndef ABSL_RANDOM_INTERNAL_MOCK_HELPERS_H_
17 #define ABSL_RANDOM_INTERNAL_MOCK_HELPERS_H_
18 
19 #include <tuple>
20 #include <type_traits>
21 
22 #include "absl/base/internal/fast_type_id.h"
23 #include "absl/types/optional.h"
24 
25 namespace absl {
26 ABSL_NAMESPACE_BEGIN
27 namespace random_internal {
28 
29 // MockHelpers works in conjunction with MockOverloadSet, MockingBitGen, and
30 // BitGenRef to enable the mocking capability for absl distribution functions.
31 //
32 // MockingBitGen registers mocks based on the typeid of a mock signature, KeyT,
33 // which is used to generate a unique id.
34 //
35 // KeyT is a signature of the form:
36 //   result_type(discriminator_type, std::tuple<args...>)
37 // The mocked function signature will be composed from KeyT as:
38 //   result_type(args...)
39 //
40 class MockHelpers {
41   using IdType = ::absl::base_internal::FastTypeIdType;
42 
43   // Given a key signature type used to index the mock, extract the components.
44   // KeyT is expected to have the form:
45   //   result_type(discriminator_type, arg_tuple_type)
46   template <typename KeyT>
47   struct KeySignature;
48 
49   template <typename ResultT, typename DiscriminatorT, typename ArgTupleT>
50   struct KeySignature<ResultT(DiscriminatorT, ArgTupleT)> {
51     using result_type = ResultT;
52     using discriminator_type = DiscriminatorT;
53     using arg_tuple_type = ArgTupleT;
54   };
55 
56   // Detector for InvokeMock.
57   template <class T>
58   using invoke_mock_t = decltype(std::declval<T*>()->InvokeMock(
59       std::declval<IdType>(), std::declval<void*>(), std::declval<void*>()));
60 
61   // Empty implementation of InvokeMock.
62   template <typename KeyT, typename ReturnT, typename ArgTupleT, typename URBG,
63             typename... Args>
64   static absl::optional<ReturnT> InvokeMockImpl(char, URBG*, Args&&...) {
65     return absl::nullopt;
66   }
67 
68   // Non-empty implementation of InvokeMock.
69   template <typename KeyT, typename ReturnT, typename ArgTupleT, typename URBG,
70             typename = invoke_mock_t<URBG>, typename... Args>
71   static absl::optional<ReturnT> InvokeMockImpl(int, URBG* urbg,
72                                                 Args&&... args) {
73     ArgTupleT arg_tuple(std::forward<Args>(args)...);
74     ReturnT result;
75     if (urbg->InvokeMock(::absl::base_internal::FastTypeId<KeyT>(), &arg_tuple,
76                          &result)) {
77       return result;
78     }
79     return absl::nullopt;
80   }
81 
82  public:
83   // Invoke a mock for the KeyT (may or may not be a signature).
84   //
85   // KeyT is used to generate a typeid-based lookup key for the mock.
86   // KeyT is a signature of the form:
87   //   result_type(discriminator_type, std::tuple<args...>)
88   // The mocked function signature will be composed from KeyT as:
89   //   result_type(args...)
90   //
91   // An instance of arg_tuple_type must be constructable from Args..., since
92   // the underlying mechanism requires a pointer to an argument tuple.
93   template <typename KeyT, typename URBG, typename... Args>
94   static auto MaybeInvokeMock(URBG* urbg, Args&&... args)
95       -> absl::optional<typename KeySignature<KeyT>::result_type> {
96     // Use function overloading to dispatch to the implemenation since
97     // more modern patterns (e.g. require + constexpr) are not supported in all
98     // compiler configurations.
99     return InvokeMockImpl<KeyT, typename KeySignature<KeyT>::result_type,
100                           typename KeySignature<KeyT>::arg_tuple_type, URBG>(
101         0, urbg, std::forward<Args>(args)...);
102   }
103 
104   // Acquire a mock for the KeyT (may or may not be a signature).
105   //
106   // KeyT is used to generate a typeid-based lookup for the mock.
107   // KeyT is a signature of the form:
108   //   result_type(discriminator_type, std::tuple<args...>)
109   // The mocked function signature will be composed from KeyT as:
110   //   result_type(args...)
111   template <typename KeyT, typename MockURBG>
112   static auto MockFor(MockURBG& m) -> decltype(
113       std::declval<MockURBG>()
114           .template RegisterMock<typename KeySignature<KeyT>::result_type,
115                                  typename KeySignature<KeyT>::arg_tuple_type>(
116               std::declval<IdType>())) {
117     return m.template RegisterMock<typename KeySignature<KeyT>::result_type,
118                                    typename KeySignature<KeyT>::arg_tuple_type>(
119         ::absl::base_internal::FastTypeId<KeyT>());
120   }
121 };
122 
123 }  // namespace random_internal
124 ABSL_NAMESPACE_END
125 }  // namespace absl
126 
127 #endif  // ABSL_RANDOM_INTERNAL_MOCK_HELPERS_H_
128