施工阶段2

CMakeLists.txt

@@ -1,6 +1,6 @@
 cmake_minimum_required(VERSION 3.18)
 project(tmp_policy)
 
-set(CMAKE_CXX_STANDARD 14)
+set(CMAKE_CXX_STANDARD 17)
 
-add_executable(tmp_policy main.cpp accpolicy.h policy_fundamentals.h)
+add_executable(tmp_policy main.cpp accpolicy.h policy_fundamentals.h accumulator.h)

accumulator.h

@@ -0,0 +1,102 @@
+// tmp_policy
+// Created by lucas on 22-8-4.
+// Re-implementation of a typical policy architecture in C++ Template Meta-Programming.
+
+
+#ifndef TMP_POLICY_ACCUMULATOR_H
+#define TMP_POLICY_ACCUMULATOR_H
+
+// A demo for integrating the policy architecture
+// into your project.
+#include <type_traits>
+#include <cmath>
+#include "accpolicy.h"
+#include "policy_fundamentals.h"
+
+// According to the C++ Standard,
+// Direct use of static_assert(false) results in undefined behavior,
+// so we define a type-dependent boolean variable whose value is false.
+template <typename T>
+constexpr bool DependencyFalse = false;
+/**
+ * @brief class Accumulator - the class performs "accumulation"
+ * and it's behavior is controlled by policy objects.
+ * @tparam TPolicies The policy objects which control the class's behavior.
+ */
+template <typename ... TPolicies>
+class Accumulator
+{
+    // Put all the policy objects into a PolicyContainer.
+    using PolicyCont = PolicyContainer<TPolicies...>;
+    // Use PolicySelect Meta-Function to get the policy selection result.
+    using PSR = PolicySelect<PolicyCont, AccPolicy>;
+    // According to the policy selection result,
+    // we get the necessary information to control the Accumulator's behavior.
+    // AccuType - controls how the "accumulation" works; Add or Multiply?
+    using AccuType = typename PSR::AccuType;
+    // IsAve - returns the average or the original result?
+    static constexpr bool IsAve = PSR::IsAveValue;
+    // ResultType - What type do you expect the Accumulator to return?
+    using ResultType = typename PSR::ResultType;
+public:
+    /**
+     * @brief eval - The function which does the "accumulation".
+     * @tparam TInput The type of parameter in - it will be judged by the compiler.
+     * @param in The input array which contains the elements waiting to be accumulated.
+     * It needs to support range-based for statement.
+     * @return The "accumulation" result. User-specified.
+     */
+    template <typename TInput>
+    static auto eval(const TInput& in)
+    {
+        ResultType res{};
+        int count{};
+        // If we want to perform "add" ...
+        if constexpr (std::is_same<AccPolicy::AccuTypeCate::Add, AccuType>::value)
+        {
+            // Add the all elements to res ...
+            for (const auto& element: in)
+            {
+                res += element;
+                // ... and also records the number of elements.
+                count += 1;
+            }
+            // If we want to calculate average...
+            if constexpr (IsAve)
+                // ... calculates the average and returns.
+                return res / static_cast<ResultType>(count);
+            else
+                // ... if not, simply returns the sum.
+                return res;
+        }
+        // Or if we want to perform "multiply" ...
+        else if constexpr (std::is_same<AccPolicy::AccuTypeCate::Mul, AccuType>::value)
+        {
+            // Because 0 times any number equals 0 ...
+            // ... we need to give res an initial value.
+            res = 1;
+            // Multiply the all elements to res ...
+            for (const auto& element: in)
+            {
+                res *= element;
+                // ... and also records the number of elements.
+                count += 1;
+            }
+            // If we want to calculate average...
+            if constexpr (IsAve)
+                // ... calculates the average and returns.
+                return std::pow(res, 1.0 / static_cast<ResultType>(count));
+            else
+                // ... if not, simply returns the sum.
+                return res;
+        }
+        else
+        {
+            // We have triggered logic that should not be triggered ...
+            // ... the solution is to generate a compile error.
+            static_assert(DependencyFalse<AccuType>, "Invalid policy argument!");
+        }
+    }
+};
+
+#endif //TMP_POLICY_ACCUMULATOR_H

cmake-build-debug/.cmake/api/v1/reply/cache-v2-7057157c536c60dfd95c.json

@@ -191,11 +191,11 @@
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@@ -319,11 +319,11 @@
 				},
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+			"type" : "UNINITIALIZED",
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+			"value" : "clang"
 		},
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 			"name" : "CMAKE_C_COMPILER_AR",

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 			"sourceGroupIndex" : 1
+		},
+		{
+			"backtrace" : 1,
+			"path" : "accumulator.h",
+			"sourceGroupIndex" : 1
 		}
 	],
 	"type" : "EXECUTABLE"

cmake-build-debug/tmp_policy.cbp

@@ -85,6 +85,9 @@
 		<Unit filename="/home/lucas/CLionProjects/tmp-policy/accpolicy.h">
 			<Option target="tmp_policy"/>
 		</Unit>
+		<Unit filename="/home/lucas/CLionProjects/tmp-policy/accumulator.h">
+			<Option target="tmp_policy"/>
+		</Unit>
 		<Unit filename="/home/lucas/CLionProjects/tmp-policy/main.cpp">
 			<Option target="tmp_policy"/>
 		</Unit>

main.cpp

@@ -1,6 +1,28 @@
+// tmp_policy
+// Created by lucas on 22-7-26.
+// Re-implementation of a typical policy architecture in C++ Template Meta-Programming.
+
 #include <iostream>
+#include "accumulator.h"
+#include <vector>
+#include <string>
 
 int main() {
-    std::cout << "Hello, World!" << std::endl;
+    // Normal demo
+    std::vector<int> intVec;
+    intVec.reserve(10);
+    for (int i = 1; i <= 10; ++i)
+        intVec.push_back(i);
+    std::cout << Accumulator<PAdd, PAve, PResultTypeIs<int>>::eval(intVec) << std::endl;
+    std::cout << Accumulator<PAve, PAdd, PResultTypeIs<int>>::eval(intVec) << std::endl;
+    std::cout << Accumulator<PAdd, PNoAve, PResultTypeIs<int>>::eval(intVec) << std::endl;
+    std::cout << Accumulator<>::eval(intVec) << std::endl;
+    std::cout << Accumulator<PMul, PNoAve>::eval(intVec) << std::endl;
+    std::cout << Accumulator<PAve>::eval(intVec) << std::endl;
+    // Punchline (gusha)
+    std::vector<std::string> strVec;
+    for (int i = 1; i <= 10; ++i)
+        strVec.emplace_back("Today is KFC crazy Thursday V me ¥50\n");
+    std::cout << Accumulator<PAdd, PNoAve, PResultTypeIs<std::string>>::eval(strVec) << std::endl;
     return 0;
 }

policy_fundamentals.h

@@ -59,8 +59,8 @@ namespace NSPolicySelect {
     };
 
     // Template specialization of MajorFilter_.
-    template <typename TEmptyContainer, typename TFirstPolicy, typename ... TOtherPolicies>
+    template <typename TFirstPolicy, typename ... TOtherPolicies>
-    struct MajorFilter_<TEmptyContainer,
+    struct MajorFilter_<PolicyContainer<>,
             PolicyContainer<TFirstPolicy, TOtherPolicies...>>
     {
         // The beginning of the recursion.
@@ -112,27 +112,99 @@ namespace NSPolicySelect {
     // For each policy object in the policy container,
     // use MinorCheckInner_ to identify if there are another policy object
     // having identical minor class.
+    // Also, the primitive template handles with the situation in which
+    // the policy container has no element.
     template <typename TPolicyContainer>
     struct MinorCheckOuter_
     {
         using type = std::true_type;
     };
 
+    // Template specialization of MinorCheckOuter_.
     template <typename TCurPolicy, typename ... TRestPolicies>
     struct MinorCheckOuter_<PolicyContainer<TCurPolicy, TRestPolicies...>>
     {
-    private:
+        // The main logic of recursion.
-        using InnerResult_ = typename MinorCheckInner_<TCurPolicy, PolicyContainer<TRestPolicies...>>::type;
-    public:
         using type = std::conditional_t<
-                InnerResult_::value,
+                // Does TCurPolicy and the rest policy objects have the same minor class?
+                MinorCheckInner_<TCurPolicy, PolicyContainer<TRestPolicies...>>::type::value,
+                // ... if no, proceed with next policy object in the policy container.
                 typename MinorCheckOuter_<PolicyContainer<TRestPolicies...>>::type,
+                // ... if yes, simply returns false and ends the recursion.
                 std::false_type
                 >;
     };
 
+    // Auxiliary template value meta-function definition: MinorCheck.
+    // Check if the TPolicyContainer has more than one element
+    // with the same minor class.
+    // If yes, returns false.
+    // If no, returns true.
     template <typename TPolicyContainer>
     constexpr bool MinorCheck = MinorCheckOuter_<TPolicyContainer>::type::value;
+
+    // Template Declaration: template <typename> struct PolicySelectionResult.
+    // The template class which describes the policy selection result.
+    template <typename TPolicyContainer>
+    struct PolicySelectionResult;
+
+    // Template specialization of PolicySelectionResult.
+    // If the PolicyContainer only has one policy object in it,
+    // the PolicySelectionResult simply inherits from the policy object.
+    template <typename TPolicy>
+    struct PolicySelectionResult<PolicyContainer<TPolicy>> :
+            public TPolicy {};
+
+    // Template specialization of PolicySelectionResult.
+    // If the PolicyContainer has more than one policy object in it,
+    // the PolicySelectionResult inherits from the first policy object
+    // and the PolicySelectionResult of other policies.
+    // The result contains a definition that is dominant and unambiguous --
+    // each declaration comes from either Policy Group's initial definition
+    // or the policy object's overwritten definition.
+    template <typename TCurPolicy, typename ... TRestPolicies>
+    struct PolicySelectionResult<PolicyContainer<TCurPolicy, TRestPolicies...>> :
+            public TCurPolicy,
+            public PolicySelectionResult<PolicyContainer<TRestPolicies...>> {};
+
+    // Template Declaration: template <typename, typename> struct Selector_.
+    // The interface for users to get the policy selection result.
+    // TPolicyContainer - the policy container which contains the policy objects.
+    // TPolicyGroup - the original policy group.
+    template <typename TPolicyContainer, typename TPolicyGroup>
+    struct Selector_;
+
+    // Template specialization of Selector_.
+    // Restricts the TPolicyContainer can only be type PolicyContainer.
+    template <typename ... TPolicies, typename TPolicyGroup>
+    struct Selector_<PolicyContainer<TPolicies...>, TPolicyGroup>
+    {
+        // Use access specifier to prevent the users to use the internal types.
+    private:
+        // Use MajorFilter (MajorFilter_) to filter out the policy objects
+        // with unmatched MajorClass.
+        using FilteredPolicy = MajorFilter<PolicyContainer<TPolicies...>>;
+        // Check if the policy objects has conflict minor classes.
+        static_assert(MinorCheck<FilteredPolicy>, "Minor Class Set Conflict!");
+    public:
+        // Generate the policy selection result.
+        using type = std::conditional_t<
+                // Is the PolicyContainer empty?
+                IsArrayEmpty<FilteredPolicy>,
+                // ... if so, returns the policy group to represent the default definitions.
+                TPolicyGroup,
+                // ... if not, generate policy selection result to represent the user-selected definitions.
+                PolicySelectionResult<FilteredPolicy>
+                >;
+    };
+
+    // Auxiliary type definition - PolicySelect.
+    // To simplify the usage of Selector_.
+    template <typename TPolicyContainer, typename TPolicyGroup>
+    using PolicySelect = typename Selector_<TPolicyContainer, TPolicyGroup>::type;
 }
 
+// Expose the NSPolicySelect::PolicySelect to global scope.
+using NSPolicySelect::PolicySelect;
+
 #endif //TMP_POLICY_POLICY_FUNDAMENTALS_H