c++ - 在此上下文中如何使用 CRTP 删除虚拟方法？

Question

我有以下类似于我的代码库的程序。执行某种算法(可能在多个线程中)的 FunctionState 类，以及控制 FunctionState 类如何使用的 Function 类，并可能执行一些算法设置/拆卸操作。

#include <iostream>
#include <vector>

class FunctionState;

class Function {
public:
    virtual FunctionState* NewFunctionState() = 0;

protected:
    std::vector<FunctionState*> states;
};

class FunctionState {
public:
    FunctionState(Function* func) : mFunc(func) {}

    virtual void RunState() = 0;
    void ExecuteFunctionLotsAndLotsOfTimes();

private:
    Function* mFunc;
};

#define VERY_BIG_NUMBER 10

void FunctionState::ExecuteFunctionLotsAndLotsOfTimes() {
    for(int i = 0; i < VERY_BIG_NUMBER; ++i) {
        RunState();
    }
};

class PrintFunction : public Function {
    FunctionState* NewFunctionState();
};

class PrintFunctionState : public FunctionState {
public:
    PrintFunctionState(PrintFunction* func) : FunctionState(func) {}

    void RunState() override {
        std::cout << "in print function state" << '\n';
    }
};

FunctionState* PrintFunction::NewFunctionState() {
    FunctionState* state = new PrintFunctionState(this);
    states.push_back(state);
    return state;
}

class AddFunction : public Function {
    FunctionState* NewFunctionState();
};

class AddFunctionState : public FunctionState {
public:
    AddFunctionState(AddFunction* func) : FunctionState(func), x(0) {}

    void RunState() override {
        ++x;
    }
private:
    int x;
};

FunctionState* AddFunction::NewFunctionState() {
    FunctionState* state = new AddFunctionState(this);
    states.push_back(state);
    return state;
}


int main() {
    Function* func = new PrintFunction();
    Function* func2 = new AddFunction();
    std::vector<Function*> vec = {func, func2};

    for(auto& func : vec) {
        func->NewFunctionState()->ExecuteFunctionLotsAndLotsOfTimes();
    }

    return 0;
}

现在我分析了我的代码，发现在 FunctionState::ExecuteFunctionLotsAndLotsOfTimes() 处有一个热点。问题在于此函数循环多次并调用 FunctionState 类上的虚函数 RunState()。在那里，我执行了很多操作，这些操作可能会从 L1 缓存中清除 vtable 指针，导致 L1 缓存错过循环的每次迭代。

所以我想消除虚拟调用的需要。我决定使用 CRTP 是一个很好的方法。 FunctionState 类将采用实现它的类类型的模板参数并调用它的适当方法，而无需对 RunState() 进行虚拟调用。

现在，当我尝试将其移动到 CRTP 时，我遇到了一些与 Function 类有关的问题:

1. 如何转发声明 FunctionState 类(因为它现在是模板化的)？

2. 我是否还需要向 Function 类添加模板参数？

   <强>3。如果我将其模板化，Function 对象的构造会是什么样子？我将如何消除对使用 Function 对象指定类型参数的类的需求？

请注意，这只是我真实代码库的一个简单版本。真正的代码库是 10K+ 行代码(并非难以管理，但完全重写是不可能的)。

此外，如果有另一种方法可以删除不涉及 CRTP 的对 RunState() 的虚拟调用，那么我们也将不胜感激。

我尝试使用 CRTP:

#include <iostream>
#include <vector>

class Function;

template<class T>
class FunctionState {
public:
    FunctionState(Function* func) : mFunc(func) {}

    void RunState() {
        static_cast<T*>(this)->RunState();
    };

    void ExecuteFunctionLotsAndLotsOfTimes();
private:
    Function* mFunc;
};

class Function {
public:
    virtual FunctionState* NewFunctionState() = 0;

protected:
    std::vector<FunctionState*> states;
};

#define VERY_BIG_NUMBER 10

template <typename T>
void FunctionState<T>::ExecuteFunctionLotsAndLotsOfTimes() {
    for(int i = 0; i < VERY_BIG_NUMBER; ++i) {
        RunState();
    }
};

class PrintFunctionState;
class PrintFunction : public Function {
    PrintFunctionState* NewFunctionState();
};

class PrintFunctionState : public FunctionState<PrintFunctionState> {
public:
    PrintFunctionState(PrintFunction* func) : FunctionState<PrintFunctionState>(func) {}

    void RunState() {
        std::cout << "in print function state" << '\n';
    }
};

PrintFunctionState* PrintFunction::NewFunctionState() {
    PrintFunctionState* state = new PrintFunctionState(this);
    states.push_back(state);
    return state;
}

class AddFunctionState;
class AddFunction : public Function {
    AddFunctionState* NewFunctionState();
};

class AddFunctionState : public FunctionState<AddFunctionState> {
public:
    AddFunctionState(AddFunction* func) : FunctionState<AddFunctionState>(func), x(0) {}

    void RunState() {
        ++x;
    }
private:
    int x;
};

AddFunctionState* AddFunction::NewFunctionState() {
    AddFunctionState* state = new AddFunctionState(this);
    states.push_back(state);
    return state;
}


int main() {
    Function* func = new PrintFunction();
    Function* func2 = new AddFunction();
    std::vector<Function*> vec = {func, func2};

    for(auto& func : vec) {
        func->NewFunctionState()->ExecuteFunctionLotsAndLotsOfTimes();
    }

    return 0;
}

Answer 1

基于类型删除和 CRTP 的混合解决方案怎么样？
它遵循一个基于问题中的片段的最小的工作示例:

#include <iostream>
#include <vector>

class PrintFunctionState;
class AddFunctionState;
class FunctionState;

class Function {
    template<typename T>
    static FunctionState * InternalNewFunctionState(Function *self, std::vector<FunctionState*> &states) {
        FunctionState* state = new T(self);
        states.push_back(state);
        return state;
    }

public:
    template<typename T>
    static Function * create() {
        Function *func = new Function;
        func->internalNewFunctionState = &InternalNewFunctionState<T>;
        return func;
    }

    FunctionState* NewFunctionState() {
        return internalNewFunctionState(this, states);
    }

private:
    FunctionState * (*internalNewFunctionState)(Function *, std::vector<FunctionState*> &);
    std::vector<FunctionState*> states;
};

class FunctionState {
public:
    FunctionState() = default;
    virtual ~FunctionState() = default;
    virtual void ExecuteFunctionLotsAndLotsOfTimes() = 0;
};

template<typename Derived>
class IntermediateFunctionState: public FunctionState {
public:
    IntermediateFunctionState(Function* func) : mFunc(func) {}

    void ExecuteFunctionLotsAndLotsOfTimes() override {
        Derived *self = static_cast<Derived *>(this);
        for(int i = 0; i < 10; ++i) {
            self->RunState();
        }
    }

private:
    Function* mFunc;
};

class PrintFunctionState : public IntermediateFunctionState<PrintFunctionState> {
public:
    PrintFunctionState(Function* func) : IntermediateFunctionState(func) {}

    void RunState() {
        std::cout << "in print function state" << '\n';
    }
};

class AddFunctionState : public IntermediateFunctionState<AddFunctionState> {
public:
    AddFunctionState(Function* func) : IntermediateFunctionState(func), x(0) {}

    void RunState() {
        std::cout << "in add function state" << '\n';
        ++x;
    }

private:
    int x;
};

int main() {
    Function* func = Function::create<PrintFunctionState>();
    Function* func2 = Function::create<AddFunctionState>();
    std::vector<Function*> vec = { func, func2 };

    for(auto& func : vec) {
        func->NewFunctionState()->ExecuteFunctionLotsAndLotsOfTimes();
    }

    return 0;
}

我删除了几个不再需要的类。
希望代码不言自明，如果我可以添加更多详细信息，请在评论中告诉我。