C++ 通过模版工厂实现简单反射机制

前言

我们知道Java/Python这种语言能够很好得支持反射。反射机制就是一种用户输入的字符串到对应实现方法的映射，比如http接口中用户传入了url，我们需要调用该url对应的方法/函数对象从而做出对应的操作。

而C++ 并没有友好得支持这样的操作，而最近工作中需要通过C++实现http接口，这个过程想要代码实现得优雅一些，就需要实现这样的功能。

最终的实现结果就是:

 int main() {Initial();JudgeName jm;auto res = jm.Judge("Zhang");res->ShowName();return 0;
}

我们只需要通过输入一个字符串，就能得到一个字符串对应的类的对象，从而调用该对象对应的方法。

实现过程

实现模版工厂
反射机制本身是依赖工厂模式实现的，也就是提供一个创建函数对象的工厂，能够创建各种类型的函数对象。
这里面的各种类型 – 其实就需要实现一个通用的模版工厂来达到创建各种类型的目的。

// 工厂类模版，创建各种类型的类对象
template <class OperationType_t>
class OperationFactory {public:// 创建一个工厂单例，只会有一个对象工厂static OperationFactory<OperationType_t>& Instance() {static OperationFactory<OperationType_t> instance;return instance;}private:// We don't allow to constructor, copy constructor and align constructorOperationFactory() = default;~OperationFactory() = default;OperationFactory(const OperationFactory&) = delete;const OperationFactory& operator= (const OperationFactory&) = delete;};

实现类的对象到类名的映射。
我们实现了能够创建所有类对象的函数工厂，但是还需要将创建好的对象和他们的类名字对应起来，做一个映射。
在模版工厂类中补充映射逻辑：

// Factory class template
template <class OperationType_t>
class OperationFactory {public:// Single pattern of the factorystatic OperationFactory<OperationType_t>& Instance() {static OperationFactory<OperationType_t> instance;return instance;}// 类名 和 类对象的映射void RegisterOperation(const std::string& op_name,OperationRegister<OperationType_t>* reg) {operationRegister[op_name] = reg;}// 获取指定类名字 对应的 对象// 也就是从映射map中查找OperationType_t* GetOperation(const std::string& op_name) {if (operationRegister.find(op_name) != operationRegister.end()) {return operationRegister[op_name]->CreateOperation(op_name);}return nullptr;}private:// We don't allow to constructor, copy constructor and align constructorOperationFactory() = default;~OperationFactory() = default;OperationFactory(const OperationFactory&) = delete;const OperationFactory& operator= (const OperationFactory&) = delete;// 类名字 和 类对象之间的映射std::map<std::string, OperationRegister<OperationType_t>* > operationRegister;
};

实现一个注册类
此时我们通过这个注册类的入口将类名和类的对象完整映射起来。

template <class OperationType_t>
class OperationRegister {public:virtual OperationType_t* CreateOperation(const std::string& op_name) = 0;protected:OperationRegister() {}virtual ~OperationRegister() {}
};// 注册一个以基类的子类对象。
template <class OperationType_t, class OperationImpl_t>
class OperationImplRegister : public OperationRegister<OperationType_t> {public:// 获取一个基类的工厂explicit OperationImplRegister(const std::string& op_name) {OperationFactory<OperationType_t>::Instance().RegisterOperation(op_name, this);}// 获取这个基类对应的子类对象OperationType_t* CreateOperation(const std::string& op_name) {return new OperationImpl_t(op_name);}
};

应用

有了上面的模版工厂以及注册类名字和类的映射模版，接下来我们看看怎么使用：

我们实现了如下一个父类以及几个父类对应的子类实现

class Father {public:virtual std::string Name() = 0;virtual void ShowName() = 0;
};class Wang : public Father{public:Wang(const std::string& name) : name_(name){}std::string Name() override { return name_; }void ShowName() override { cout << "Name: " << name_ << endl; }private:std::string name_;
};class Zhang : public Father {public:Zhang(const std::string& name) : name_(name) {}std::string Name() override { return name_; }void ShowName() override { cout << "Name: " << name_ << endl; }private:std::string name_;
};class Li : public Father {public:Li(const std::string& name) : name_(name) {}std::string Name() override { return name_; }void ShowName() override { cout << "Name: " << name_ << endl; }private:std::string name_;
};

接下来需要统一注册一下这几个子类的名字以及他们的对象：

// Construct the string name with there object's map
void Initial() {static bool init = false;if (init == false) {static OperationImplRegister<Father, Wang> wang("Wang");static OperationImplRegister<Father, Zhang> zhang("Zhang");static OperationImplRegister<Father, Li> li("Li");init = true;}
}

最后就是一个通过字符串获取对象

class JudgeName {public:JudgeName(){}Father* Judge(const std::string& name) {OperationFactory<Father>& fac = OperationFactory<Father>::Instance();return fac.GetOperation(name);}
};

这个类的函数中，我们用户只需要传入一个字符串，就能够得到这个字符串的类的对象，从而进行后续的对象方法相关的操作。

能够极大得简化我们的if-else if 这样的分支代码，同时提升了代码的可扩展性。用户只需要继续补充想要使用的类，并将这个类的名字和对象注册到映射map中就好了，后续只需要使用类名就能够创建出这个类的对象。

完整代码实现

#include <iostream>
#include <map>using namespace std;// Register the operation
// The 'OperationTyple_t' is the abstract class
template <class OperationType_t>
class OperationRegister {public:virtual OperationType_t* CreateOperation(const std::string& op_name) = 0;protected:OperationRegister() {}virtual ~OperationRegister() {}
};// Factory class template
template <class OperationType_t>
class OperationFactory {public:// Single pattern of the factorystatic OperationFactory<OperationType_t>& Instance() {static OperationFactory<OperationType_t> instance;return instance;}void RegisterOperation(const std::string& op_name,OperationRegister<OperationType_t>* reg) {operationRegister[op_name] = reg;}OperationType_t* GetOperation(const std::string& op_name) {if (operationRegister.find(op_name) != operationRegister.end()) {return operationRegister[op_name]->CreateOperation(op_name);}return nullptr;}private:// We don't allow to constructor, copy constructor and align constructorOperationFactory() = default;~OperationFactory() = default;OperationFactory(const OperationFactory&) = delete;const OperationFactory& operator= (const OperationFactory&) = delete;std::map<std::string, OperationRegister<OperationType_t>* > operationRegister;
};// An template class to create the detail Operation
template <class OperationType_t, class OperationImpl_t>
class OperationImplRegister : public OperationRegister<OperationType_t> {public:explicit OperationImplRegister(const std::string& op_name) {OperationFactory<OperationType_t>::Instance().RegisterOperation(op_name, this);}OperationType_t* CreateOperation(const std::string& op_name) {return new OperationImpl_t(op_name);}
};class Father {public:virtual std::string Name() = 0;virtual void ShowName() = 0;
};class Wang : public Father{public:Wang(const std::string& name) : name_(name){}std::string Name() override { return name_; }void ShowName() override { cout << "Name: " << name_ << endl; }private:std::string name_;
};class Zhang : public Father {public:Zhang(const std::string& name) : name_(name) {}std::string Name() override { return name_; }void ShowName() override { cout << "Name: " << name_ << endl; }private:std::string name_;
};class Li : public Father {public:Li(const std::string& name) : name_(name) {}std::string Name() override { return name_; }void ShowName() override { cout << "Name: " << name_ << endl; }private:std::string name_;
};// Construct the string name with there object's map
void Initial() {static bool init = false;if (init == false) {static OperationImplRegister<Father, Wang> wang("Wang");static OperationImplRegister<Father, Zhang> zhang("Zhang");static OperationImplRegister<Father, Li> li("Li");init = true;}
}class JudgeName {public:JudgeName(){}Father* Judge(const std::string& name) {OperationFactory<Father>& fac = OperationFactory<Father>::Instance();return fac.GetOperation(name);}
};int main() {Initial();JudgeName jm;auto res = jm.Judge("Zhang");res->ShowName();return 0;
}