代码环境为VScode + CMake + GCC 8.1.0

首先，如何才能做到给我一个名字我就能拿到这个对象的这个值，也是比较难的一个地方，方法如下

#define OFFSET(className,fieldName) (size_t)&(((className*)0)->fieldName)

这个能够得到该成员变量与该对象的偏移量，之后根据这个偏移量来获取成员的值

正题开始

首先这个反射类应该只有一个，要不然反射就会变得很混乱，这一个反射对象，那里一个反射对象。所以应该将该反射类变为一个单例。全局内只允许出现一个。单例类如下：

Singleton.h

#pragma once
#include "Singleton.h"
#ifndef _SINGLETON_
#define _SINGLETON_
#include "Util.h"

NAME_SPACE_START(myUtil)
//单例模式
template<typename T>
class Singleton{
public:
    static T* Instance(){
        if(m_instance==nullptr){
            m_instance = new T();
        }
        return m_instance;
    }
private:
    Singleton();
    Singleton(const Singleton<T>&);
    ~Singleton();
    Singleton<T>& operator=(const Singleton<T>&);

private:
    static T* m_instance;
};

template<typename T>
T* Singleton<T>::m_instance=nullptr;

#define SINGLETON_DECLARE(className)                    \
        friend class Singleton<className>;              \
        className(){};                                  \
        className(const className&){};                  \
        ~className(){};                                 \
        className& operator=(const className&);

NAME_SPACE_END()
#endif //!_SINGLETON_

我在这个头文件中写了一个单例声明SINGLETON_DECLARE，只要将这个声明放到私有部分就行了，这个类就变为一个单例类了。

反射类

如何才能做到反射呢，应该在这个反射类中保存注册表，传入了对应的类名，返回对应的信息，然后使用基类实现对应的方法即可。代码如下：

Reflex.h

#pragma once
#include "Util.h"
#include <cstdint>
#include <vadefs.h>
#include <vector>
#include <functional>
#ifndef _REFLEX_
#define _REFLEX_

NAME_SPACE_START(myUtil)
#include "Singleton.h"
#include <map>
#include <string>

//因为编译器不支持类模板和实现分开写，所以放到一起了
class Field;
class Reflex;
class RObject{
public:
    RObject(){}
    virtual ~RObject(){}
    std::string _className;

    template<typename T>
    T get(const std::string& fieldName);

    template<typename T>
    void set(const std::string& fieldName, const T& fieldValue);

    void Call(const std::string& methodName);
    template<typename T,typename... Args>
    T Call(const std::string& methodName, Args... args);
};

typedef RObject* (*construct)(void);

//使用方法,使用REGISTER_REFLEX注册，然后直接使用createClass即可
class Reflex{
    SINGLETON_DECLARE(Reflex)
public:
    void ReflexRegister();
    RObject* createClass(const std::string& className);
    void RegisterClass(const std::string& className, construct constructMethod);

    void RegisterField(const std::string& className, const std::string& FieldName, const std::string& FieldType, const size_t& offset);
    void RegisterMethod(const std::string& className, const std::string& methodName, const uintptr_t& lpMethod);
    template<typename T>
    T getClassField(void* originPos, const std::string& className, const std::string& fieldName);
    template<typename T>
    void setClassField(void* originPos, const std::string& className, const std::string& fieldName, const T& fieldValue);

    uintptr_t getClassMethod(const std::string& className, const std::string& methodName);
private:
    std::map<std::string, construct> m_classMap;
    std::map<std::string, std::map<std::string, Field>> m_fieldMap;
    std::map<std::string, std::map<std::string, uintptr_t>> m_methodMap;
};

//仅仅用来在reflex中注册使用
class RegisterClass{
public:
    RegisterClass(const std::string& className,construct constructMethod)
    {
        Reflex* factory = myUtil::Singleton<Reflex>::Instance();
        factory->RegisterClass(className, constructMethod);
    }
    RegisterClass(const std::string& className,const std::string& fieldName,const std::string& fieldType,const size_t& offset)
    {
        Reflex* factory = myUtil::Singleton<Reflex>::Instance();
        factory->RegisterField(className, fieldName, fieldType, offset);
    }
    RegisterClass(const std::string& className,const std::string& methodName,const uintptr_t& lpMethod)
    {
        Reflex* factory = myUtil::Singleton<Reflex>::Instance();
        factory->RegisterMethod(className, methodName, lpMethod);
    }
};

class Field{
private:
    std::string m_fieldType{""};
    //std::string m_fieldName{""};
    size_t m_offset{0};
public:
    Field(){}
    Field(const std::string& fieldType,const size_t& offset):
        m_fieldType(fieldType),m_offset(offset){}
    ~Field(){}
    
    inline std::string getFieldType(){return m_fieldType;}
    //inline std::string getFieldName(){return m_fieldName;}
    inline size_t getOffSet(){return m_offset;}

    inline void setFieldType(const std::string& type){ m_fieldType = type;}
    //inline void setFieldName(const std::string& name){ m_fieldName = name;}
    inline void setOffSet(const size_t& offset){ m_offset = offset;}
};

class Method{
public:
    Method(){}
    Method(const std::string& name,const std::uintptr_t& method):
        methodName(name),lpMethod(method){}
    ~Method(){}
    inline void setMethodName(const std::string& name) { methodName = name;}
    inline void setLpMethod(const uintptr_t& lp) { lpMethod = lp;}
    inline std::string getMethodName(){return methodName;}
    inline uintptr_t getLpMethod(){return lpMethod;}
private:
    std::string methodName{""};
    std::uintptr_t lpMethod{0};
};


#define REGISTER_REFLEX(className)                                      \
        RObject* construct##className()                                 \
        {                                                               \
            RObject* obj = new className();                             \
            obj->_className = #className;                               \
            return obj;                                                 \
        }                                                               \
        RegisterClass registerClass##className(#className,construct##className);

#define REGISTER_REFLEX_FIELD(className,fieldType,fieldName)            \
        RegisterClass registerClass##className##fieldType##fieldName(#className,#fieldName,#fieldType,OFFSET(className,fieldName));

#define REGISTER_REFLEX_METHOD(className,methodName)                    \
        std::function<void(className)> className##methodName = &className::methodName;\
        RegisterClass registerClass##className##method(#className,#methodName,(std::uintptr_t)&className##methodName);

#define REGISTER_REFLEX_METHOD_ARGS(className,methodName,returnType,...)                    \
        std::function<returnType(className,##__VA_ARGS__)> className##methodName = &className::methodName;\
        RegisterClass registerClass##className##method##returnType(#className,#methodName,(std::uintptr_t)&className##methodName);

template<typename T>
T RObject::get(const std::string &fieldName)
{
    Reflex* factory = myUtil::Singleton<Reflex>::Instance();
    return factory->getClassField<T>(this, _className, fieldName);
}

template<typename T>
void RObject::set(const std::string &fieldName, const T &fieldValue)
{
    Reflex* factory = myUtil::Singleton<Reflex>::Instance();
    factory->setClassField(this, _className, fieldName, fieldValue);
}

void RObject::Call(const std::string& methodName)
{
    Reflex* factory = myUtil::Singleton<Reflex>::Instance();
    std::uintptr_t temp = factory->getClassMethod(_className, methodName);
    if (temp == 0) return;
    typedef std::function<void(decltype(this))> class_method;
    class_method* method = (class_method*)temp;
    (*method)(this);
}

template<typename T,typename... Args>
T RObject::Call(const std::string& methodName,Args... args)
{
    Reflex* factory = myUtil::Singleton<Reflex>::Instance();
    std::uintptr_t temp = factory->getClassMethod(_className, methodName);
    if(temp == 0) return T();
    typedef std::function<T(decltype(this),Args...)> class_method;
    class_method* method = (class_method*)temp;
    return (*method)(this,args...);
}

RObject* Reflex::createClass(const std::string &className)
{
    if(m_classMap.find(className)==m_classMap.end()) return nullptr;
    return m_classMap[className]();
}

void Reflex::RegisterClass(const std::string &className, construct constructMethod)
{
    if(m_classMap.find(className)!=m_classMap.end()){
        throw std::exception();
        return;
    }
    m_classMap.insert(std::pair<std::string, construct>(className,constructMethod));
    m_fieldMap[className] = std::map<std::string, Field>();
    m_methodMap[className] = std::map<std::string, uintptr_t>();
}

void Reflex::RegisterField(const std::string &className, const std::string &FieldName, const std::string &FieldType, const size_t &offset)
{
    m_fieldMap[className][FieldName] = Field(FieldType,offset);
}

void Reflex::RegisterMethod(const std::string &className, const std::string &methodName, const uintptr_t &lpMethod)
{
    m_methodMap[className][methodName] = lpMethod;
}

template<typename T>
T Reflex::getClassField(void* originPos, const std::string &className, const std::string &fieldName)
{
    if(m_fieldMap.find(className) == m_fieldMap.end()){
        return T();
    }
    if(m_fieldMap[className].find(fieldName) == m_fieldMap[className].end()){
        return T();
    }
    size_t offset = m_fieldMap[className][fieldName].getOffSet();
    return *(T*)((size_t)originPos + offset);
}

template<typename T>
void Reflex::setClassField(void* originPos, const std::string &className, const std::string &fieldName, const T &fieldValue)
{
    if(m_fieldMap.find(className) == m_fieldMap.end()){
        return;
    }
    if(m_fieldMap[className].find(fieldName) == m_fieldMap[className].end()){
        return;
    }
    size_t offset = m_fieldMap[className][fieldName].getOffSet();
    *(T*)((size_t)originPos + offset) = fieldValue;
}

uintptr_t Reflex::getClassMethod(const std::string &className, const std::string &methodName)
{
    if(m_fieldMap.find(className) == m_fieldMap.end()){
        return 0;
    }
    if(m_methodMap[className].find(methodName) == m_methodMap[className].end()){
        return 0;
    }
    return m_methodMap[className][methodName];
}

NAME_SPACE_END()
#endif //!_REFLEX_

该反射类使用方法如下：

#include <iostream>
#include <string>
#include "Util.h"
#include "Singleton.h"
#include "Reflex.h"
using namespace std;
using namespace myUtil;

class A:public RObject{
public:
    void show(){
        cout<<"hello world"<<endl;
    }
    int add(int a,int b){
        return a+b;
    }
    int m_age;
    A():m_age(10){}
};

REGISTER_REFLEX(A)
REGISTER_REFLEX_FIELD(A, int, m_age)
REGISTER_REFLEX_METHOD(A, show)
REGISTER_REFLEX_METHOD_ARGS(A, add, int,int,int)

int main(){
    Reflex* factory=Singleton<Reflex>::Instance();
    A* a=(A*)factory->createClass("A");
    cout<<a->get<int>("m_age")<<endl;
    a->set<int>("m_age", 30);
    cout << a->get<int>("m_age") << endl;
    a->Call("show");
    int b = a->Call<int,int,int>("add",1,5);
    cout << b << endl;
    A* c=(A*)factory->createClass("A");
    cout<<c->get<int>("m_age")<<endl;
    c->set<int>("m_age", 40);
    cout << c->get<int>("m_age") << endl;
    c->Call("show");
    b = c->Call<int,int,int>("add",2,5);
    cout << b << endl;
    return 0;
}

结果截图

最后讲解一下是怎么用的，见注释

//首先要使用反射的类要继承RObject
//要使用反射的类和成员方法都要声明为public
class A:public RObject{
public:
    void show(){
        cout<<"hello world"<<endl;
    }
    int add(int a,int b){
        return a+b;
    }
    int m_age;
    A():m_age(10){}
};
//这里在反射类中注册A这个类，原理是把重复工作用宏展开来替代
REGISTER_REFLEX(A)
//注册类中的成员变量，一定要先注册类再注册成员变量，原理是将成员变量与对象的偏移量保存起来，用到的时候解引用来获取值
REGISTER_REFLEX_FIELD(A, int, m_age)
//注册类的成员方法，此宏是声明没有返回值和入参的成员方法的，原理是使用function能够调用成员函数的功能，将function的地址保存到注册表中（转为uintptr_t），需要时通过传入的参数转换回来，再调用
REGISTER_REFLEX_METHOD(A, show)
//注册类的成员方法，此宏是声明有返回值和多参数的成员方法的，原理同上
REGISTER_REFLEX_METHOD_ARGS(A, add, int,int,int)

int main(){
	//在使用类时，要先获取这个全局唯一的反射对象，使用它来创建对象
    Reflex* factory=Singleton<Reflex>::Instance();
    A* a=(A*)factory->createClass("A");
    //为了能够得到准确的类型值，这里使用模板来获取
    cout<<a->get<int>("m_age")<<endl;
    //设置同获取
    a->set<int>("m_age", 30);
    cout << a->get<int>("m_age") << endl;
    //调用无参且无返回值的成员函数时使用没有模板的Call，反之使用有模板的Call
    a->Call("show");
    int b = a->Call<int,int,int>("add",1,5);
    cout << b << endl;
    return 0;
}