C++ 是否有一种方法可以从保存其类名的字符串实例化对象？

我有一个文件：Base.h

class Base;
class DerivedA : public Base;
class DerivedB : public Base;

/*etc...*/

还有另一个文件：BaseFactory.h

#include "Base.h"

class BaseFactory
{
public:
  BaseFactory(const string &sClassName){msClassName = sClassName;};

  Base * Create()
  {
    if(msClassName == "DerivedA")
    {
      return new DerivedA();
    }
    else if(msClassName == "DerivedB")
    {
      return new DerivedB();
    }
    else if(/*etc...*/)
    {
      /*etc...*/
    }
  };
private:
  string msClassName;
};

/*etc.*/

有没有办法将这个字符串转换成实际的类型（类），这样BaseFactory就不必知道所有可能的派生类，并且每个派生类都有if（）？我可以从这个字符串生成一个类吗

---

我认为这可以通过C#反射来实现。C++中有类似的东西吗？

< p>意义反射，如java。 这里有一些信息：

---

一般来说，在谷歌上搜索“c++反射”

不，没有。我对这个问题的首选解决方案是创建一个字典，将名称映射到创建方法。希望像这样创建的类然后向字典注册一个创建方法。这将在中详细讨论。

这是出厂模式。参见维基百科（和示例）。如果没有一些惊人的技巧，就无法从字符串创建类型本身。你为什么需要这个？

< p>我在另一个关于C++工厂的问题上回答了这个问题。请看是否有兴趣建立一个灵活的工厂。我试图描述一种从ET++到使用宏的老方法，这种方法对我来说非常有效

---

是将旧Mac App移植到C++和X11的项目。在it的努力中，Eric Gamma等人开始思考设计模式，简单的回答是你不能。请参见以下问题了解原因：

---

没有，没有，除非你自己做映射。C++没有机制来创建在运行时确定类型的对象。不过，您可以使用地图自己进行映射：

template<typename T> Base * createInstance() { return new T; }

typedef std::map<std::string, Base*(*)()> map_type;

map_type map;
map["DerivedA"] = &createInstance<DerivedA>;
map["DerivedB"] = &createInstance<DerivedB>;

获取一个新实例。另一个想法是让类型自己注册：

// in base.hpp:
template<typename T> Base * createT() { return new T; }

struct BaseFactory {
    typedef std::map<std::string, Base*(*)()> map_type;

    static Base * createInstance(std::string const& s) {
        map_type::iterator it = getMap()->find(s);
        if(it == getMap()->end())
            return 0;
        return it->second();
    }

protected:
    static map_type * getMap() {
        // never delete'ed. (exist until program termination)
        // because we can't guarantee correct destruction order 
        if(!map) { map = new map_type; } 
        return map; 
    }

private:
    static map_type * map;
};

template<typename T>
struct DerivedRegister : BaseFactory { 
    DerivedRegister(std::string const& s) { 
        getMap()->insert(std::make_pair(s, &createT<T>));
    }
};

// in derivedb.hpp
class DerivedB {
    ...;
private:
    static DerivedRegister<DerivedB> reg;
};

// in derivedb.cpp:
DerivedRegister<DerivedB> DerivedB::reg("DerivedB");

一个

boost:：variant

就像一个联合体。它通过查看用于初始化或分配给它的对象来知道存储在其中的类型。看看它的文档。最后，原始函数指针的使用也有点陈旧。现代C++代码应该与特定的函数/类型进行解耦。你可能想寻找更好的方法。它看起来是这样的（地图）：

typedef std:：map\u type；

---

<>代码> STD:：函数也将在下一个版本的C++中可用，包括<代码> STD:：SyrdYPPTR < /代码> . Tor Brede Vekterli提供了一个增强扩展，它提供了您想要的功能。目前，使用当前的boostlibs有点不方便，但在更改了1.48_0的基本名称空间后，我能够让它使用1.48_0

回答那些质疑为什么这样的事情（如反射）对C++有用的人——我用它来处理UI和引擎之间的交互——用户在UI中选择一个选项，引擎取UI选择字符串，并产生一个所需类型的对象。 在这里使用框架（而不是在某处维护水果列表）的主要好处是，注册函数位于每个类的定义中（每个注册类只需要一行代码调用注册函数），而不是包含水果列表的文件，每次派生新类时都必须手动将其添加到

---

我使工厂成为基类的静态成员。

boost:：functional有一个非常灵活的工厂模板：

不过，我的首选是生成隐藏映射和对象创建机制的包装器类。我遇到的常见场景是需要将某些基类的不同派生类映射到键，其中派生类都有一个可用的公共构造函数签名。这是我到目前为止提出的解决方案

#ifndef GENERIC_FACTORY_HPP_INCLUDED

//BOOST_PP_IS_ITERATING is defined when we are iterating over this header file.
#ifndef BOOST_PP_IS_ITERATING

    //Included headers.
    #include <unordered_map>
    #include <functional>
    #include <boost/preprocessor/iteration/iterate.hpp>
    #include <boost/preprocessor/repetition.hpp>

    //The GENERIC_FACTORY_MAX_ARITY directive controls the number of factory classes which will be generated.
    #ifndef GENERIC_FACTORY_MAX_ARITY
        #define GENERIC_FACTORY_MAX_ARITY 10
    #endif

    //This macro magic generates GENERIC_FACTORY_MAX_ARITY + 1 versions of the GenericFactory class.
    //Each class generated will have a suffix of the number of parameters taken by the derived type constructors.
    #define BOOST_PP_FILENAME_1 "GenericFactory.hpp"
    #define BOOST_PP_ITERATION_LIMITS (0,GENERIC_FACTORY_MAX_ARITY)
    #include BOOST_PP_ITERATE()

    #define GENERIC_FACTORY_HPP_INCLUDED

#else

    #define N BOOST_PP_ITERATION() //This is the Nth iteration of the header file.
    #define GENERIC_FACTORY_APPEND_PLACEHOLDER(z, current, last) BOOST_PP_COMMA() BOOST_PP_CAT(std::placeholders::_, BOOST_PP_ADD(current, 1))

    //This is the class which we are generating multiple times
    template <class KeyType, class BasePointerType BOOST_PP_ENUM_TRAILING_PARAMS(N, typename T)>
    class BOOST_PP_CAT(GenericFactory_, N)
    {
        public:
            typedef BasePointerType result_type;

        public:
            virtual ~BOOST_PP_CAT(GenericFactory_, N)() {}

            //Registers a derived type against a particular key.
            template <class DerivedType>
            void Register(const KeyType& key)
            {
                m_creatorMap[key] = std::bind(&BOOST_PP_CAT(GenericFactory_, N)::CreateImpl<DerivedType>, this BOOST_PP_REPEAT(N, GENERIC_FACTORY_APPEND_PLACEHOLDER, N));
            }

            //Deregisters an existing registration.
            bool Deregister(const KeyType& key)
            {
                return (m_creatorMap.erase(key) == 1);
            }

            //Returns true if the key is registered in this factory, false otherwise.
            bool IsCreatable(const KeyType& key) const
            {
                return (m_creatorMap.count(key) != 0);
            }

            //Creates the derived type associated with key. Throws std::out_of_range if key not found.
            BasePointerType Create(const KeyType& key BOOST_PP_ENUM_TRAILING_BINARY_PARAMS(N,const T,& a)) const
            {
                return m_creatorMap.at(key)(BOOST_PP_ENUM_PARAMS(N,a));
            }

        private:
            //This method performs the creation of the derived type object on the heap.
            template <class DerivedType>
            BasePointerType CreateImpl(BOOST_PP_ENUM_BINARY_PARAMS(N,const T,& a))
            {
                BasePointerType pNewObject(new DerivedType(BOOST_PP_ENUM_PARAMS(N,a)));
                return pNewObject;
            }

        private:
            typedef std::function<BasePointerType (BOOST_PP_ENUM_BINARY_PARAMS(N,const T,& BOOST_PP_INTERCEPT))> CreatorFuncType;
            typedef std::unordered_map<KeyType, CreatorFuncType> CreatorMapType;
            CreatorMapType m_creatorMap;
    };

    #undef N
    #undef GENERIC_FACTORY_APPEND_PLACEHOLDER

#endif // defined(BOOST_PP_IS_ITERATING)
#endif // include guard

---

假定通用工厂头文件名为GenericFactory.hpp

详细解决方案，用于注册对象并使用字符串名称访问它们

common.h

：

#ifndef COMMON_H_
#define COMMON_H_


#include<iostream>
#include<string>
#include<iomanip>
#include<map>

using namespace std;
class Base{
public:
    Base(){cout <<"Base constructor\n";}
    virtual ~Base(){cout <<"Base destructor\n";}
};
#endif /* COMMON_H_ */

/*
 * test1.h
 *
 *  Created on: 28-Dec-2015
 *      Author: ravi.prasad
 */

#ifndef TEST1_H_
#define TEST1_H_
#include "common.h"

class test1: public Base{
    int m_a;
    int m_b;
public:
    test1(int a=0, int b=0):m_a(a),m_b(b)
    {
        cout <<"test1 constructor m_a="<<m_a<<"m_b="<<m_b<<endl;
    }
    virtual ~test1(){cout <<"test1 destructor\n";}
};



#endif /* TEST1_H_ */

3. test2.h
#ifndef TEST2_H_
#define TEST2_H_
#include "common.h"

class test2: public Base{
    int m_a;
    int m_b;
public:
    test2(int a=0, int b=0):m_a(a),m_b(b)
    {
        cout <<"test1 constructor m_a="<<m_a<<"m_b="<<m_b<<endl;
    }
    virtual ~test2(){cout <<"test2 destructor\n";}
};


#endif /* TEST2_H_ */

main.cpp

：

#include "test1.h"
#include "test2.h"

template<typename T> Base * createInstance(int a, int b) { return new T(a,b); }

typedef std::map<std::string, Base* (*)(int,int)> map_type;

map_type mymap;

int main()
{

    mymap["test1"] = &createInstance<test1>;
    mymap["test2"] = &createInstance<test2>;

     /*for (map_type::iterator it=mymap.begin(); it!=mymap.end(); ++it)
        std::cout << it->first << " => " << it->second(10,20) << '\n';*/

    Base *b = mymap["test1"](10,20);
    Base *b2 = mymap["test2"](30,40);

    return 0;
}

---

我需要这个，因为我从文件中读取字符串，如果我有这个，那么我可以让工厂非常通用，它不需要知道任何东西来创建正确的实例。这是非常强大的。那么，你是说你不需要为公共汽车和汽车定义不同的类，因为它们都是汽车吗？但是，如果您这样做，添加另一行应该不是问题：）映射方法也有同样的问题——您更新映射内容。宏thingy适用于普通类。我是说，在我的例子中，为了创建一辆公共汽车或汽车，我不需要不同的定义，否则工厂设计模式将永远不会被使用。我的目标是让工厂尽可能地愚蠢。但是我在这里看到，没有什么可以逃避的：-）您所提到的页面上的内容与标准c++非常不同，我们喜欢派生类将自己注册的想法。这正是我所寻找的，一种从工厂中删除派生类存在的硬编码知识的方法。最初由somedave在另一个问题中发布，该代码在VS2010上失败，由于make_pair导致模棱两可的模板错误。若要修复，请将make_pair更改为std:：pair，它应该修复这些错误。我还得到了一些链接错误，通过添加BaseFactory:：map_type BaseFactory:：map=new map_type（）修复了这些错误；到base.cpp如何确保

DerivedB:：reg

已实际初始化？我的理解是，根据3.6.2.1，如果翻译单元

derivedb.cpp

中未定义任何功能或对象，则可能根本无法构建自注册。要编译，我需要一个

BaseFactory:：map\u类型*BaseFactory:：map=NULL在我的cpp文件中。没有这个，链接器抱怨未知的符号映射。不幸的是，这不起作用。正如musiphil已经指出的那样，
DerivedB:：reg
如果在翻译单元
DerivedB.cpp
中未定义其任何函数或实例，则不会初始化。这意味着类在实际实例化之前不会被注册。有人知道吗
#ifndef GENERIC_FACTORY_HPP_INCLUDED

//BOOST_PP_IS_ITERATING is defined when we are iterating over this header file.
#ifndef BOOST_PP_IS_ITERATING

    //Included headers.
    #include <unordered_map>
    #include <functional>
    #include <boost/preprocessor/iteration/iterate.hpp>
    #include <boost/preprocessor/repetition.hpp>

    //The GENERIC_FACTORY_MAX_ARITY directive controls the number of factory classes which will be generated.
    #ifndef GENERIC_FACTORY_MAX_ARITY
        #define GENERIC_FACTORY_MAX_ARITY 10
    #endif

    //This macro magic generates GENERIC_FACTORY_MAX_ARITY + 1 versions of the GenericFactory class.
    //Each class generated will have a suffix of the number of parameters taken by the derived type constructors.
    #define BOOST_PP_FILENAME_1 "GenericFactory.hpp"
    #define BOOST_PP_ITERATION_LIMITS (0,GENERIC_FACTORY_MAX_ARITY)
    #include BOOST_PP_ITERATE()

    #define GENERIC_FACTORY_HPP_INCLUDED

#else

    #define N BOOST_PP_ITERATION() //This is the Nth iteration of the header file.
    #define GENERIC_FACTORY_APPEND_PLACEHOLDER(z, current, last) BOOST_PP_COMMA() BOOST_PP_CAT(std::placeholders::_, BOOST_PP_ADD(current, 1))

    //This is the class which we are generating multiple times
    template <class KeyType, class BasePointerType BOOST_PP_ENUM_TRAILING_PARAMS(N, typename T)>
    class BOOST_PP_CAT(GenericFactory_, N)
    {
        public:
            typedef BasePointerType result_type;

        public:
            virtual ~BOOST_PP_CAT(GenericFactory_, N)() {}

            //Registers a derived type against a particular key.
            template <class DerivedType>
            void Register(const KeyType& key)
            {
                m_creatorMap[key] = std::bind(&BOOST_PP_CAT(GenericFactory_, N)::CreateImpl<DerivedType>, this BOOST_PP_REPEAT(N, GENERIC_FACTORY_APPEND_PLACEHOLDER, N));
            }

            //Deregisters an existing registration.
            bool Deregister(const KeyType& key)
            {
                return (m_creatorMap.erase(key) == 1);
            }

            //Returns true if the key is registered in this factory, false otherwise.
            bool IsCreatable(const KeyType& key) const
            {
                return (m_creatorMap.count(key) != 0);
            }

            //Creates the derived type associated with key. Throws std::out_of_range if key not found.
            BasePointerType Create(const KeyType& key BOOST_PP_ENUM_TRAILING_BINARY_PARAMS(N,const T,& a)) const
            {
                return m_creatorMap.at(key)(BOOST_PP_ENUM_PARAMS(N,a));
            }

        private:
            //This method performs the creation of the derived type object on the heap.
            template <class DerivedType>
            BasePointerType CreateImpl(BOOST_PP_ENUM_BINARY_PARAMS(N,const T,& a))
            {
                BasePointerType pNewObject(new DerivedType(BOOST_PP_ENUM_PARAMS(N,a)));
                return pNewObject;
            }

        private:
            typedef std::function<BasePointerType (BOOST_PP_ENUM_BINARY_PARAMS(N,const T,& BOOST_PP_INTERCEPT))> CreatorFuncType;
            typedef std::unordered_map<KeyType, CreatorFuncType> CreatorMapType;
            CreatorMapType m_creatorMap;
    };

    #undef N
    #undef GENERIC_FACTORY_APPEND_PLACEHOLDER

#endif // defined(BOOST_PP_IS_ITERATING)
#endif // include guard

#include "GenericFactory.hpp"

typedef GenericFactory_3<std::string, std::shared_ptr<BaseClass>, int, int int> factory_type;

factory_type factory;
factory.Register<DerivedClass1>("DerivedType1");
factory.Register<DerivedClass2>("DerivedType2");
factory.Register<DerivedClass3>("DerivedType3");

factory_type::result_type someNewObject1 = factory.Create("DerivedType2", 1, 2, 3);
factory_type::result_type someNewObject2 = factory.Create("DerivedType1", 4, 5, 6);

class SomeBaseFactory : public GenericFactory_2<int, BaseType*, std::string, bool>
{
    public:
        SomeBaseFactory() : GenericFactory_2()
        {
            Register<SomeDerived1>(1);
            Register<SomeDerived2>(2);
        }
}; 

SomeBaseFactory factory;
SomeBaseFactory::result_type someObject = factory.Create(1, "Hi", true);
delete someObject;

#define BOOST_PP_FILENAME_1 "GenericFactory.hpp"

#ifndef COMMON_H_
#define COMMON_H_


#include<iostream>
#include<string>
#include<iomanip>
#include<map>

using namespace std;
class Base{
public:
    Base(){cout <<"Base constructor\n";}
    virtual ~Base(){cout <<"Base destructor\n";}
};
#endif /* COMMON_H_ */

/*
 * test1.h
 *
 *  Created on: 28-Dec-2015
 *      Author: ravi.prasad
 */

#ifndef TEST1_H_
#define TEST1_H_
#include "common.h"

class test1: public Base{
    int m_a;
    int m_b;
public:
    test1(int a=0, int b=0):m_a(a),m_b(b)
    {
        cout <<"test1 constructor m_a="<<m_a<<"m_b="<<m_b<<endl;
    }
    virtual ~test1(){cout <<"test1 destructor\n";}
};



#endif /* TEST1_H_ */

3. test2.h
#ifndef TEST2_H_
#define TEST2_H_
#include "common.h"

class test2: public Base{
    int m_a;
    int m_b;
public:
    test2(int a=0, int b=0):m_a(a),m_b(b)
    {
        cout <<"test1 constructor m_a="<<m_a<<"m_b="<<m_b<<endl;
    }
    virtual ~test2(){cout <<"test2 destructor\n";}
};


#endif /* TEST2_H_ */

#include "test1.h"
#include "test2.h"

template<typename T> Base * createInstance(int a, int b) { return new T(a,b); }

typedef std::map<std::string, Base* (*)(int,int)> map_type;

map_type mymap;

int main()
{

    mymap["test1"] = &createInstance<test1>;
    mymap["test2"] = &createInstance<test2>;

     /*for (map_type::iterator it=mymap.begin(); it!=mymap.end(); ++it)
        std::cout << it->first << " => " << it->second(10,20) << '\n';*/

    Base *b = mymap["test1"](10,20);
    Base *b2 = mymap["test2"](30,40);

    return 0;
}