是否有一种方法可以从保存其类名的字符串实例化对象?

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>;

然后你就可以做了

return map[some_string]();

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");

您可以决定为注册创建宏

#define REGISTER_DEC_TYPE(NAME) \
    static DerivedRegister<NAME> reg

#define REGISTER_DEF_TYPE(NAME) \
    DerivedRegister<NAME> NAME::reg(#NAME)

不过我相信这两个人还有更好的名字。在这里使用另一个可能有意义的东西是 shared_ptr

如果有一组没有公共基类的不相关类型,则可以为函数指针提供 boost::variant<A, B, C, D, ...>

typedef boost::variant<Foo, Bar, Baz> variant_type;
template<typename T> variant_type createInstance() { 
    return variant_type(T()); 
}

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

A. boost::variant 就像一个联盟。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 here . 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。你可能想调查一下 Boost.Function 寻找更好的方法。它看起来是这样的(地图):

typedef std::map<std::string, boost::function<variant_type()> > map_type;

std::function 将在下一版本的C++中也可用,包括 std::shared_ptr .

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。本节将对此进行详细讨论 GoF patterns book .

1. Why does C++ not have reflection?
2. How can I add reflection to a C++ application?

我在另一个关于C++工厂的问题上回答了问题。请看 there 如果一个灵活的工厂感兴趣。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。

ET++ 是一个将旧Mac App移植到C++和X11的项目。在它的努力下,Eric Gamma等开始思考 设计模式

functional有一个非常灵活的工厂模板: http://www.boost.org/doc/libs/1_54_0/libs/functional/factory/doc/html/index.html

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

我通常反对大量使用宏,但我在这里破例。上面的代码为0和泛型工厂最大数量(含)之间的每个N生成名为GenericFactory\u N的类的泛型工厂最大数量+1个版本。

使用生成的类模板很容易。假设您希望工厂使用字符串映射创建基类派生对象。每个派生对象都采用3个整数作为构造函数参数。

#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);

GenericFactory\N类析构函数是虚拟的,允许执行以下操作。

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;

请注意,这一行是通用factory generator宏的

#define BOOST_PP_FILENAME_1 "GenericFactory.hpp"

注册对象并使用字符串名称访问它们的详细解决方案。

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 :

/*
 * 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;
}

编译并运行它(使用Eclipse完成此操作)

输出:

Base constructor
test1 constructor m_a=10m_b=20
Base constructor
test1 constructor m_a=30m_b=40

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。

http://arcticinteractive.com/static/boost/libs/factory/doc/html/factory/factory.html#factory.factory.reference

对于那些质疑为什么这样的事情(如反射)对C++有用的人来说,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。

在Java中表示反射。 http://msdn.microsoft.com/en-us/library/y0114hz2(VS.80).aspx

一般来说,在谷歌搜索“c++反射”

这是工厂模式。参见维基百科(和 this 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?