c++ - 如何通过映射从派生类实例化对象

Question

我有一个问题，关于如何通过映射对识别对象，实例化用该对识别的类型的对象，然后将它存储在某种容器(可能是 vector )中。这里的问题是，我正在寻找的对象应该都是某个基类的派生类。

这是一个例子:

class BaseClass {
public:
    BaseClass() {cout << "BaseClass constructor...\n"};
    ~BaseClass() {cout << "BaseClass destructor...\n"};
};


class A : public BaseClass {
public:
    A() {cout << "A constructor...\n"};
    ~A() {cout << "A destructor...\n"};
};


class B : public BaseClass {
public:
    B() {cout << "B constructor...\n"};
    ~B() {cout << "B destructor...\n"};
};


class C : public BaseClass {
public:
    C() {cout << "C constructor...\n"};
    ~C() {cout << "C destructor...\n"};
};


int main(int argc, char *argv[])
{
    map <string, BaseClass*> my_map; // Map used to compare a string in order to identify the object type I'd like to make

    vector<BaseClass*> keyword_vct; // Vector to store the objects of the different derived class types

    BaseClass* ptr;
    my_map.insert (make_pair ("A", ptr = new A ));
    my_map.insert (make_pair ("B", ptr = new B));
    my_map.insert (make_pair ("C", ptr = new C));

    string testStr "B";

    map <string, BaseClass*> ::const_iterator it = my_map.find(testStr);
    if (it == oscards_map.end()) {
        cout << "String not found in map." << endl;
    }
    else {
        cout << it->first << "\t keyword found in map!" << endl;
        BaseClass* pSomekey;
        pSomekey = it->second; // This is where I'm lost

        keyword_vct.push_back(pSomekey); // Once I instantiate the derived object in the line above, I want to store it in a container.
    }
}   

所以我的主要问题是:

1. 如何将 pSomekey 变成 A、B 或 C 类型的对象>?

2. 如果我能够实例化其中一个派生类，我是否能够将这些不同类型的对象存储到同一个 vector 中，因为它们是 BaseClass 的派生类？

我注意到，当我为 map 制作对时，它们似乎在各自的派生类中构造了一个对象。

我还注意到，当执行 pSomekey = it->second; 时，没有构造任何对象。

请记住，这只是一个示例。在我的真实代码中，我将比较数百个 testStr 以生成数百个不同的对象。

如有任何帮助或建议，我们将不胜感激。谢谢!

Answer 1

1. 在基类中添加一个虚成员函数来复制对象。
2. 在派生类中适本地实现它们。
3. 在需要时调用复制函数。

这是您的代码，已在正确的位置更新。

class BaseClass
    {
    public:
        BaseClass() {cout << "BaseClass constructor...\n"};
        ~BaseClass() {cout << "BaseClass destructor...\n"};
        virtual BaseClass* clone() const = 0;
    };


class A : public BaseClass
    {
    public:
        A() {cout << "A constructor...\n"};
        ~A() {cout << "A destructor...\n"};
        virtual A* clone() const { return new A();}
    };


class B : public BaseClass
    {
    public:
        B() {cout << "B constructor...\n"};
        ~B() {cout << "B destructor...\n"};
        virtual B* clone() const { return new B();}
    };


class C : public BaseClass
    {
    public:
        C() {cout << "C constructor...\n"};
        ~C() {cout << "C destructor...\n"};
        virtual C* clone() const { return new C();}
    };


int main(int argc, char *argv[])
    {

    map <string, BaseClass*> my_map; // Map used to compare a string in order to identify the object type I'd like to make

    vector<BaseClass*> keyword_vct; // Vector to store the objects of the different derived class types

    BaseClass* ptr;
    my_map.insert (make_pair ("A", ptr = new A ));
    my_map.insert (make_pair ("B", ptr = new B));
    my_map.insert (make_pair ("C", ptr = new C));

    string testStr "B";

    map <string, BaseClass*> ::const_iterator it = my_map.find(testStr);
    if (it == oscards_map.end()) {
        cout << "String not found in map." << endl;
        } else {
        cout << it->first << "\t keyword found in map!" << endl;
        BaseClass* pSomekey;
        pSomekey = it->second;

        // Make a copy of the object and store it in keyword_vct.
        keyword_vct.push_back(pSomekey->clone());
        }

    }