在运行时从没有虚拟函数的基类方法访问派生类方法

问题描述

|| 我有以下代码示例。一个基类和两个派生类具有各自的功能（分别为function1和function2）。在基类中，function1和function2都不是虚拟的。我无法更改此设置，因为这些类已经实现。

#include <iostream>    
class Base
{
public:
    Base(){}
    Base(int val) : m_base(val){}
    virtual ~Base(){}
    //base class methods
private:
    int m_base;
};

class Derived1 : public Base
{
public:
    Derived1(int val) : m_derived1(val){}
    ~Derived1(){}
    void print1(){std::cout << \"Using Derived 1 class method\" << std::endl;};
private:
    int m_derived1;
};

class Derived2 : public Base
{
public:
    Derived2(int val) : m_derived2(val){}
    ~Derived2(){}
    void print2(){std::cout << \"Using Derived 2 class method\" << std::endl;};
private:
    int m_derived2;
};

我正在努力实现以下目标。我想在运行时决定要使用哪个派生类。 此外，我想仅通过使用对象b从基类方法中调用它们。否则，我将不得不为运行时允许输入的每个选项重写程序（实际上，我可以从中选择很多类）。

int main()
{
    int option;
    std::cin >> option;

Base* b = new Base(5);

Derived1* d1 = new Derived1(5);
Derived2* d2 = new Derived2(5);

d1->print1(); //obvIoUsly this works
d2->print2(); //obvIoUsly this works

//In reality I thus have a function in d1 and d2 which is not present in b
//I have to decide on runtime which class I need to use

if(option == 1)
{
    b = d1;
}
else if(option == 2)
{
    b = d2;
}

/*
Rest of program ...
    b->invokeMethod;
    // ... //
    b->invokeMoreMethods;
*/

//Call derived functions on base object

//b->print1(); //fails obvIoUsly
if(option == 1)
{
    dynamic_cast<Derived1*>(b)->print1(); //will work if option == 1 is specified (*)
}
if(option == 2)
{
    dynamic_cast<Derived2*>(b)->print2(); //will work if option == 2 is specified (*)
}

return 0;
}

是否可以在没有if（option == 1）和if（option == 2）循环的情况下进行（*）代码行？由于无法实现，我无法使用任何虚拟功能...是否有针对此问题的更优雅的解决方案？     

解决方法

可能您可以编写一个包装器，该包装器使用任一指针初始化，并在一天结束时在内部解析动态调度，但是我不确定这样做是否值得。如果您可以修改这三种类型，那就是您应该要做的，那么其他一切都只是一个讨厌的hack。 用std :: // boost :: function破解：

int main() {
    Base * b = 0;                    // You were leaking a Base object here
    boost::function< void () > f;
    ...
    if ( option == 1 ) {
       Derived1 d1 = new Derived1;   // Only instantiate the object you will use
       b = d1;
       f = boost::bind( &Derived1::print1,d1 );
    } else if ( option == 2 ) {
       Derived2 d2 = new Derived2;
       b = d2;
       f = boost::bind( &Derived2::print2,d2 );
    }
    ...
    f(); // will call either d1->print1(),or d2->print2()
    ...
    delete b;                      // should really use a smart pointer but I left
                                   // the raw pointer to minimize changes
}

请注意，这是一个令人讨厌的把戏，很难轻易维护。