c++ - 如何使用协变返回从基类中检索派生类？

Question

该代码示例只是我实际程序的一个简单模型，它试图将来自单个基类的各种不同类全部保存在基 vector 中。然后使用虚函数调用获取*this，返回派生。这样我就不需要多个容器。

    #include "stdafx.h"
#include <iostream>
#include <vector>

class Base
{
public:
    virtual Base* getThis() { return this; }
    virtual void printClass() const { std::cout << "Base" << std::endl; }
};

class Derived : public Base
{
public:
    virtual Derived* getThis() { return this; }
    virtual void printClass() const { std::cout << "Derived" << std::endl; }
};


int main(int argc, _TCHAR* argv[])
{   
    Base Bar;
    Derived Foo;

    typedef std::vector<Base*> vContainer;

    vContainer Objects;
    Objects.push_back(new Derived);

    for (vContainer::iterator it = Objects.begin(); it != Objects.end(); ++it)
    {
        Bar = **it; // works to get Base but not wanted

        // attempts     
        //Foo = it->getThis(); // the pointer selector doesnt work...
        //Foo = static_cast<Derived>(**it);  // utterly confused!       
    }

    Bar.printClass(); // prints base as expected
    //Foo.printClass();  // Mean't to print Derived

    std::cin.get();
    return 0;
}

几个小时以来，我一直在寻找对此的更好理解，但每个人都只谈论克隆，这不是我所追求的。 任何帮助将不胜感激。

尼尔

Answer 1

为了安全起见，请使用dynamic_cast。

for (vContainer::iterator it = Objects.begin(); it != Objects.end(); ++it)
{
    Bar* basePtr = *it;
    Derived* derivedPtr = dynamic_cast<Derived*>(basePtr);
    if ( derivedPtr ) // Check whether the dynamic_cast was successful.
    {
       // Use derivedPtr
    }
}