在基类向量上调用虚函数 [英] Calling a virtual function on a vector of base classes

问题描述

我创建了一些代码来重现问题:

I created some code to reproduce the problem:

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

class A
{
protected:
    int m_X;
public:
    A() { 
        std::cout << "in A ctor" << std::endl; 
        m_X = 0;
    }
    virtual void printX(){ std::cout << "in A " << m_X << std::endl; }
};

class B : public A
{
public:
    B() {
        std::cout << "in B ctor" << std::endl; 
        m_X = 1;
    }
    virtual void printX(){ std::cout << "in B " << m_X << std::endl; }
};

class As
{
public:
    void AddA( const A &a ){ m_As.push_back( a ); }
    void PrintXs()
    {
        for ( auto a : m_As )
        {
            a.printX();
        }
    }
private:
    std::vector<A> m_As;
};

int _tmain(int argc, _TCHAR* argv[])
{
    As as;
    B b;
    as.AddA( b );
    as.PrintXs();
    system("pause");
    return 0;
}

输出结果是:

在演员

在 B 中

在 A 1

我想要在 B 1"而不是在 A 1".我确信我对虚拟的理解是有缺陷的.我必须如何更改代码以调用 B PrintX()?请注意，会有其他类从 A 继承，所以我真的不想编写静态调用.

I want "in B 1" instead of "in A 1". I'm sure my understanding of virtual is flawed. How must I change the code to call the B PrintX()? Note that there will be other classes that inherit from A so I really don't want to code a static call.

谢谢.

推荐答案

你正在做的事情叫做 切片.这是您获取派生类的对象并修剪掉父类中不存在的所有对象并将其分配给父类的地方.

What you're doing is called slicing. This is where you take an object of a derived class and trim off everything that is not in the parent and assign it to the parent.

您想要做的是使用多态来做您所解释的事情.为此，请将向量从对象的副本更改为对象的 ptr.

What you want to do is use polymorphism to do what you explained. To do this, change your vector from a copy of the object, to a ptr to the object.

如果对更多细节感兴趣，请使用提供的链接，其中包含的信息似乎非常完整.

If interested in more details, please use the links provided, the information included in them seems to be very complete.

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