即使通过引用传递，也将调用基类方法而不是派生方法 [英] Base class method being called instead of derived, even when passed by reference

问题描述

我有3个类别:A，B和AnotherClass.其中B是从A派生的:

class A {
public:
    A(){}
    virtual void method() {//default action}
};

然后我有一个派生类B:

class B : public A {
public:
    B(){}
    void method() {//redefine action}
};

和AnotherClass:

class AnotherClass {
public:
    AnotherClass(A& a);
    A a;
    anotherMethod(){ a.method()}
};
AnotherClass :: AnotherClass(A& a) : a(a) //initialization

因此，如果我用B的对象构造了AnotherClass的对象:

B b();
AnotherClass myObj(b);

请记住，由于B是从A继承的，并且AnotherClass接受A的对象，因此我能够成功地将B对象作为参数传递.

然后我打电话给

myObj.anotherMethod();

我希望它执行anotherMethod()，并且当它执行时，我希望它调用属于B的REDEFINED method()，但是它将调用A中定义的默认method()

我当时在想我的问题是因为我将AnotherClass的参数指定为class A的对象.但是，我不想将此参数更改为class B的对象，因为我也有类C，D和E，它们也直接从A继承.因此，我想使用基类作为参数类型，因此我不仅限于只能传递b对象.但是，我在该站点上阅读了一些较旧的文章，大多数建议的解决方案是通过引用传递派生对象(b).

有人可以解释为什么会这样吗?

解决方案

构造函数的参数很好.它是类型为B的对象，该对象通过引用传递给A子对象.问题是您将A子对象仅(而不是派生对象)复制到成员变量a中.然后，当您调用a.method()时，那里只有一个基础对象，因此将调用基础函数.

解决方案是使成员变量成为参数的指针或引用(而不是副本). (请注意，无论哪种情况，您都需要确保传入的对象的生存时间至少与AnotherClass对象一样长.

I have 3 classes: A, B, and AnotherClass. Where B is derived from A:

class A {
public:
    A(){}
    virtual void method() {//default action}
};

Then I have a derived class, B:

class B : public A {
public:
    B(){}
    void method() {//redefine action}
};

And AnotherClass:

class AnotherClass {
public:
    AnotherClass(A& a);
    A a;
    anotherMethod(){ a.method()}
};
AnotherClass :: AnotherClass(A& a) : a(a) //initialization

So, if I construct an object of AnotherClass with an object of B:

B b();
AnotherClass myObj(b);

Keep in mind, since B inherits from A, and AnotherClass accepts an object of A, I am able to successfully pass in a B object as the argument.

And I call:

myObj.anotherMethod();

I expect this to execute anotherMethod(), and when it does, I expect it to call the REDEFINED method() that belongs to B, but instead it calls the default method() defined in A

I was thinking my problem is because I specify the argument of AnotherClass as an object of class A. However, I do not want to change this argument to an object of class B because I also have classes C, D, and E, that also inherit directly from A. So I want to use the base class as the argument type so I am not limited to only being able to pass in a b object. But, I read some older posts on this site and most proposed solutions was to pass the derived object (b) by reference, which I am doing.

Can someone explain why this is happening?

解决方案

The argument to the constructor is fine. It is an object of type B passed by reference to the A subobject. The problem is that you copy the A subobject only (and not the derived object) into the member variable a. Then when you call a.method() there is only a base object there, so the base function is called.

The solution is to make the member variable be a pointer or a reference (and not a copy) of the argument. (Note that in either case, you will then need to make sure that the object you pass in, will live at least as long as the AnotherClass object.

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