使用取消引用对象的虚函数调用 [英] Virtual function calling using dereference object

问题描述

我有一个基类指针指向一个派生类对象。我在下面的代码中使用两种不同的方式调用 foo（）函数。为什么 Derived :: foo（）在第一种情况下被调用？不应该（* obj）.foo（）调用 Base :: foo（）取消引用？

I have a base class pointer pointing to a derived class object. I am calling foo() function by using two different ways in the code below. Why does Derived::foo() get called in the first case? Shouldn't (*obj).foo() call Base::foo() function as it has already been dereferenced?

    class Base
    {
    public:
        Base() {}
        virtual void foo() { std::cout << "Base::foo() called" << std::endl; }
        virtual ~Base() {};
    };

    class Derived: public Base
    {
    public:
        Derived() : Base() {}
        virtual void foo() {  std::cout << "Derived::foo() called" << std::endl; }
        virtual ~Derived() {};
    };

    int main() {
        Base* obj = new Derived();
   // SCENARIO 1
        (*obj).foo();
// SCENARIO 2
        Base obj1 = *obj;
        obj1.foo();

        return 0;
    }

推荐答案

// SCENARIO 1
(*obj).foo();

请注意

1. obj 在这里是错误的，因为它不是指向一个对象，而是一个指针，


2. （* ptr）.foo（）只是一个迂回的方式来执行 ptr- ; foo（）。

1. obj is a misnomer here, since it doesn't refer to an object, but to a pointer,
2. (*ptr).foo() is just a roundabout way to do ptr->foo().

* ptr 不会导致对象， strong> 基本& 并且通过引用的虚拟函数调用经受动态分派，就像通过指针的调用一样。

*ptr doesn't result in an object, but in a reference Base& to the object. And a virtual function call through a reference is subject to dynamic dispatch, just as such a call through a pointer.

// SCENARIO 2
Base obj1 = *ptr;
obj1.foo();

你在这里做的是创建一个全新的对象通过< a href =http://stackoverflow.com/questions/274626/what-is-the-slicing-problem-in-c> 切片 ：它只有基类部分 * ptr 。你想要的是这样：

What you do here is you create a totally new object through slicing: it just has the base class parts of *ptr. What you want instead is this:

Base& ref = *ptr;
ref.foo();

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