downcast问题在c ++

问题描述

  #include< iostream> 
 using std :: cout; 
 using std :: endl; 
 class Base 
 {
 public：
 void f（）; 
 void g（）; 
 
 int mBaseData1; 
 
}; 
 
 class Derived：public Base 
 {
 public：
 
 
 int mDerivedData1; 
}; 
 
 void main（）
 {
 
 Base * base = new Base（）; 
 Derived * derived =（Derived *）（base）; // DownCast 
 derived-> mDerivedData1 = 6; 
 cout<< derived-> mDerivedData1<< endl; // Result = 6; 
} 
   
 
 
在此代码 new base（）在堆中分配内存
 
 
 
和 Derived * derived =（Derived *）（base）到派生
 
 
 
我们如何使用mDerivedData1？我不能找到我们为mDerivedData1分配内存的地方，或者当我们调用Derived的构造函数来分配mDerivedData1？
解决方案
 t找到 mDerivedData1 的内存分配位置是因为没有分配内存。您已执行无效的类型投放。存储在 base 中的东西是指向 Base 实例的指针。使用类型转换告诉编译器它实际上是一个指向 Derived 实例的指针不会这样（但编译器会相信你反正因为你是一个负责）。对象仍然只是一个 Base 。如果你想要一个 Derived ，那么你需要实例化一个 Derived 。您可以使用 dynamic_cast 将 Base 指针转换为派生指针。
  Base * base = new Derived; 
 Derived * derived = dynamic_cast< Derived *>（base）; 
 derived-> mDerivedData1 = 6; 
  
 
#include <iostream>
using std::cout;
using std::endl;
class Base
{
public :
    void f();
    void g();

    int mBaseData1;

};

class Derived : public Base
{
public : 


    int mDerivedData1;
};

void main()
{

    Base* base = new Base();
    Derived* derived = (Derived*)(base); // DownCast
    derived->mDerivedData1 = 6;
    cout<< derived->mDerivedData1<<endl; // Result = 6;
}
in this code  new base() allocate memory in heap 
and Derived* derived = (Derived*)(base) cast base to derived 
how we can use mDerivedData1? i cant find where we allocate memory for mDerivedData1  or when we call constructor of Derived for allocate mDerivedData1 ?
 解决方案 
The reason you can't find where memory for mDerivedData1 was allocated is because no memory was allocated. You have performed an invalid type-cast. The thing stored in base is a pointer to a Base instance. Using a type-cast to tell the compiler that it's actually a pointer to a Derived instance doesn't make it so (but the compiler will believe you anyway because you're the one in charge). The object is still just a Base. If you want a Derived, then you'll need to instantiate a Derived. You can use dynamic_cast to convert the Base pointer into a Derived pointer.
Base* base = new Derived;
Derived* derived = dynamic_cast<Derived*>(base);
derived->mDerivedData1 = 6;
                        
这篇关于downcast问题在c ++的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！