将 parent 静态转换为 child C++

Question

我有一个 parent class 和一个 child class，它具有 parent class 的特定功能没有。我创建了一个 Parent 类型的向量，并向其添加了一个 Child 类型的 object。我确定该项目是 child，因此我尝试使用 static_cast 创建指向它的指针，以便使用其 child 特定功能，但它拒绝转换给出下面代码中描述的错误。

#include <vector>

//Parent Class
class Parent {
public:
    Parent() {};
};

//Child Class
class Child : public Parent
{
public:
    void childSpecificFunction() {};
};

int main()
{
    //I know for a fact that the item is a child
    std::vector<Parent> items;
    items.push_back(Child());

    //Static cast to use child specific function doesn't work
    Child* childPtr = static_cast<Child*>(items[0]);
    childPtr->childSpecificFunction();

    //Gives intellisence error: no suitable conversion from "Parent" to "Child *" exists
}

为什么会这样？有没有更好的方法来处理这种事情？

谢谢。

Answer 1

Why is this happening

我们来剖析一下：

int main()
{
    //I know for a fact that the item is a child
    std::vector<Parent> items;
    items.push_back(Child()); // <<<< You are copying and slicing here

    //Static cast to use child specific function doesn't work
    Child* childPtr = static_cast<Child*>(items[0]);
    childPtr->childSpecificFunction(); // <<<<< undefined behavior

    //Gives intellisence error: no suitable conversion from "Parent" to "Child *" exists
}

一个合适的修复

int main()
{
    std::vector<std::unique_ptr<Parent>> items;
    items.push_back(std::make_unique<Child>());  // <<< working with pointers,
                                                 //     no slicing.

    Child* childPtr = static_cast<Child*>(items[0].get());
    childPtr->childSpecificFunction();
 }

Answer 2

实际上，您正在尝试存储对象，然后将其转换为指向子对象的指针*

#include <vector>

//Parent Class
class Parent {
public:
    Parent() {};
};

//Child Class
class Child : public Parent
{
public:
    void childSpecificFunction() {};
};

int main()
{
    std::vector<Parent*> items;
    items.push_back(new Child());

    Child* childPtr = static_cast<Child*>(items[0]);
    childPtr->childSpecificFunction();


}

另一种方法是转换子对象的地址

#include <vector>

//Parent Class
class Parent {
public:
    Parent() {};
};

//Child Class
class Child : public Parent
{
public:
    void childSpecificFunction() {};
};

int main()
{
    std::vector<Parent> items;
    items.push_back(Child());

    Child* childPtr = static_cast<Child*>(&(items[0]));
    childPtr->childSpecificFunction();

}

将 parent 静态转换为 child C++

statically casting parent to child C++

c++

polymorphism

casting

static-cast