c++ - 基类 -> 派生类和 C++ 中的反之转换

Question

我有以下示例代码:

#include <iostream>
#include <string>

using namespace std;

class Event
{
public:
    string type;
    string source;
};

class KeyEvent : public Event
{
public:
    string key;
    string modifier;
};

class MouseEvent : public Event
{
public:
    string button;
    int x;
    int y;
};

void handleEvent(KeyEvent e)
{
    if(e.key == "ENTER")
        cout << "Hello world! The Enter key was pressed ;)" << endl;
}

Event generateEvent()
{
    KeyEvent e;
    e.type = "KEYBOARD_EVENT";
    e.source = "Keyboard0";
    e.key = "SPACEBAR";
    e.modifier = "none";

    return e;
}

int main()
{
    KeyEvent e = generateEvent();

    return 0;
}

我无法编译它，G++ 抛出一个错误:

main.cpp: In function 'int main()':
main.cpp:47:29: error: conversion from 'Event' to non-scalar type 'KeyEvent' requested

我知道这个错误对于 C++ 大师来说是显而易见的，但我不明白为什么我不能将基类对象转换为派生对象。有人可以建议我解决我遇到的问题吗？谢谢你的建议

Answer 1

您的函数 generateEvent 执行以下操作:

• 创建一个KeyEvent
• 通过复制和切片将对象转换(向上转换)为事件
• 返回事件对象

然后您尝试获取该 Event 对象拷贝并将其再次放入 KeyEvent。

你试图使用多态性但实际上只是切片。考虑(谨慎!)动态分配:

boost::shared_ptr<Event> generateEvent() {
    KeyEvent* e = new KeyEvent;
    e->type = "KEYBOARD_EVENT";
    e->source = "Keyboard0";
    e->key = "SPACEBAR";
    e->modifier = "none";

    return boost::shared_ptr<Event>(static_cast<Event*>(e));
}

int main() {
    boost::shared_ptr<Event> e = generateEvent();
    // you can now use dynamic_cast and/or virtual
    // function calls to treat e as a pointer-to-KeyEvent
}

另请注意，return 0; 隐含在入口函数中。