将子类对象传递给采用超类对象的函数 [英] Passing a subclass object to a function that takes superclass object

问题描述

假设以下代码:

class Event {
public:
    virtual void execute() {
        std::cout << "Event executed.";
    }
}

class SubEvent : public Event {
    void execute() {
        std::cout << "SubEvent executed.";
    }
}

void executeEvent(Event e) {
    e.execute();
}

int main(int argc, char ** argv) {
    SubEvent se;
    executeEvent(se);
}

执行时，程序输出事件已执行."，但我想执行SubEvent.我该怎么办?

When executed, the program outputs "Event executed.", but I want to execute SubEvent. How can I do that?

推荐答案

您正在按值传递Event.该函数获取其自己的参数副本，这是一个Event对象，而不是SubEvent.您可以通过传递参考来解决此问题:

You are passing the Event by value. The function gets its own copy of the argument, and this is an Event object, not a SubEvent. You can fix this by passing a reference:

void executeEvent(Event& e) 
{//                    ^
  e.execute();
}

这称为 对象切片 .等效于此:

This is called object slicing. It is the equivalent of this:

SubEvent se;
Event e{se};
e.execute();

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