問題描述
我是一名 Java 程序員，最近開始學習 C++.我被某些事情搞糊涂了.
I'm a Java programmer and recently started studying C++. I'm confused by something.
我知道在 C++ 中，要實現多態行為，您必須使用指針或引用.例如，考慮一個帶有實現方法 getArea() 的 Shape 類.它有幾個子類，每個子類都以不同的方式覆蓋 getArea().比考慮以下功能:
I understand that in C++, to achieve polymorphic behavior you have to use either pointers or references. For example, consider a class Shape with an implemented method getArea(). It has several subclasses, each overriding getArea() differently. Than consider the following function:
void printArea(Shape* shape){
cout << shape->getArea();
}
函數根據指針指向的具體Shape調用正確的getArea()實現.
The function calls the correct getArea() implementation, based on the concrete Shape the pointer points to.
效果相同:
void printArea(Shape& shape){
cout << shape.getArea();
}
但是，以下方法不能多態地工作:
However, the following method does not work polymorphicaly:
void printArea(Shape shape){
cout << shape.getArea();
}
不管函數中傳入什么樣的Shape，都會調用相同的getArea()實現:Shape<中的默認實現/代碼>.
Doesn't matter what concrete kind of Shape is passed in the function, the same getArea() implementation is called: the default one in Shape.
我想了解這背后的技術推理.為什么多態適用于指針和引用，而不適用于普通變量?(而且我想這不僅適用于函數參數，而且適用于任何東西.
I want to understand the technical reasoning behind this. Why does polymorphism work with pointers and references, but not with normal variables? (And I suppose this is true not only for function parameters, but for anything).
請解釋這種行為的技術原因，以幫助我理解.
Please explain the technical reasons for this behavior, to help me understand.
推薦答案
答案是復制語義.
當您在 C++ 中按值傳遞對象時，例如printArea(Shape shape) 一個副本是你傳遞的對象.如果將派生類傳遞給該函數，則復制的只是基類 Shape.仔細想想，編譯器不可能做任何其他事情.
When you pass an object by value in C++, e.g. printArea(Shape shape) a copy is made of the object you pass. And if you pass a derived class to this function, all that's copied is the base class Shape. If you think about it, there's no way the compiler could do anything else.
Shape shapeCopy = circle;
shapeCopy 被聲明為 Shape，而不是 Circle，所以編譯器所能做的就是構造一個 的副本對象的形狀部分.
shapeCopy was declared as a Shape, not a Circle, so all the compiler can do is construct a copy of the Shape part of the object.
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