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继承编译时还是运行时？

发布于 2024-12-26 03:32:17 字数 39 浏览 3 评论 0原文

在c++中，继承发生在运行时还是编译时？

例子？

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羁绊已千年 2025-01-02 03:32:17

在 C++ 中，自身继承（无多态性）是编译时功能。在编译后的代码中，和之间几乎没有区别

struct foo : bar {};

，

struct foo { bar b; };

除了访问其成员的偏移量之外，不会有任何关于 bar 与 foo 关系的“知识”在编译的二进制文件中。

但是，当您添加多态性（即虚拟函数，允许dynamic_cast<>()) 到图片。它允许后期绑定：将调用哪个函数是在运行时决定的。当然，这需要数据结构来执行此操作（通常使用所谓的虚拟表），并且这些数据结构在运行时访问以便确定要调用哪个函数。

此外，虚拟基类需要运行时支持才能使用派生对象访问它们。

如果您对某些 C++ 功能的运行时成本感兴趣，您可能需要尝试获取 C++ 对象模型内部 作者：Stanley Lippman。这是一本老书，但如果您想了解虚拟成员函数通常是如何实现的，以及在多继承场景中基对象通常如何在内存中布局，以及所有这些如何影响性能，您可以在本书中找到深入的讨论此类主题。

In C++, inheritance in itself (without polymorphism) is a compile-time feature. In the compiled code, there will be little or no difference between

struct foo : bar {};

and

struct foo { bar b; };

Except for offsets to access their members, there will not be any "knowledge" about how bar relates to foo in the compiled binary.

This is different, however, when you add polymorphism (i.e., virtual functions, allowing dynamic_cast<>()) to the picture. It allows late binding: which exact function will be called is decided at runtime. Of course, this requires data structures to perform this (usually employing so-called virtual tables), and those data structures are accessed at runtime in order to determine which function to call.

Also, virtual base classes require runtime support in order to access them withing derived objects.

If you are interested in the runtime costs of certain C++ features, you might want to try to get hold of a copy of Inside the C++ Object Model by Stanley Lippman. It's an old book, but if you want to know how virtual member functions are commonly implemented and how base objects are commonly laid out in memory in a multi-inheritance scenario, and how all this affects performance, this is where you will find thorough discussions of such topics.

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