blocks|key|1203905|text|这意味着指针所指向的对象在其继承层次结构中没有Okular::Part+(我忽略了涉及多重继承的某些边缘情况)。因此，动态转换失败。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1203906|这就是它的意思。不过，它并不能真正帮助您理解最初引用的bug。|1203907|entityMap^0|N|C|0|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|M|8|@]|D|@]|E|$]]|$1|H|3|-4|5|6|7|N|8|@]|D|@]|E|$]]]|I|$]]

This means that the object that the pointer is pointing to does not have <code>Okular::Part</code> in its inheritance hierarchy (I'm ignoring certain edge cases involving multiple inheritance). Therefore, the dynamic cast fails.

That's what it means. It doesn't really help you in understanding the originally referenced bug, though.

blocks|key|157405|text|除了这里的传统演员之外，dynamic_cast还能做些什么？|type|blockquote|depth|inlineStyleRanges|entityRanges|data|157406|它执行运行时检查转换是否有效；指针确实指向正确类型的对象。|unstyled|157407|我怎么知道它为什么会失败呢？|157408|如果Part确实是从ViewInterface派生的，那么如果指针指向有效的Part对象，则不会失败。所以这个物体一定是被破坏了。像瓦兰这样的动态分析工具可以帮助诊断这类问题。|offset|length|style|CODE|157409|这将是运行时的一个问题，libc%2B%2B，不是吗？|157410|几乎可以肯定不是。管理对象的生存期很可能是一个问题，结果是一个悬空指针。或者，它可能是某个其他地方的bug，破坏了对象，因此它不再包含有效的RTTI信息。或者，由于似乎涉及到线程，所以它可能是在没有足够的同步的情况下共享的。|157411|静态石膏将是穷人的难题|157412|这不会解决任何问题。通过不同的方式访问无效对象，您只会得到一种不同的未定义行为。|157413|entityMap|0|LINK|mutability|MUTABLE|url|http://valgrind.org/^0|0|0|0|2|4|A|D|12|4|1U|2|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|12|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|13|8|@]|9|@]|A|$]]|$1|E|3|F|5|6|7|14|8|@]|9|@]|A|$]]|$1|G|3|H|5|D|7|15|8|@$I|16|J|17|K|L]|$I|18|J|19|K|L]|$I|1A|J|1B|K|L]]|9|@$I|1C|J|1D|1|1E]]|A|$]]|$1|M|3|N|5|6|7|1F|8|@]|9|@]|A|$]]|$1|O|3|P|5|D|7|1G|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|1H|8|@]|9|@]|A|$]]|$1|S|3|T|5|D|7|1I|8|@]|9|@]|A|$]]|$1|U|3|-4|5|D|7|1J|8|@]|9|@]|A|$]]]|V|$W|$5|X|Y|Z|A|$10|11]]]]

<blockquote>
 what does a dynamic_cast do in addition to a traditional cast here?
</blockquote>

It performs a runtime check that the conversion is valid; that the pointer really does point to an object of the correct type.

<blockquote>
 how would I figure out why it fails?
</blockquote>

If <code>Part</code> really is derived from <code>ViewInterface</code>, then it can't fail if the pointer points to a valid <code>Part</code> object. So the object must have been corrupted or destroyed. A dynamic analysis tool like <a href="http://valgrind.org" rel="nofollow">Valgrind</a> can help diagnose this kind of problem.

<blockquote>
 This would be an issue with the runtime, libc++, no?
</blockquote>

Almost certainly not. It's most likely an issue with managing the lifetime of the object, ending up with a dangling pointer. Or it might be a bug somewhere else corrupting the object so it no longer contains valid RTTI information. Or, since there seem to be threads involved, perhaps it's being shared without adequate synchronisation.

<blockquote>
 static cast would be a poor man's fix
</blockquote>

It wouldn't fix anything. You'd just get a different flavour of undefined behaviour from accessing the invalid object in a different way.

blocks|key|157419|text|让我解决这个问题的答案是蒂亚戈·马西埃拉()在Qt兴趣ML上提供的。我在这里复制它：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|157420|这类问题的常见怀疑是相同的:您的虚拟表没有正确地锚定在共享库中。
请确保参加此派对的所有类都选中以下所有框：
*在类声明中有一个导出宏
*主虚拟函数位于.cpp中，从不内联
主虚拟函数是以父类的顺序排列的第一个新覆盖函数，如果没有重写，则是第一个新的虚拟函数。通常，多态类有虚拟析构函数，析构函数总是被覆盖，所以这是最好的选择。|blockquote|157421|在这种情况下，问题是由于目标类(Okular::ViewerInterface)没有在OS上导出(在默认情况下也可能是因为Linux上的全局编译器开关)。KDocumentViewer类的Idem，但这可能与此无关。|157422|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$]]|$1|E|3|F|5|6|7|K|8|@]|9|@]|A|$]]|$1|G|3|-4|5|6|7|L|8|@]|9|@]|A|$]]]|H|$]]

The answer that allowed me to resolve the issue was provided by Thiago Macieira on the Qt Interest ML. I'm copying it here:

<blockquote>
 the usual suspect in this 
 kind of problem is the same: your virtual tables are not properly anchored in 
 shared libraries.
 
 Make sure that all classes participating in this party check all these boxes: 
 * have an export macro in the class declaration 
 * the primary virtual function is in a .cpp, NEVER inline
 
 The primary virtual function is the first newly overriden function in the 
 parent classes' order or, if none are overridden, the first new virtual. 
 Usually, polymorphic classes have virtual destructors and destructors are 
 always overridden, so this is your best bet.
</blockquote>

In this case, the issue was due to the fact that the target class (Okular::ViewerInterface) was not exported on OS X (and probably by default or because of a global compiler switch on Linux). Idem for the KDocumentViewer class, but that might not have been relevant.

I've encountered a few cases in KDE software where a dynamic_cast of something that's a KPart fails on OS X. I'm not that much of a C++ expert, so I wouldn't even know where to begin to debug such a situation.
I've seen this happen with ktimetracker, and the latest (and better discussed) case is with Okular: <a href="https://bugs.kde.org/show_bug.cgi?id=345765" rel="nofollow">https://bugs.kde.org/show_bug.cgi?id=345765</a>

In a nutshell: the <code>Okular::Part</code> class inherits the <code>Okular::ViewInterface</code> class (as the last parent in a list). Yet when the code retrieves an <code>Okular::Part*</code> instance (<code>part</code>), <code>dynamic_cast&lt;Okular::ViewInterface*&gt;(part)</code> returns NULL.

The last comment in the BKO ticket above is that using a static cast would be a poor man's fix in this case, but that I should try to figure out why the dynamic_cast fails. Which leads me to 2 questions:

<ul>
<li>what does a dynamic_cast do in addition to a traditional cast here?</li>
<li>how would I figure out why it fails? This would be an issue with the runtime, libc++, no?</li>
</ul>

What does it mean when a dynamic_cast fails?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

文章

问答

视频

学习中心

腾讯云实验室

直播

竞赛

腾讯云代码分析专区

腾讯iOA零信任安全管理系统专区

腾讯云架构师技术同盟交流圈

腾讯云数据库专区

腾讯云顾问专区

腾讯云原生专区

腾讯混元专区

腾讯云TCE专区

腾讯云Lighthouse专区

腾讯云HAI专区

腾讯云Edgeone专区

腾讯云存储专区

腾讯云智能专区

腾讯轻联专区 

腾讯云开发专区

TAPD专区

腾讯轻量云游戏服专区

腾讯云最具价值专家

腾讯云架构师技术同盟

腾讯云创作之星

腾讯云开发者先锋

腾讯云代码助手

云原生构建

TAPD 敏捷项目管理

Cloud Studio

SDK中心

API中心

命令行工具

涵盖代码开发、场景应用、自动测试全流程，助你从零构建专属AI助手

一站式MCP教程库，解锁AI应用新玩法

我在KDE软件中遇到过几个例子，其中dynamic_cast的KPart在OS上失败了。我并不是一个C++专家，所以我甚至不知道从哪里开始调试这种情况。我在ktimetracker中看到了这种情况，最近(也是讨论得更多的)案例是Okular：简而言之：Okular::Part类继承了Okular::ViewInterf...

问当dynamic_cast失败时，这意味着什么？
EN

Stack Overflow用户

社区

活动

圈层

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问当dynamic_cast失败时，这意味着什么？EN