dynamic_cast返回NULL，但它不应该 [英] dynamic_cast returns NULL but it shouldn't

问题描述

我有以下类层次结构：

  class IStorage {
 [...] 
} 
 Q_DECLARE_INTERFACE（IStorage，ch.gorrion.smssender.IStorage / 1.0）
 
 
类ISQLiteStorage：public IStorage {
 Q_INTERFACES（IStorage）
 
 [...] 
} 
 Q_DECLARE_INTERFACE（ISQLiteStorage，ch.gorrion.smssender.ISQLiteStorage / 1.0）
 
 
 DASQLiteStorage：public QObject，public ISQLiteStorage {
 Q_OBJECT 
 Q_INTERFACES（ISQLiteStorage）
 
 [...] 
} 
  

我正在使用QT，我想用QtPlugin创建一个插件（对于我的应用程序）。
我创建一个DASQLiteStorage的实例，我给这个实例从一个对象FROM WITHIN插件：

  /下一行是在我的主要应用程序。 
 // storage是DASQLiteStorage实例。 
 // gateway是插件中的一个对象。 
 gateway-> setDefaultStorage（storage）; 
 
 //此方法位于插件内
 void AbstractGateway :: setDefaultStorage（IStorage * storage）{
 defaultStorage_ = dynamic_cast< ISQLiteStorage *>（storage）;问题是，dynamic_cast返回一个空指针（不是一个空指针）预期），而在我的主应用程序（即在gateway-> setDefaultStorage（storage）;）之前做dynamic_cast给我有效的指针（预期）。
 
 
 
任何人都知道为什么会发生这种情况？程序是否在与插件不同的内存范围内运行？这会导致这样的问题吗？任何想法如何解决这个问题？
 
 
 
非常感谢！ 
 
 
 
 
 
 
 
 
 
编辑：
我已经尝试了一些建议： 
  //此方法位于插件内
 void AbstractGateway :: setDefaultStorage（IStorage * storage）{
 ISQLiteStorage * s = dynamic_cast< ISQLiteStorage *>（storage）; 
 s = static_cast< ISQLiteStorage *>（storage）; 
 s = qobject_cast< ISQLiteStorage *>（（QObject *）storage）; 
 
 defaultStorage_ = s; 
} 
  
在方法的第一行，s等于NULL，包含正确的指针，并在第三个其他指针。为什么这些指针不相等？
 
为什么我现在使用dynamic_cast仍然不工作：
  pluginLoader（） - > setLoadHints（QLibrary :: ResolveAllSymbolsHint | QLibrary :: ExportExternalSymbolsHint）; 
  
 
 
 
 
 
 
编辑2：
我注意到，分段错误我得到一点进一步的代码也与此相关。我有以下结构：
  //以下类在主应用程序中定义。 
 class ILoginAccount：public IAccount [...] 
 
类AbstractAccountStroageOfficer {
 public：
 AbstractAccountStroageOfficer（IAccount * account）[...] } 
 
 
 //这些类在我的插件中定义，并从插件中创建。 
类BCAccount：public ILoginAccount {
 public：
 BCAccount（）
：ILoginAccount（new DAAccountStorageOfficer（this））
 {}; 
} 
 
 class DAAccountStorageOfficer：public AbstractAccountStorageOfficer {
 public：
 DAAccountStorageOfficer（ILoginAccount * account）
：AbstractAccountStorageOfficer segfault。 
 {
 IAccount * a = account; //这行也引发一个segfault。 
 a = dynamic_cast< IAccount *>（account）; //这也是。 
 a = static_cast< IAccount *>（account）; //这也是。 
} 
} 
  
这些分段错误不应该发生， 
解决方案
基本上，RTTI在模块边界是不可靠的。不同的编译器在这里有不同的行为;你必须研究你的编译器/版本在这种情况下的行为。当然，如果你有一个不同的编译器/版本的主要应用程序和插件，它显然没有机会工作。
 
 
 
使用static_cast作为一个工作。 p> 
I'm having the following class hierarchy:
class IStorage {
    [...]
}
Q_DECLARE_INTERFACE(IStorage, "ch.gorrion.smssender.IStorage/1.0")


class ISQLiteStorage: public IStorage { 
    Q_INTERFACES(IStorage)

    [...] 
}
Q_DECLARE_INTERFACE(ISQLiteStorage, "ch.gorrion.smssender.ISQLiteStorage/1.0")


class DASQLiteStorage: public QObject, public ISQLiteStorage {
    Q_OBJECT
    Q_INTERFACES(ISQLiteStorage)

    [...]
}
I'm using QT and am trying to create a plugin (for my app) with QtPlugin.
I'm creating an instance of DASQLiteStorage and I give this instance to an object FROM WITHIN the plugin:
// the next line is within my main app.
// storage is the DASQLiteStorage instance.
// gateway is an object from within the plugin.
gateway->setDefaultStorage(storage);

// this method lies within the plugin
void AbstractGateway::setDefaultStorage(IStorage* storage) {
    defaultStorage_ = dynamic_cast<ISQLiteStorage*>(storage);
}
The problem is, that the dynamic_cast is returning me a null-pointer (not expected), while doing the dynamic_cast within my main app (i.e. before "gateway->setDefaultStorage(storage);") gives me the valid pointer (expected).

Does anyone know why this could happen? Is the program operating in a different memory range as the plugin? Could this lead to such problems? Any ideas how to fix this?

Thanks a lot!







EDIT:
I've tried out some suggestions:
// this method lies within the plugin
void AbstractGateway::setDefaultStorage(IStorage* storage) {
    ISQLiteStorage* s = dynamic_cast<ISQLiteStorage*>(storage);
    s = static_cast<ISQLiteStorage*>(storage);
    s = qobject_cast<ISQLiteStorage*>((QObject*)storage);

    defaultStorage_ = s;
}
In the first line of the method, s equals NULL, in the second s contains the correct pointer and in the third an other pointer. Why aren't these pointers equal?

And why could the dynamic_cast be still not working although I'm using now:
pluginLoader()->setLoadHints(QLibrary::ResolveAllSymbolsHint | QLibrary::ExportExternalSymbolsHint);






EDIT2:
I noticed, that the segmentation fault I get a little further in the code is also related to this. I have the following construct:
// The following classes are defined within the main app.
class ILoginAccount: public IAccount [...]

class AbstractAccountStroageOfficer {
public:
    AbstractAccountStroageOfficer(IAccount* account)[...]
}


// These classes are defined within my plugin and are created from within the plugin.
class BCAccount: public ILoginAccount {
public:
    BCAccount()
      : ILoginAccount(new DAAccountStorageOfficer(this))
    {};
}

class DAAccountStorageOfficer: public AbstractAccountStorageOfficer {
public:
    DAAccountStorageOfficer(ILoginAccount* account)
      : AbstractAccountStorageOfficer(account) // This line raises a segfault.
    {
        IAccount* a = account; // This line raises a segfault as well.
        a = dynamic_cast<IAccount*>(account); // This as well.
        a = static_cast<IAccount*>(account); // This as well.
    }
}
These segmentation faults should not occur, should they? But why do they?
解决方案
Basically, RTTI is unreliable across module boundaries. Different compilers have different behaviors here; you'll have to research how your compiler/version acts in this case. Of course, if you have a different compiler/version for the main app and plugin, it clearly has no chance of working.

Use static_cast as a work around.

                        
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