Question

我正在努力确认我已经在实践中工作的背后的理论。完整设置有点扭曲，因为功能在不同的dll之间分配，但我会尝试描述这种情况：

class __declspec( dllexport ) BaseClass
/** This class' definition is available to everything,
 *  via .h file, .dll and .lib. */
{
    protected:
        std::string name;

    public:
        std::string GetName();
        /** This is implemented in BaseClass, and just returns this->name. */
}

class DerivedClass: public BaseClass
/** This class is created within the executable, but is not 'visible' to other
 *  dlls - either through .h files, .lib, or anything else. */
{
    public:
        DerivedClass();
        /** This sets this->name based on its own propertied. */
}

此向上转换有效，但它需要完全访问DerivedClass的定义：

void* pointer;
DerivedClass* derived_pointer = reinterpret_class<DerivedClass*>(pointer);
BaseClass* base_pointer = dynamic_cast<BaseClass*>(derived_pointer);
base_pointer->GetName();

但是，以下操作不起作用：

void* pointer;
BaseClass* base_pointer = reinterpret_class<BaseClass*>(pointer);
base_pointer->GetName();

为了解决这个问题，我实现了一个接口：

class __declspec( dllexport ) IBaseClass
/** Fully virtual 'interface' class, in same file as BaseClass. */
{
    public:
        virtual std::string GetName() = 0;
}

class __declspec( dllexport ) BaseClass: public IBaseClass
/** This class' definition is available to
 *  everything, via .h file, .dll and .lib. */
{
    protected:
        std::string name;

    public:
        std::string GetName();
        /** This is implemented in BaseClass, and just returns this->name. */
}

class DerivedClass: public BaseClass
/** This class is created within the executable, but is not 'visible'
 *  to other dlls - either through .h files, .lib, or anything else. */
{
    public:
        DerivedClass();
        /** This sets this->name based on its own propertied. */
}

现在以下代码工作：

void* pointer;
IBaseClass* ibase_pointer = reinterpret_class<IBaseClass*>(pointer);
ibase_pointer->GetName();

我有一些朦胧的记忆，有人告诉我，投入一个完全虚拟的课是一个特例 - 但我不记得为什么，或在网上找到任何关于它的东西。

请帮帮我 - 为什么我的代码有用？！

Answer 1

这完全依赖于类布局，这是实现定义的，不能依赖它。特别是对于MSVC，类布局的一个很好的介绍是http://www.openrce.org/articles/full_view/23，值得知道你可以要求带有/d1reportSingleClassLayout标志的类布局。

在您的情况下，由于第一个BaseClass没有虚拟成员，因此它将被放置在DerivedClass内的非指定位置。我猜DerivedClass有一些虚拟成员，否则我希望BaseClass位于DerivedClass的开头且reinterpret_cast可以正常工作。如果它有虚拟成员你就有：

+--------------------+
|DerivedClass vtable |
|BaseClass::name     |
|DerivedClass members|

通过添加接口IBaseClass没有任何变化; DerivedClass仍然列为：

+--------------------+
|DerivedClass vtable |
|BaseClass::name     |
|DerivedClass members|

但DerivedClass vtable以IBaseClass vtable开头：

+---------------------+
|IBaseClass::GetName  |
|DerivedClass virtuals|

所以通过DerivedClass vtable调用。

向上升级到超类或接口？

1 个答案: