在 C++ 类中使用 dllimport 和 dllexportUsing dllimport and dllexport in C++ Classes

11/04/2016

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Microsoft 专用Microsoft Specific

可以用或属性声明 c + + 类 dllimport dllexport 。You can declare C++ classes with the dllimport or dllexport attribute. 这些形式表示已导入或导出整个类。These forms imply that the entire class is imported or exported. 以这种方式导出的类称为可导出类。Classes exported this way are called exportable classes.

以下示例定义了可导出类。The following example defines an exportable class. 将导出其所有成员函数和静态数据：All its member functions and static data are exported:

#define DllExport __declspec( dllexport )

class DllExport C {

int i;

virtual int func( void ) { return 1; }

};

请注意， dllimport dllexport 禁止对可导出类的成员显式使用和属性。Note that explicit use of the dllimport and dllexport attributes on members of an exportable class is prohibited.

dllexport 类dllexport Classes

声明类时 dllexport ，将导出其所有成员函数和静态数据成员。When you declare a class dllexport, all its member functions and static data members are exported. 您必须在同一程序中提供所有此类成员的定义。You must provide the definitions of all such members in the same program. 否则，将生成链接器错误。Otherwise, a linker error is generated. 此规则有一个例外情况，即对于纯虚函数，您无需为其提供显式定义。The one exception to this rule applies to pure virtual functions, for which you need not provide explicit definitions. 但是，由于基类的析构函数始终在调用抽象类的析构函数，因此纯虚拟析构函数必须始终提供定义。However, because a destructor for an abstract class is always called by the destructor for the base class, pure virtual destructors must always provide a definition. 请注意，这些规则对不可导出的类是相同的。Note that these rules are the same for nonexportable classes.

如果导出类类型的数据或返回类的函数，请务必导出类。If you export data of class type or functions that return classes, be sure to export the class.

dllimport 类dllimport Classes

声明类时 dllimport ，会导入其所有成员函数和静态数据成员。When you declare a class dllimport, all its member functions and static data members are imported. 与 dllimport 非类类型上和的行为不同 dllexport ，静态数据成员不能在定义类的同一程序中指定定义 dllimport 。Unlike the behavior of dllimport and dllexport on nonclass types, static data members cannot specify a definition in the same program in which a dllimport class is defined.

继承和可导出类Inheritance and Exportable Classes

可导出类的所有基类都必须是可导出的。All base classes of an exportable class must be exportable. 否则，会生成编译器警告。If not, a compiler warning is generated. 此外，同样是类的所有可访问成员必须是可导出的。Moreover, all accessible members that are also classes must be exportable. 此规则允许 dllexport 类从类继承 dllimport ，使用类 dllimport 从类继承， dllexport 但不建议将后者) (。This rule permits a dllexport class to inherit from a dllimport class, and a dllimport class to inherit from a dllexport class (though the latter is not recommended). 通常来说，对 DLL 客户端可访问的所有内容(根据 C++ 访问规则)都应该是可导出接口的一部分。As a rule, everything that is accessible to the DLL's client (according to C++ access rules) should be part of the exportable interface. 这包括在内联函数中引用的私有数据成员。This includes private data members referenced in inline functions.

选择性成员导入/导出Selective Member Import/Export

由于类中的成员函数和静态数据隐式具有外部链接，因此你可以使用 dllimport 或特性声明它们 dllexport ，除非导出整个类。Because member functions and static data within a class implicitly have external linkage, you can declare them with the dllimport or dllexport attribute, unless the entire class is exported. 如果导入或导出整个类，则禁止将成员函数和数据显式声明为 dllimport 或 dllexport 。If the entire class is imported or exported, the explicit declaration of member functions and data as dllimport or dllexport is prohibited. 如果在类定义中将静态数据成员声明为 dllexport ，则定义必须出现在同一程序中的某处 (与非类外部链接) 相同。If you declare a static data member within a class definition as dllexport, a definition must occur somewhere within the same program (as with nonclass external linkage).

同样，可以用或特性声明成员函数 dllimport dllexport 。Similarly, you can declare member functions with the dllimport or dllexport attributes. 在这种情况下，您必须在 dllexport 同一程序中的某处提供定义。In this case, you must provide a dllexport definition somewhere within the same program.

有关选择性成员导入和导出的某些要点值得注意：It is worthwhile to note several important points regarding selective member import and export:

选择性成员导入/导出最适合用于提供具有更强限制的导出类接口版本；即，您可以为该版本设计一个 DLL，该 DLL 公开的公用和专用功能比本应允许的语言公开的更少。Selective member import/export is best used for providing a version of the exported class interface that is more restrictive; that is, one for which you can design a DLL that exposes fewer public and private features than the language would otherwise allow. 这对于优化可导出接口也很有用：当通过定义知道客户端无法访问某些私有数据时，您不需要导出整个类。It is also useful for fine-tuning the exportable interface: when you know that the client, by definition, is unable to access some private data, you need not export the entire class.

如果导出了某个类中的一个虚函数，则必须导出其中的所有虚函数，或者至少必须提供客户端可直接使用的版本。If you export one virtual function in a class, you must export all of them, or at least provide versions that the client can use directly.

如果有在其中将选择性成员导入/导出用于虚函数的类，则这些函数必须在可导出接口或已定义内联中(对客户端可见)。If you have a class in which you are using selective member import/export with virtual functions, the functions must be in the exportable interface or defined inline (visible to the client).

如果将成员定义为 dllexport ，但不将其包括在类定义中，则会生成编译器错误。If you define a member as dllexport but do not include it in the class definition, a compiler error is generated. 必须在类头中定义成员。You must define the member in the class header.

尽管允许将类成员定义为 dllimport 或 dllexport ，但不能重写在类定义中指定的接口。Although the definition of class members as dllimport or dllexport is permitted, you cannot override the interface specified in the class definition.

如果在声明的类定义的主体以外的位置定义成员函数，并且 dllexport dllimport 如果此定义与类声明) 中指定的不同，则会生成警告(如果该函数定义为或 ()。If you define a member function in a place other than the body of the class definition in which you declared it, a warning is generated if the function is defined as dllexport or dllimport (if this definition differs from that specified in the class declaration).

结束 Microsoft 专用END Microsoft Specific

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