视窗C++: 外部 &__declspec(dllimport)

Windows amp; C++: extern amp; __declspec(dllimport)(视窗C++: 外部 amp;__declspec(dllimport))

问题描述

extern"和__declspec(dllimport") 之间有什么区别/关系?我发现有时需要同时使用它们，有时一个就足够了.

What is the difference/relationship between "extern" and "__declspec(dllimport")? I found that sometimes it is necessary to use both of them, sometimes one is enough.

我说的对吗:

1. extern"用于静态链接库，
2. "__declspec(dllimport)" 用于 DLL(动态链接库)，
3. 对于各自的链接类型，两者实际上都做了相同的工作，
4. 在使用导入库(有助于与 dll 链接的小 .lib 文件)时，您需要同时使用这两者吗?

推荐答案

extern 表示实体具有外部链接，即在其翻译单元(C 或 CPP 文件)之外可见.这意味着相应的符号将被放置在目标文件中，因此如果该目标文件成为静态库的一部分，它也将是可见的.但是，extern 本身并不意味着一旦目标文件成为 DLL 的一部分，该符号也将可见.

extern means that the entity has external linkage, i.e. is visible outside its translation unit (C or CPP file). The implication of this is that a corresponding symbol will be placed in the object file, and it will hence also be visible if this object file is made part of a static library. However, extern does not by itself imply that the symbol will also be visible once the object file is made part of a DLL.

__declspec(dllexport) 表示符号应该从 DLL 中导出(如果它确实是 DLL 的一部分).它在编译进入 DLL 的代码时使用.

__declspec(dllexport) means that the symbol should be exported from a DLL (if it is indeed made part of a DLL). It is used when compiling the code that goes into the DLL.

__declspec(dllimport) 表示符号将从 DLL 中导入.在编译使用 DLL 的代码时使用它.

__declspec(dllimport) means that the symbol will be imported from a DLL. It is used when compiling the code that uses the DLL.

因为在编译 DLL 本身以及将使用 DLL 的客户端代码时通常使用相同的头文件，所以习惯上定义一个解析为 __declspec(dllexport) 的宏编译 DLL 时和 __declspec(dllimport) 时编译其客户端，如下所示:

Because the same header file is usually used both when compiling the DLL itself as well as the client code that will use the DLL, it is customary to define a macro that resolves to __declspec(dllexport) when compiling the DLL and __declspec(dllimport) when compiling its client, like so:

#if COMPILING_THE_DLL
    #define DLLEXTERN __declspec(dllexport)
#else
    #define DLLEXTERN __declspec(dllimport)
#endif

回答您的具体问题:

1. 是的，仅 extern 就足以用于静态库.
2. 是的 -- 声明还需要一个 extern (看这里的解释).
3. 不完全——见上文.
4. 您并不严格需要带有 __declspec(dllimport) 的 extern(请参阅上面链接的说明)，但由于您通常会使用相同的标头extern 文件，因为在编译 DLL 时需要它.

1. Yes, extern alone is sufficient for static libraries.
2. Yes -- and the declaration also needs an extern (see explanation here).
3. Not quite -- see above.
4. You don't strictly need the extern with a __declspec(dllimport) (see explanation linked to above), but since you'll usually be using the same header file, you'll already have the extern in there because it's needed when compiling the DLL.

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