窗户&C++:extern &__declspec(dllimport)

2021-12-25 00:00:00 windows dll linker c++

extern"和__declspec(dllimport") 之间的区别/关系是什么?我发现有时候需要两个都用，有时候一个就够了.

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.

我说得对吗:

  1. extern"用于静态链接库，
  2. "__declspec(dllimport)" 用于 DLL(动态链接库)，
  3. 对于各自类型的链接，两者实际上都做同样的工作，
  4. 当您使用导入库(有助于与 dll 链接的小型 .lib 文件)时，您需要同时使用两者吗?

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) 表示应该从 DLL 导出符号(如果它确实是 DLL 的一部分).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) 表示将从 DLL 导入符号.在编译使用 DLL 的代码时使用.

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

因为通常在编译 DLL 本身以及将使用该 DLL 的客户端代码时使用相同的头文件，所以习惯上定义一个解析为 __declspec(dllexport) 的宏编译 DLL 时，__declspec(dllimport) 编译其客户端时，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，对于静态库，仅 extern 就足够了.
  2. 是的――声明还需要一个 extern (参见此处的说明).
  3. 不完全――见上文.
  4. 您并不严格需要带有 __declspec(dllimport) 的 extern(请参阅上面链接的解释)，但因为您通常会使用相同的标头文件，您将在其中拥有 extern，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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