Windows& C ++：extern& __declspec（dllimport） [英] Windows & C++: extern & __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.

我是对的：

1. extern用于静态链接库，


2. __ declspec（dllimport）用于DLL（动态链接库），


3. 实际上对于它们各自的链接类型都有相同的作业，


4. 您需要在使用导入库（帮助链接到dll的小型.lib文件）？

1. "extern" is for statically linked libraries,
2. "__declspec(dllimport)" is for DLL (dynamically linked libraries),
3. both do actually the same job for their respective type of linking,
4. you need to use both when you use import libraries (small .lib files that help linking with dll)?

推荐答案

extern 实体具有外部链接，即在其翻译单元（C或CPP文件）之外是可见的。这意味着，相应的符号将被放置在目标文件中，因此，如果该目标文件是静态库的一部分，它也将是可见的。然而， extern 本身并不意味着一旦将目标文件作为DLL的一部分，该符号也将是可见的。

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）表示该符号应从DLL导出（如果它确实是DLL的一部分）。它用于编译进入DLL的代码。

__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）从DLL导入。它用于编译使用DLL的代码。

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

因为同样的头文件通常用于编译DLL本身以及将使用的客户端代码DLL，通常在编译DLL和 __ declspec（dllimport） __ declspec（dllexport） c>当编译其客户端时，像这样：

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

要回答您的具体问题：

1. 是， extern


2. 是 - 且声明还需要 extern （请点击此处）。


3. 不完全 - 见上文。


4. 您不需要使用 __ declspec（dllimport）严格需要 extern 到上面），但是因为你通常使用相同的头文件，你已经有了 extern ，因为它在编译DLL时是需要的。

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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