这个宏在类定义的开头有什么用?

2022-01-11 00:00:00 class macros c++ c-preprocessor

我正在查看一个库的源代码，许多类是使用以下形式定义的

I'm looking through the source of a library and many classes are defined using the following form

class THING_API ClassName 
{
...

跳转到宏定义...

#ifndef THING_API
 #define THING_API   /**< This macro is added to all public class declarations. */
#endif

这有什么用，它是一种常见的技术吗?

What could this be for, and is it a common technique?

推荐答案

它看起来很像导出宏，在 Windows 上构建共享库 (.dll) 时需要它.使用 MSVC 编译时，您必须在构建库时将 __declspec(export) 放在该位置，在构建其客户端时将 __declspec(import) 放在该位置.这是这样实现的:

It looks to me very much like export macro, which is required when building a shared library (.dll) on Windows. When compiling with MSVC, You have to put __declspec(export) in that spot when building a library, and __declspec(import) when building its client. This is achieved like so: 

#if COMPILING_DLL
    #define THING_API __declspec(dllexport)
#else
    #define THING_API __declspec(dllimport)
#endif

然后您为库项目定义COMPILING_DLL，并为所有其他项目保留未定义.如果您不在 Windows 上或编译静态库，则需要将其定义为空白，就像在您的问题中所做的那样.

Then you define COMPILING_DLL for the library project, and leave it undefined for all other projects. And if you're not on Windows or compiling a static library, you need to define it blank like it's done in your question.

P.S.其他Windows编译器使用自己的关键字代替__declspec(dllimport)，但原理不变.

P. S. Other Windows compilers use their own keywords instead of __declspec(dllimport), but the principle remains.

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