在类定义的开始，这个宏是什么？ [英] What is this macro for at the beginning of a class definition?

问题描述

我正在查找库的源代码，并且许多类使用以下形式定义：

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。 。 。

Jumping to the macro definition . . .

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

这是什么可以做到的，

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

推荐答案

它对我来说非常喜欢导出宏，这是在Windows上构建共享库当使用MSVC编译时，在构建库时，必须在该位置插入 __ declspec（export）， __ declspec（import）当构建其客户端时。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

code> COMPILING_DLL 用于库项目，对所有其他项目保留未定义。如果你不是在Windows或编译一个静态库，你需要定义它的空白，就像在你的问题。

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。其他Windows编译器使用自己的关键字，而不是 __ declspec（dllimport），但原则仍然存在。

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

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