c++ - 类声明中的BOOST_FILESYSTEM_DECL是什么意思？

This question already has answers here:

Macros in the middle of a class or function declaration

(4个答案)


4年前关闭。




我发现以下代码:

class BOOST_FILESYSTEM_DECL path
{

};

通常，类是通过以下方式定义的:

class Baloon
{

};

在类声明中如何有两个术语？有人知道BOOST_FILESYSTEM_DECL用于什么(增强库)吗？

最佳答案

如果您看看how it is defined，那么一切都会很清楚:

#ifdef BOOST_HAS_DECLSPEC // defined in config system
// we need to import/export our code only if the user has specifically
// asked for it by defining either BOOST_ALL_DYN_LINK if they want all boost
// libraries to be dynamically linked, or BOOST_FILESYSTEM_DYN_LINK
// if they want just this one to be dynamically liked:
#if defined(BOOST_ALL_DYN_LINK) || defined(BOOST_FILESYSTEM_DYN_LINK)
// export if this is our own source, otherwise import:
#ifdef BOOST_FILESYSTEM_SOURCE
# define BOOST_FILESYSTEM_DECL __declspec(dllexport)
#else
# define BOOST_FILESYSTEM_DECL __declspec(dllimport)
#endif  // BOOST_FILESYSTEM_SOURCE
#endif  // DYN_LINK
#endif  // BOOST_HAS_DECLSPEC
//
// if BOOST_FILESYSTEM_DECL isn't defined yet define it now:
#ifndef BOOST_FILESYSTEM_DECL
#define BOOST_FILESYSTEM_DECL
#endif

如您所见，它是一个宏。它扩展为__declspec(dllexport)或__declspec(dllimport)或为空，具体取决于其他宏。有关decspec标识符的更多详细信息，请参见this answer。简而言之，动态链接需要平台特定的功能。