C ++方法声明包括宏 [英] C++ method declaration including a macro

问题描述

我使用QuickFAST库，而检查它，我发现这个类声明，我似乎不真的得到！我的意思是类名前有一个宏名称！

 类QuickFAST_Export消息：public FieldSet 
  
 
 
 
也找到这个声明
  friend void QuickFAST_Export intrusive_ptr_add_ref（const Field * ptr）; 
  
，我再也没有使用这个声明！
 
 
 
如需更多资讯，请参阅这里的QuickFAST_Export.hpp 
  #ifdef _MSC_VER 
＃pragma once 
 #endif 
 #ifndef QUICKFAST_EXPORT_H 
 #define QUICKFAST_EXPORT_H 
 
 //编译库生成的时间控件。使用/ D或#define 
定义//生成或使用静态库：#define QUICKFAST_HAS_DLL = 0 
 //默认生成/使用DLL 
 //构建QUICKFAST_库：#define QUICKFAST_BUILD_DLL 
 //默认是从预建的QUICKFAST DLL导出符号
 // 
 //在QUICKFAST中使用QuickFAST_Export宏，其中需要__declspec。 
 
 #if defined（_WIN32）
 
＃if！defined（QUICKFAST_HAS_DLL）
＃define QUICKFAST_HAS_DLL 1 
＃endif / *！ QUICKFAST_HAS_DLL * / 
 
＃如果已定义（QUICKFAST_HAS_DLL）&& （QUICKFAST_HAS_DLL == 1）
＃如果定义（QUICKFAST_BUILD_DLL）
＃define QuickFAST_Export __declspec（dllexport）
＃else / * QUICKFAST_BUILD_DLL * / 
＃define QuickFAST_Export __declspec（dllimport）
＃endif / * QUICKFAST_BUILD_DLL * / 
＃else / * QUICKFAST_HAS_DLL == 1 * / 
＃define QuickFAST_Export 
＃endif / * QUICKFAST_HAS_DLL == 1 * / 
 
＃else / *！_WIN32 * / 
  
 
 
解决方案
 
 
 
 
如果定义了 QUICKFAST_HAS_DLL ，那么该类将被导出或导入并且等于1，则意味着该模块被构建为DLL。要使用外部函数，必须导出类和方法。
 
 
 
在模块中定义了 QUICKFAST_BUILD_DLL  。因此，当您构建模块时， QuickFAST_Export 扩展为 __ declspec（dllexport）。你的类定义变成：
  class __declspec（dllexport）消息：public FieldSet 
  
当您包含来自不同模块的标题时，未定义 QUICKFAST_BUILD_DLL   __ declspec（dllimport），您的类别定义为：
  class __declspec （dllimport）消息：public FieldSet 
  
 
I'm using QuickFAST library and while checking it I found this class declaration which I don't seem to really get ! I mean what does a macro name before the class name !
class QuickFAST_Export Message : public FieldSet
also I found this declaration 
friend void QuickFAST_Export intrusive_ptr_add_ref(const Field * ptr);
and again I don't get the use of this declaration !

for more info here's the QuickFAST_Export.hpp
#ifdef _MSC_VER
# pragma once
#endif
#ifndef QUICKFAST_EXPORT_H
#define QUICKFAST_EXPORT_H

// Compile time controls for library generation.  Define with /D or #define
// To produce or use a static library: #define QUICKFAST_HAS_DLL=0
//   Default is to produce/use a DLL
// While building the QUICKFAST_ library: #define QUICKFAST_BUILD_DLL
//   Default is to export symbols from a pre-built QUICKFAST DLL
//
// Within QUICKFAST use the QuickFAST_Export macro where a __declspec is needed.

#if defined (_WIN32)

#  if !defined (QUICKFAST_HAS_DLL)
#    define QUICKFAST_HAS_DLL 1
#  endif /* ! QUICKFAST_HAS_DLL */

#  if defined (QUICKFAST_HAS_DLL) && (QUICKFAST_HAS_DLL == 1)
#    if defined (QUICKFAST_BUILD_DLL)
#      define QuickFAST_Export __declspec(dllexport)
#    else /* QUICKFAST_BUILD_DLL */
#      define QuickFAST_Export __declspec(dllimport)
#    endif /* QUICKFAST_BUILD_DLL */
#  else /* QUICKFAST_HAS_DLL == 1 */
#    define QuickFAST_Export
#  endif /* QUICKFAST_HAS_DLL == 1 */

#  else /* !_WIN32 */

解决方案
It means that the class is either exported or imported, depending on which module is built.

If QUICKFAST_HAS_DLL is defined and equal to 1, it means that the module is built as a DLL. To use functionalities from the outside, classes and methods have to be exported.

Inside the module, QUICKFAST_BUILD_DLL is defined. So when you build the module, QuickFAST_Export expands to __declspec(dllexport). Your class definition becomes:
class __declspec(dllexport) Message : public FieldSet
When you include the header from a different module, QUICKFAST_BUILD_DLL is not defined, so the macro expands to __declspec(dllimport), and your class definition to:
class __declspec(dllimport) Message : public FieldSet


                        
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