Why/when is __declspec( dllimport ) not needed?

问题

In a project using a server.dll and a client.exe, I have dllexported a server symbol from the server dll, and not dllimported it into the client exe.

Still, the application links, and starts, without any problem. Is dllimport not needed, then???

Details:

I have this 'server' dll:

// server.h
#ifdef SERVER_EXPORTS
  #define SERVER_API __declspec(dllexport)
#else
  #define SERVER_API // =====> not using dllimport!
#endif
class  SERVER_API CServer {
   static long s;
   public:
   CServer();
};

// server.cpp
CServer::CServer(){}

long CServer::s;

and this client executable:

#include <server.h>
int main() {
   CServer s;
}

The server command line:

cl.exe /Od  /D "WIN32" /D "_DEBUG" /D "_WINDOWS" /D "_USRDLL" 
 /D "SERVER_EXPORTS" /D "_UNICODE" /D "UNICODE" /D "_WINDLL" 
 /Gm /EHsc /RTC1 /MDd /Yu"stdafx.h" 
 /Fp"Debug\server.pch" /Fo"Debug\\" /Fd"Debug\vc80.pdb" 
 /W3 /nologo /c /Wp64 /ZI /TP /errorReport:prompt

cl.exe /OUT:"U:\libs\Debug\server.dll" /INCREMENTAL:NO /NOLOGO /DLL 
/MANIFEST /MANIFESTFILE:"Debug\server.dll.intermediate.manifest" 
/DEBUG /PDB:"u:\libs\Debug\server.pdb" 
/SUBSYSTEM:WINDOWS /MACHINE:X86 /ERRORREPORT:PROMPT 
kernel32.lib user32.lib gdi32.lib winspool.lib comdlg32.lib advapi32.lib 
shell32.lib ole32.lib oleaut32.lib uuid.lib odbc32.lib odbccp32.lib

Client command line:

cl.exe /Od /I "..\server" 
 /D "WIN32" /D "_DEBUG" /D "_CONSOLE" /D "_UNICODE" /D "UNICODE" 
 /Gm /EHsc /RTC1 /MDd /Fo"Debug\\" /Fd"Debug\vc80.pdb" /W3 /c /Wp64 /ZI /TP 
 .\client.cpp

cl.exe /OUT:"U:\libs\Debug\Debug\client.exe" /INCREMENTAL 
/LIBPATH:"U:\libs\Debug" 
/MANIFEST /MANIFESTFILE:"Debug\client.exe.intermediate.manifest" 
/DEBUG /PDB:"u:\libs\debug\debug\client.pdb" 
/SUBSYSTEM:CONSOLE /MACHINE:X86 
server.lib kernel32.lib user32.lib gdi32.lib winspool.lib comdlg32.lib 
advapi32.lib shell32.lib ole32.lib oleaut32.lib uuid.lib odbc32.lib odbccp32.lib

回答1:

It isn't required. It is an optimization, a hint to the compiler that the DLL is going to export the function pointer directly rather than just an entry in the IAT of the DLL. The exported function pointer for a function named foo() will be __imp_foo. Which allows it to generate better code, saving a function pointer load from the IAT and an indirect jump. It is a time optimization, not space.

This blog post has the details.

来源：https://stackoverflow.com/questions/4489441/why-when-is-declspec-dllimport-not-needed

标签

c++

dllimport

visual-c++-2005

declspec