What does the code "DECLDIR __declspec(dllexport)" really do? - c

Question

#ifndef _DLL_TUTORIAL_H_
#define _DLL_TUTORIAL_H_
#include <iostream>
#if defined DLL_EXPORT
#define DECLDIR __declspec(dllexport)
#else
#define DECLDIR __declspec(dllimport)
#endif
extern "C"
{
DECLDIR int Add( int a, int b );
DECLDIR void Function( void );
}
#endif
What does the code DECLDIR __declspec(dllexport) really do?

Answer 1

In the Microsoft world, __declspec(dllexport) makes a function or class callable from outside the DLL.
When you create a DLL, by default, any functions defined within the DLL are only callable from that same DLL. You cannot call that function from an executable or a different DLL.
If you want your a function to be called from outside the DLL, you need to export it by adding __declspec(dllexport).
One way to think about it is that __declspec(dllexport) marks a function as being part of a DLL's public interface.
While you didn't ask about __declspec(dllimport) that is sort of the opposite. When calling a function in a different DLL, your DLL needs to know that it's part of a different DLL's public interface so it can properly handle the call (calling a function in a different DLL requires more complex code that calling a function in yourself).

Answer 2

It defines the DECLDIR macro constant to be __declspec(dllexport). dllexport is for exporting functions from DLLs. Here's a quote from this page:
These attributes explicitly define the
DLL's interface to its client, which
can be the executable file or another
DLL. Declaring functions as dllexport
eliminates the need for a
module-definition (.DEF) file, at
least with respect to the
specification of exported functions.
Note that dllexport replaces the
__export keyword.
If a class is marked
declspec(dllexport), any
specializations of class templates in
the class hierarchy are implicitly
marked as declspec(dllexport). This
means templates are explicitly
instantiated and its members must be
defined.
__declspec, by the way, is explained here.