Purpose of code '#define'

[sudhansukanungo]

HI All.
Im new in c++. some wher i saw the code

#define DECLAREDLL
 
class DECLAREDLL myclass
{
};
 

why DECLAREDLL before class name?
can any body solve my doubt?
THANK U.

[gpraghuram]

HI All.
Im new in c++. some wher i saw the code

#define DECLAREDLL

class DECLAREDLL myclass
{
};

why DECLAREDLL before class name?
can any body solve my doubt?
THANK U.

I cant figure out why they have used this but the DECLAREDLL will be nothing when the proprocessing happens and the declartion changes to class myclass{};

Raghuram

[sicarie]

HI All.
Im new in c++. some wher i saw the code

#define DECLAREDLL

class DECLAREDLL myclass
{
};

why DECLAREDLL before class name?
can any body solve my doubt?
THANK U.

That is a declaratory statement. I have seen them usually be

#ifndef DECLAREDLL
#define DECLAREDLL
class....
 

This means "if not defined" and then it will search for another declaration of whatever is in the space of 'DECLAREDLL', then it will define it for the project. I'm not sure of the scope of this - but I believe it will be for the namespace. The code you posted will not check for another instance of 'declaredll' so another version might be overwritten.

PS - In the future, please use code tags. Thanks!

[kreagan]

HI All.
Im new in c++. some wher i saw the code

#define DECLAREDLL
 
class DECLAREDLL myclass
{
};
 

why DECLAREDLL before class name?
can any body solve my doubt?
THANK U.

I am also new to c/c++, but have worked with a couple of instances where you need to define the file. Usually you will use:

#ifndefine DECLAREDLL
#define DECLAREDLL

As someone said in this post, you define DECLAREDLL if DECLAREDLL is not defined. What this does, if this file is not loaded in memory, the compiler will do so. If there is an instance where this file is loaded in memory, the compiler will access the file that has been loaded instead of loading the code again. (At least that's how it has been told to me.

Another use can be when you create DLLs.

#ifdef DECLAREDLL
    #define    DECLAREDLL_DLL __export __cdecl
#else
    #define DECLAREDLL_DLL __import __cdecl
#endif

which basically states that if DECLAREDLL is define (see above), insert __export __cdecl where DECLAREDLL_DLL typed, otherwise, use __import __cdecl.

Hope that make sense.
Good luck.

[dmjpro]

All these are for Conditional Compilation.
You know it?

Kind regards,
Dmjpro.

[sicarie]

Found this today, it has another explanation of that in there.