class definition syntax question

I found this format in some, but not all, of the class heads in the svn source code for the PLplot plotting library (specifically, in headers and source files in the bindings/c++ and bindings/wxwidgets directories).

Code:

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class PLDLLIMPEXP_CXX plstream {

// ...
};

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I don't recall ever having seen anything between the keyword "class" and the classname. What is that term "PLDLLIMPEXP_CXX" and what does it do?

It is a macro. Something like this can be found by googling:

Code:

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> #ifdef USINGDLL
>  #if defined(WIN32)
>    /* Visual C/C++, Borland, MinGW and Watcom */
>    #if defined(__VISUALC__) || defined(_MSC_VER) ||
> defined(__BORLANDC__) || defined(__GNUC__) || defined(__WATCOMC__)
>      #define PLDLLEXPORT __declspec(dllexport)
>      #define PLDLLIMPORT __declspec(dllimport)
>    #else
>      #define PLDLLEXPORT
>      #define PLDLLIMPORT
>    #endif
>  #elif defined(__CYGWIN__)
>    #define PLDLLEXPORT __declspec(dllexport)
>    #define PLDLLIMPORT __declspec(dllimport)
>  #elif defined(__GNUC__) && __GNUC__ > 3
>    /* Follow ideas in http://gcc.gnu.org/wiki/Visibility for GCC
> version 4.x
>      * The following forces exported symbols specifically designated 
> with
>      * PLDLLEXPORT to be visible.  */
>    #define PLDLLEXPORT __attribute__ ((visibility("default")))
>    #define PLDLLIMPORT
>  #endif
> #endif

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How and why would a macro be expanded in the middle of a class head? I've only seen

Code:

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class [classname] { ... };

---

I've never seen

Code:

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class [blah, blah, blah] [classname] { ... };

---

What sort of "blah, blah, blah" can go in there?

OK, I can answer the "what", but not the "how" or the "why". I found various definitions in another file, include/pldll.h. Depending on the compiler, the resulting code may be any of these:

Code:

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class _declspec( dllexport ) plstream { //...
class _declspec( dllimport ) plstream { //...
class __attribute__ (( visibility( "default" ))) plstream { //...
// and so on...

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but I'm still left with my original question. What are these terms that are placed between "class" and "classname", and (in general) what purpose do they serve?

Or, to put it another way, what should I google to find out more about this?

They export or import classes from or in a dll, basically.
You should be able to find easy reference of __declspec(dllexport) (and import respectively). The only way they differ from functions is that they export all the functions and variables inside the class so the class can be used in an application from the DLL.

These are compiler-specific ways to compile that class into a DLL, or something.

dllexport, dllimport (C++)

Hmm. I'm familiar enough with using libraries, but not clear on the meaning of importing and exporting wrt libraries, so I guess I'll have to read up on how libraries work under the hood. Thanks.

The basic idea is that the code is stored inside a file of its own.
Multiple processes can load this and use this shared code. Moreover, a dynamic linked library is shared between processes, so the dll is loaded into memory only once. So multiple processes can use the code without having the code stored in their executables.

But processes can only access what you "export" from the dll. And to use it, the processes need to "import" it from the dll. That's the basic story.

Quote:

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Originally Posted by Elysia

The basic idea is that the code is stored inside a file of its own.
Multiple processes can load this and use this shared code. Moreover, a dynamic linked library is shared between processes, so the dll is loaded into memory only once. So multiple processes can use the code without having the code stored in their executables.

But processes can only access what you "export" from the dll. And to use it, the processes need to "import" it from the dll. That's the basic story.

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That's helpful. Thanks again.