Re: [anjuta-list] Is something like this exists for gcc/configure & friends

[Igor Korot <ikorot01 gmail com>]

Hi, Jens,

On Fri, Apr 15, 2016 at 5:25 AM, Jens Mühlenhoff <j muehlenhoff gmx de> wrote:
> Hello Igor.
>
> Am 14.04.2016 um 19:11 schrieb Igor Korot:
> > extern "C" makes it work as now the function will be exported without
> > name mangling.
> >
> > However, since I need to return a C++ object this will not work as the
> > function has C-linkage.
>
> What do you mean with will not work?
>
> A pointer is a pointer. You can return a pointer to a C++ object using an extern "C" function.

Consider following piece of code:

class Base
{
public:
    virtual void MyPureVirtual() = 0;
};

class Derived : public Base
{
public:
    virtual void MyPureVirtual() { };
};

extern "C" Base *MyFunc()
{
    Base *pd = new Derived();
    return pd;
}

(NOTE: MSVC-specifics removed)

Using extern "C" doesn't help since C does not know anything about
inheritance, C++
objects and C++ interfaces.

So when running this code I will get just a plain old pointer in the
memory (an address).

Now when I remove the 'extern "C"' it will work because I'm using C++
features with the
C++ code.

I can see it clearly in the debugger - running the original code when
I get out of MyFunc()
all I see is an untyped pointer to memory (an address). When I comment
out ' extern "C" '
the caller of MyFunc() does see a pointer to base which is a Derived object.

> > So with MSVC I can use "/EXPORT:" link option to assign a name to the
> > exported function.
> >
> > So, if I give a linker following "/EXPORT:MyFunc", MSVC will find the
> > appropriate function
> > and will export it with the name "_MyFunc", as if I use 'extern "C"'
> > instead of "@v MyFunc@v".
>
> Isn't this even more complicated then declaring a extern "C" block?

Nope - it is actually easier.
Instead of writing ' extern "C" ' on each function I just give a
linker " /EXPORT:MyFunc "
option (without a quotes). Then the code will behave as if ' extern
"C" ' was used.

I just need to know if something like this exists for gcc/link.

> > You mean I can write something like this:
> >
> > #ifdef WIN32
> > __declspec(dllexport)
> > #endif
> > MyClass *MyFunc()
> > {
> > }
> >
> > and then do this:
> >
> > void *dl_ptr = dlopen("my_dll" );
> > MYFUNC func = (MYFUNC) dlsym( dl_ptr, "MyFunc" );
> >
> > and it will work. This code will find MyFunc inside libmy_dll.so
> > library and everything will be good?
>
> That won't work.
>
> PS: Have you read this page?
>
> http://www.tldp.org/HOWTO/html_single/C++-dlopen/
>
> You can either flatten your class and make the instance an opaque pointer (which is great for using C++ 
> classes in languages other than C++) or export an abstract class (also known as an interface) to be 
> consumed by C++ clients.

I just went thru the article mentioned.
It appears it is for *nix only.

I need a generic solution for the problem explained above.

However I will look at it more when I come home from work.

Thank you.

> --
> Mit freundlichen Grüßen
> Jens Mühlenhoff
>
>
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