Question

I’m trying to compile some other people’s wrapper program on a large system, in which it’s using a global object from another file, the definition of the class is like:

class __declspec(dllexport) A
{
...
static A * instance;
}

And in my separate source file (which I want to build to a .exe), I included the header file A.h, and declare at the beginning like this

A * A::instance;

to access the data in A.

The problem is, this piece of code can be built successfully on Linux, now when I try to build it in windows NT, it will have issues.

1. If I leave it like this, the compiler will complain with a C2491 error, definition of dllimport function not allowed.

2. If I remove this line of A declaration, I’ll get a lot of error LNK2001: unresolved external symbol.

Not quite sure what is the problem here, I did some research online, but couldn’t find a good answer, I myself is not that familiar with dll import/export and some C++ tricks either. Can some one provide some insights on the problem or which direction should I spend my time on to figure this out?

Or say, if I already have a static object like this, and I want to write another program to access it, how exactly should I do?

Please also correct me if I made any mistakes here. Thanks!

Answer 1

I think the keyword you need is extern, as in:

extern A *A::instance;

This says the variable A::instance is defined somewhere else, but you’re telling the compiler about it so you can use it by name in this file.