<p>I've been reading many a tutorial/article on unmanaged DLLs in C++. For the life of me, however, I cannot seem to grasp the concept. I'm easily confused by the seeming disagreement about whether it needs a header file, how to export it, whether I need a .lib file and what have you.</p>

<p>So, let's assume I have just a function like so:</p>

<pre><code>public int calculateSquare(int num)
{
    return num*num;
}
</code></pre>

<p>Ignoring the actual code, what do I require to make this simple function, by itself, into a DLL which I can then call? Do I just add __dllexport or whatever it is to the first line or do I require a header? I am perplexed by all of this.</p>

I cannot stress this enough, the C++ compiler does not see header files, after the preprocessor is done, there's just one big source file ( also called the compilation unit ). So strictly you don't need a header to export this function from a dll.
What you do need is some form of conditional compilation to export the function in the dll that you are compiling and to import it in the client code.
Typically this is done with a combination of macros and header files. You create a macro called MYIMPORTEXPORT and through the use of macro conditional statements you make it work like __declspec ( dllexport ) in the dll, and __declspec( dllimport ) in the client code.
in file MYIMPORTEXPORT.h
```
```
#ifdef SOME_CONDITION
#define MYIMPORTEXPORT __declspec( dllexport )
#else
#define MYIMPORTEXPORT __declspec( dllimport )
#endif
```
```
in file MyHeader.h
```
```
#include <MyImportExport.h>

MYIMPORTEXPORT public int calculateSquare(int num)
{
    return num*num;
}
```
```
in dll .cpp file
```
```
#define SOME_CONDITION

#include <MyHeader.h>
```
```
in client code .cpp file
```
```
#include <MyHeader.h>
```
```
Of course you also need to signal to the linker that you are building a dll with the
/DLL option
.
The build process will also make a .lib file, this is a static lib - called the stub in this case - which the client code needs to link to as if it were linking to a real static lib. Automagically, the dll will be loaded when the client code is run. Of course the dll needs to be found by the OS through its lookup mechanism, which means you cannot put the dll just anywhere, but in a specific location.
Here
is more on that.
A very handy tool to see whether you exported the correct function from the dll, and whether the client code is correctly importing is
dumpbin
. Run it with /EXPORTS and /IMPORTS respectively.