dllexport and dllimport question for shared libraries on windows - c++

Question

I encountered a strange behavior for an msvc c++ build while compiling two shared libraries and an executable. The complete setup can be found on GitHub
Information:
platform toolset is v143
4 projects: 1 header only, 2 DLL`s, 1 executable
The 4 projects are:
HeaderOnlyInterface (header only)
Implementation (DLL)
Factory (DLL)
SharedLibsTest (EXE)
Projects 2 - 4 can include all header files from HeaderOnlyInterface.
HeaderOnlyInterface consists of two header files:
DLLExport.h
#pragma once
#ifdef MAKEDLL
# define EXPORT __declspec(dllexport)
#else
# define EXPORT __declspec(dllimport)
#endif
IFoo.h
#pragma once
#include "DLLExport.h"
class EXPORT IFoo
{
public:
virtual ~IFoo() = 0 {}
virtual void Bar() = 0;
};
The next project is Implementation which implements the defined abstract class. This class has MAKEDLL defined as PREPROCESSOR macro. I only show the header because the source is unnecessary:
Foo.h
#pragma once
#include <IFoo.h>
class EXPORT Foo : public IFoo
{
public:
void Bar() override;
};
Going on to Factory, which also defines MAKEDLL and links against Implementation.lib, I have these two files:
FooFactory.h
#pragma once
#include <IFoo.h>
class EXPORT FooFactory
{
public:
static IFoo* Create();
};
FooFactory.cpp
#include "FooFactory.h"
#include <Foo.h>
IFoo* FooFactory::Create()
{
return new Foo();
}
And at the last, I have a main method in an Executable, which links against Factory.lib:
#include <FooFactory.h>
int main()
{
auto* foo = FooFactory::Create();
foo->Bar();
delete foo;
}
For me, the strange thing is, that everything compiles and works fine until now. I have no compile or linker warnings.
The strange thing for me is the compilation and linkage of Factory.lib (and NOT against Implementation.lib).
This library imports a class and exports it also and I do not know why it works. I try to explain my thinking by providing in (pseudo-) code what happens if FooFactory.cpp is pre-processed:
class __declspec(dllexport) IFoo
{
public:
virtual ~IFoo() = 0 {}
virtual void Bar() = 0;
};
class __declspec(dllexport) FooFactory
{
public:
static IFoo* Create();
};
class __declspec(dllexport) Foo : public IFoo
{
public:
void Bar() override;
};
IFoo* FooFactory::Create()
{
return new Foo();
}
The macro is the same for all compiled libs meaning that Factory.lib is exporting all classes. But it does not compile the implementation of Foo::Bar(). Also, it is somehow able to import the implementation because the code works fine. I am not sure why it works, and what really happens under the hood.
If someone could lighten me up on this behavior and also maybe could explain why I should do it not like this or maybe why this is a good idea to do it like this, that would be really helpful.
Thanks!