Correctly defining DLL-interfaces with C++11/14 - c++

Question

I've read several times that passing STL objects like vector and string outside of a DLL boundary is bad practice because different compiler versions can generate different code for STL objects. Therefore, you should design a C-style interface and not pass STL objects at all. However, there are still some things unclear to me:
1. What is the 'boundary' of a DLL?
Is it right to say, that the boundary is where code is beeing compiled on DLL side? What if I define a .h file inside a DLL (f.e. to write a factory class) and use that header file in a different project? Is that .h file inside or outside the boundary of the DLL and why?
2. What is contained in a DLL?
Let' say I have a class Foo:
class Foo
{
public:
__declspec(dllexport) void f1(); //instantiates v1 inside function
private:
unique_ptr<vector<int>> v1 = nullptr;
}
If I only mark the function f1() with __declspec(dllexport), only this function should be contained in the DLL. How does the code inside f1() know what v1 is if v1 isn't contained in the DLL?
3. Passing objects out of a DLL-boundary using unique_ptr
I'm using unique_ptr almost everytime in my project. From what I understand, returning a unique_ptr from a DLL would be bad practice because unique_ptr is an STL object. How can I instantiate an object inside the DLL and return a unique_ptr to it?
4. Why does defining interfaces or using PIMPL help to define an DLL interface?
I still have to convert my STL classes to C-style objects. And in the project using the DLL, I would have to somehow wrap the C-style objects inside STL classes again. I don't see any advantage of using interfaces or PIMPL in this case.
Also, if I define an interface (class with pure virtual functions), wouldn't this have the same effect as just declaring the functions in my class with __declspec(dllexport)?
class IFoo
{
public:
virtual ~IFoo() = 0 {};
virtual void f1() = 0;
}
class Foo : public IFoo
{
public:
void f1();
//__declspec(dllexport) void f1(); //why use an interface if I can just declare the functions like this?
}
How is the DLL-STL problematic solved in modern C++ 11/14 libraries? Are there any modern open-source libraries that I can have a look at?

Answer 1

Unfortunately STL types aren't consistent across compilers. Even different versions of Visual Studio have differences.
The boundary is where the code is compiled. If you have an implementation in a header file in your library, then the compiler used to compile the EXE will compile the code. This is potentially very bad because what the code in the EXE thinks is the data is different to what the code in the DLL thinks is the data. (You need to look out for differences like this especially if you have #ifs in a struct definition and you need to be explicit about packing).
The only way to be sure is to define all your own types (being careful of packing) and not use STL. This is what DLL libraries usually do.
Interfaces can enable the user to dynamically link to the library. Using __declspec(dllexport) requires a static linking; that is the EXE has to link to the .lib generated when you compiled the DLL to be able to access all the functions. This means amongst other things you can't update the DLL without the EXE having to be recompiled (probably - you can get away with this in some circumstances, but it's not a good idea).
By dynamically linking you can update the DLL or add functionality to the DLL without relinking the EXE as long as you don't change your interfaces. The EXE might call LoadLibrary() on the DLL and GetProcAddress() to access one function that returns an interface. Everything else including data types passed as parameters are interfaces (i.e. contain only pure virtual functions) or simple structs. This is how the basic level of COM works.

Answer 2

To answer question 2, when you declare something as __declspec(dllexport) you are stating that this is part of the interface to the DLL - something that is accessible to the component that loads the DLL. Anything declared without __declspec(dllexport) should be present within the DLL but will not be available to be called/used by an external component.