Reuse C++ Header files

Question

Guys, I have a Visual C++ solution with 2 projects AlgorithmA & AlgorithmB and both share a common header file RunAlgo.h with the class declaration. Each project in the solution has its own unique implementation for the header file.

I am trying to compile a DLL out of the common header file RunAlgo.h and add reference to this DLL in the projects AlgorithmA & AlgorithmB. I have then included separate RunAlgo.cpp definition file in both my projects. The problem is that I am getting linker errors while compiling the new DLL project which has only the header file.

So, the question is

1. Can a header file with only class declaration be compiled into a DLL (Similar to class library containing an Interface in C#)?
2. For the above scenario, is there a better approach to reuse the common Header file among projects?
3. Should the above method work (re-check my code?)

Answer 1

To 1.:

No, free-standing header files never end up in a dll. Header files are included in implementation files and that's how they are compiled. Header files are usually distributed along with a dll or library if you want to allow third parties to link against it.

To 2.:

Why don't you declare an abstract base class with the interface for the algorithm and provide two different implementations by defining two subclasses (AlgorithmA and AlgorithmB) deriving from the base class? I don't get why you want to different DLLs.

To 3.:

No, it shouldn't. See point 1.

Answer 2

I think that to make the header file stand-alone (an interface), you'll need to make all the methods pure virtual.

Answer 3

Can a header file with only class declaration be compiled into a DLL

No, headers typically include only declarations. Declarations when compiled don't produce any machine code, so resulting DLL would be empty.

For the above scenario, is there a better approach to reuse the common Header file among projects?

Reusing header is fine. In fact, every library has it's set of headers that you need to include in projects using that library.

I don't know much Visual C++, but I think you could make third project, containing common parts (i.e. RunAlgo.h header), and mark it as a dependency for AlgorithmA and AlgorithmB projects.

Answer 4

1 & 3: No, that doesn't make sense in C++.

Libraries (dynamic or otherwise) are only used during linking. During compilation declarations must be visible to the compiler in source-code form. This is why, for example, you have to explicitly #include standard library headers in addition to linking against the standard library.

2: What you're already doing is basically the only solution. Put the common header files in their own directory, and add that directory to the include path of each of the two projects.

Answer 5

Use 2 namespaces in C++ to write 2 different implementation with the same header file

namespace ImplementationA
{
}
namespace ImplementationB
{
}

When you want to use 1st implementation

using implementationA;

or

implementationA::function1();