I've tried to find my answer here but the other "similar topics" were different or just wasn't able to find the right one.
This is mainly a design (and some c++) question about the dependency between the different projects. Let me describe my problem:
So I've already used Unity and UE4 and I like the approach(es) they're using. They have a (game) engine an editor application and a user-created gameplay code. You can use your own gameplay code in the editor - you can assign scripts to objects, run/pause/step/stop (and debug!) the actual game in the editor.
I've checked the UE4 source code (which is actually pretty big) and the way this works is they're using custom build and preprocess tools (like Unreal Header Tool) and they've implemented everything they're using. As I could see, they're not using any STL class/function, they've implemented their own. This way they can make the "engine project" shareable, so they can build the libraries to dlls and that's it.
I'm using the same component-based approach. Different components can be attached to an object, and an important component base class is the Behavior. The "gameplay components" inherits from the Behavior base class.
So back to the problem: I (will) have 3 different projects:
The Engine is the core, that is a static library with every required thing except the game-specific code, of course. The Gameplay depends on the Engine and is built as an executable (*.exe). This contains the actual gameplay code while using the engine.
And here comes the real problem: the Editor depends on both the Engine and the Gameplay projects. Why? The Editor is using the Engine to create the window, draw buttons, log messages, etc. However it would be nice to be able to assign the "scripts" (in the terms of Unity) to objects and that would be the best if I could "play the game" in the editor, like in Unity.
Why is this a problem at all? Because I'm not able to use a shared library for the Engine project. Why? Because I should dll-export a lot of things which is not even possible while using the STL. It's a known "fact" that most of the STL objects cannot be used with dll-export (and I should write a lot of export code now, heheh). Also I don't want to use any bigger library like Boost or Qt.
However if I'm using static library for the Engine project then the Editor and the Gameplay will have its "own copy" of the Engine.
Maybe a half-solution could be the following: I could have a 4th project for the gameplay code exports which would be a dll. This project should only contain some functions which would be used in the Editor - eg. query the list of the available scripts with the visible variables, etc. However the game code is not actually available in the editor, so any "play in the editor" wouldn't be possible.