Questions :-

• To sell a C++ game library/framework/engine, does it usually have to be open-sourced? 
• For a C++ template class (real template), it is quite hard not to be open-sourced, correct?
• If it is open-sourced, how to protect the copyright?
• How to enforce people to just use it, but not copy?
• ... I think it is practically very hard, because people can see the source code, and "rewrite" it.
• Does a shop like Unity's asset store make thing safer?  If so, how?

Arrogant assumption :-

• The game library/framework/engine is high-quality, well-documented, and it would be marvellously sell well.
• Besides some free libraries like Bullet and Ogre, it is developed all by myself.
• The library/framework/engine is not bound to any legal obligation (e.g. the most restriction is MIT).

I have these questions after reading a news  :-

• Muhagames develop a game named Thea Awakening (C#).
• The company use its own Honey Hex Framework (C#) as a library to develop the game.
• The company sell Honey Hex Framework via Unity's asset store to boost its financial position.

There are also others old examples e.g. Unity itself or Unreal, which is now open-source ... How do they protect it?

Actually, I don't plan to sell any library. I am just curious.

• To sell a C++ game library/framework/engine, does it usually have to be open-sourced? No.
• For a C++ template class (real template), it is quite hard not to be open-sourced, correct? Yes, in the sense of "the code is visible". 'Open source' usually implies the use of certain licenses, and using those is entirely your choice (unless you're using someone else's code too...)
• If it is open-sourced, how to protect the copyright? Keep an eye out for infringement, then send lawyers.
• How to enforce people to just use it, but not copy? "Open source" usually implies that copying is okay, since most open source licenses say so. But if you're selling a library with full source code but don't want it copied, it's just down to the lawyers again.
• ... I think it is practically very hard, because people can see the source code, and "rewrite" it. That's not a question.
• Does a shop like Unity's asset store make thing safer?  If so, how? No. You can sell libraries there without including the source, but you could do that anyway.

Thank.  

As a follow-up question, assume a below situation :-

• Mr.B1,B2,B3,..., and Mr.B1000 signs contract to use a game engine from Mr.C.
• In the contract, Mr.C get 10% of revenue of every game that Mr.Bx developed using the game engine. 
• Mr.C send header files and .lib to every Mr.B via email.
• Some Bs successfully deployed the game and paid C, but some Bs abandoned and used other engines instead.
• However, B1 is different from other Bs.
• Using C's game engine, B1 has developed a game without any collaborator.
• (^ Thus evidence is very hard to find.)
• After finishing the game, Mr.B1 wanted to cheat. 
• Mr.B1 obfuscated C's header and .lib, and sell a game using the engine without paying C.
• Furthermore, Mr.B1 accidentally leaked C's obfuscated game engine in the internet. 
• A random Mr.D found the copy. 
• Mr.D knew that it is actually C's, but decided to use it.  It is free.

How can C detect such case?   It is practically impossible to detect, right? 

Reason :-

• C can't access to source of every game in the market, including B1's game and D's game.
• Suppose that C can, there are a lot of game sold every day!
• If C's engine is also open-sourced (not free; it is like some contract of Unity/Unreal; easier to obfuscate), possibility to detect is almost zero.  Correct?

On 11/16/2017 at 4:58 PM, LorenzoGatti said:

... remember that your business is selling a game engine to the largest possible number of well-behaved B's, not wasting effort on quantitatively unimportant and/or imaginary pirates like B1 and D.

That totally sounds reasonable, thank.