如何在编译时关闭功能？

Question

I have a C++ application I'm developing. I want to release a free version and a paid version, with the production version adding features on top of the existing free version.

What's the best way to compile features depending on the release type? Ideally I would pass a flag to CMake so while building the binary it would also include any code I need in the production version.

Answer 1

You answered your question yourself :) Pass a flag to your compiler and in your code gate the features with:

#ifdef YOUR_FLAG
//your extra logic
#elif
//in case YOUR_FLAG is not defined
#endif

I used exactly the same approach in a game I developed - in free version ads are enabled and in paid version that code gets excluded by preprocessor.

Also you can use run-time checks in case it's more appropriate for some specific task, but as it's mentioned in comments to the question - code is there and tampering with memory (flipping your flag) will possible allow to gain access to restricted features.

Answer 2

You add them as licensed plugins.

That is:

1. Keep the code seperate. Heck, you can even license the "free" code as open-source and still keep your business model.
2. Make them phone home or verify some crypt key so that you know said person is licensed.
3. Make sure the rest of the code can live with its paid for guts removed. Comfort if necessary.