BUILD_SHARED_LIBS

As far as I know it’s best practice to not specify SHARED or STATIC for a library, but rely on BUILD_SHARED_LIBS instead.

I have to questions regarding this.

  1. Should one specify BUILD_SHARED_LIBS as an option? Up to know I never did. But some Libraries do this and when using FetchContent BUILD_SHARED_LIBS will be set to that default value as it is by default empty → This lead to nasty bugs as some libs where build static, then one lib had option(BUILD_SHARED_LIBS ON) and set it therefore to On. All a second run then all Libraries where build shared.
    → Is it wrong from the libraries to declare the option?
    → Should I declare in my project the option, so BUILD_SHARED_LIB always has a value? → It will then not be overwritten, right?

  2. Often Libraries delcare my-library and my-library-shared, so the just have two targets for both static and dynamic. This is often done so they can pass set some different values depending on whether it’s shared or not. But I think it’s also done to make packaging and building from source easier.
    → Is this bad practice? → It makes my life harder to keep track which libraries use BUILD_SHARED_LIBS and which have two targets
    → What are arguments against it? How can be argued against the “pro” arguments → I believe the different logic could just be done with if(BUILD_SHARED_LIBS), so that’s no argument. What about the “ease of use” argument?

Thanks,
Leon

It’s fine to declare it as an option if you want to change the default. As long as the minimum required CMake version is 3.13, a parent project can override it with a normal variable.

There is always a danger from FetchContent / add_subdirectory that the child project will set a cache variable that affects global state. It is always a requirement to audit the code before including it, so you know what it does.

I think having two separate targets for static and shared is an absolutely terrible idea and I wish people would stop doing it. So much do I feel this way, I wrote a blog post about it:

2 Likes

Thanks for your answer! Especially the detailed blog post, which I will use in the future to convience library authors :smiley: