Let us imagine that the small library that we have introduced has become hugely popular, with many people using it. However, some clients would also like a static library to be available with the installation. Other clients have noticed that all symbols are visible in the shared library. Best practices dictate that shared libraries only expose the minimal amount of symbols, thus limiting the visibility to the outside world of objects and functions defined in the code. We want to make sure that by default all symbols defined in our shared library are hidden from the outside world. This will force contributors to the project to clearly delimit the interface between the library and external codes, since they will have to explicit mark all symbols that also meant to be used outside of the project. As such, we wish to do the following:

• Build both a shared and static library from the same set of source files.
• Ensure that only the visibility of symbols in the shared library is properly delimited.

Recipe 3, Building and linking static and shared libraries, in Chapter 1, From a Simple Executable to Libraries, already showed that CMake offers functionality to achieve the first point in a platform-independent way. We did not, however, address the issue of symbol visibility. We will revisit both points with the present recipe.