CppPatterns | The web front-end for C Patterns | Architecture library

You're haven't added any of the code from foo.cpp to your compiler. Compiling with clang++ -std=c+14 main.cpp foo.cpp -o main should fix your problem.

P0142R0

5.2.3 Exported Class Properties

An occasionally vexing rule of standard C++ is that of controls access, not visibil- ity. E.g. a private member of a class is visible to, but not accessible to non-member entities. In particular, any change to a private member of a class is likely to trigger e-processing of any translation unit that depends on that class’s definition even if the change does not affect the validity of dependent units. It is tempting to solve that problem with a module system. However, having two distinct sets of rules (visibility and accessibility) for class members strikes us as undesirable and poten- tially fertile source of confusion. Furthermore, we want to support mass-migration of existing codes to modules without programmers having to worry about class member name lookup rules: if you understand those rules today, then you do not have to learn new rules when you move to modules and you do not have to worry about how the classes you consume are provided (via modules or non-modules).

Rule 3 In general, any property of a class (e.g. completeness) that is computed in the export declaration part of a module is made available to importing modules as is.

As all properties of a class are visible to the importer, any change to those properties would be visible in the importer. I don't see modules solving the problem that PIMPL solves.