How it's done...

In this recipe, we create an example class that could suit as a member of a typical header-only library. The target is to give it a static member and instantiate it in a globally available manner using the inline keyword, which would not be possible like this before C++17:

1. The process_monitor class should both contain a static member and be globally accessible itself, which would produce double-defined symbols when included from multiple translation units:

       // foo_lib.hpp 
       
       class process_monitor { 
       public: 
           static const std::string standard_string 
               {"some static globally available string"}; 
       };
       
       process_monitor global_process_monitor;

1. If we now include this in multiple .cpp files in order to compile and link them, this would fail at the linker stage. In order to fix this, we add the inline keyword:

       // foo_lib.hpp 

       class process_monitor { 
       public: 
           static const inline std::string standard_string 
               {"some static globally available string"}; 
       };

       inline process_monitor global_process_monitor;

Voila, that's it!