To provide multiple versions of a library and let the user decide what version to use, do the following:

• Define the content of the library inside a namespace.
• Define each version of the library or parts of it inside an inner inline namespace.
• Use preprocessor macros and #if directives to enable a particular version of the library.

The following example shows a library that has two versions that clients can use:

    namespace modernlib 
    { 
      #ifndef LIB_VERSION_2 
      inline namespace version_1 
      { 
        template<typename T> 
        int test(T value) { return 1; } 
      } 
      #endif 

      #ifdef LIB_VERSION_2 
      inline namespace version_2 
      { 
        template<typename T> 
        int test(T value) { return 2; } 
      } 
      #endif 
    }