Where do I put constant strings in C++: static class members or anonymous namespaces?

Question

I need to define some constant strings that will be used only by one class. It looks like I have three options:

1. Embed the strings directly into locations where they are used.

2. Define them as private static constant members of the class:

   //A.h  
   class A {  
   private:  
      static const std::string f1;  
      static const std::string f2;  
      static const std::string f3;  
   };  
   
   //A.cpp  
   const std::string f1 = "filename1";  
   const std::string f2 = "filename2";  
   const std::string f3 = "filename3";  
   
   //strings are used in this file  
   

3. Define them in an anonymous namespace in the cpp file:

   //A.cpp  
   namespace {  
     const std::string f1 = "filename1";  
     const std::string f2 = "filename2";  
     const std::string f3 = "filename3";  
   }  
   
   //strings are used in this file  
   

Given these options, which one would you recommend and why? Thanks.

Answer 1

I'd place them in anonymous namespace in the CPP file. It makes them private to the implementation and at the same moment makes it visible to the non-member functions that are part of implementation (such as operator<<).