Purpose of reference template arguments

Question

You can use a reference to a global object as a template parameter. For example thus:

class A {};  template<A& x> void fun() { }  A alpha;  int main() {     fun<alpha>(); } 

In what situation might a reference template argument be useful?

Answer 1

One scenario could be a strong typedef with an identity token that shouldn't be of integral type, but instead a string for ease of use when serializing stuff. You can then leverage empty base class optimization to eliminate any additional space requirements a derived type has.

Example:

// File: id.h #pragma once #include <iosfwd> #include <string_view>  template<const std::string_view& value> class Id {     // Some functionality, using the non-type template parameter...     // (with an int parameter, we would have some ugly branching here)     friend std::ostream& operator <<(std::ostream& os, const Id& d)     {         return os << value;     }      // Prevent UB through non-virtual dtor deletion:     protected:       ~Id() = default; };  inline const std::string_view str1{"Some string"}; inline const std::string_view str2{"Another strinng"}; 

And in some translation unit:

#include <iostream> #include "id.h"  // This type has a string-ish identity encoded in its static type info, // but its size isn't augmented by the base class: struct SomeType : public Id<str2> {};  SomeType x;  std::cout << x << "\n";