#ifndef CPR_AUTH_H
#define CPR_AUTH_H

#include <string>

namespace cpr {

    class Authentication {
      public:
        Authentication(const std::string& username, const std::string& password) :
            username_{username}, password_{password}, auth_string_{username_ + ":" + password_} {}

        const char* GetAuthString() const;

      private:
        std::string username_;
        std::string password_;
        std::string auth_string_;
    };

}

#endif
#ifndef CPR_AUTH_H
#define CPR_AUTH_H

#include <string>

namespace cpr {

    class Authentication {
      public:
        Authentication(const std::string& username, const std::string& password) :
            username_{username}, password_{password}, auth_string_{username + ":" + password} {}

        const char* GetAuthString() const;

      private:
        std::string username_;
        std::string password_;
        std::string auth_string_;
    };

}

#endif
--- 
+++ 
@@ -8,7 +8,7 @@
     class Authentication {
       public:
         Authentication(const std::string& username, const std::string& password) :
-            username_{username}, password_{password}, auth_string_{username + ":" + password} {}
+            username_{username}, password_{password}, auth_string_{username_ + ":" + password_} {}
 
         const char* GetAuthString() const;
 
Use the member strings since their types on concrete
