util: Enhance Join()

Author: hebasto

src/util/string.h

@@ -30,20 +30,28 @@ NODISCARD inline std::string TrimString(const std::string& str, const std::strin
  * @param separator  The separator
  * @param unary_op   Apply this operator to each item in the list
  */
-template <typename T, typename UnaryOp>
-std::string Join(const std::vector<T>& list, const std::string& separator, UnaryOp unary_op)
+template <typename T, typename BaseType, typename UnaryOp>
+auto Join(const std::vector<T>& list, const BaseType& separator, UnaryOp unary_op)
+    -> decltype(unary_op(list.at(0)))
 {
-    std::string ret;
+    decltype(unary_op(list.at(0))) ret;
     for (size_t i = 0; i < list.size(); ++i) {
         if (i > 0) ret += separator;
         ret += unary_op(list.at(i));
     }
     return ret;
 }
 
+template <typename T>
+T Join(const std::vector<T>& list, const T& separator)
+{
+    return Join(list, separator, [](const T& i) { return i; });
+}
+
+// Explicit overload needed for c_str arguments, which would otherwise cause a substitution failure in the template above.
 inline std::string Join(const std::vector<std::string>& list, const std::string& separator)
 {
-    return Join(list, separator, [](const std::string& i) { return i; });
+    return Join<std::string>(list, separator);
 }
 
 /**