Error: Add std::ostream Operator to Write Error

error/include/error/error.hpp

@@ -3,6 +3,7 @@
 #include <fmt/core.h>
 
 #include <exception>
+#include <ostream>
 #include <string>
 #include <utility>
 
@@ -20,6 +21,26 @@ struct Error : public std::exception {
    */
   Error(const std::string& msg);
 
+  /**
+   * @brief Writes the string representation of an error object to the given
+   * output stream.
+   * @param os The output stream.
+   * @param err The error object.
+   * @return The output stream.
+   *
+   * This operator allows an error object to be printed to the output stream
+   * using the << operator. The error message will be written to the output
+   * stream.
+   *
+   * @code{.cpp}
+   * const error::Error err("unknown error");
+   *
+   * // Print "error: unknown error"
+   * std::cout << err << std::endl;
+   * @endcode
+   */
+  friend std::ostream& operator<<(std::ostream& os, const error::Error& err);
+
   /**
    * @brief Returns the explanatory string.
    * @return Pointer to a null-terminated string with explanatory information.

error/src/error.cpp

@@ -4,6 +4,10 @@ namespace error {
 
 Error::Error(const std::string& msg) : message(msg) {}
 
+std::ostream& operator<<(std::ostream& os, const error::Error& err) {
+  return os << "error: " << err.what();
+}
+
 const char* Error::what() const noexcept { return message.c_str(); }
 
 bool operator==(const Error& lhs, const Error& rhs) {

error/test/error_test.cpp

@@ -1,5 +1,6 @@
 #include <catch2/catch_test_macros.hpp>
 #include <error/error.hpp>
+#include <sstream>
 #include <string>
 
 TEST_CASE("Error Construction") {
@@ -43,3 +44,9 @@ TEST_CASE("Error Comparison") {
   CHECK_FALSE(err == other_err);
   CHECK(err != other_err);
 }
+
+TEST_CASE("Error Printing") {
+  const error::Error err("unknown error");
+  const auto ss = std::stringstream() << err;
+  REQUIRE(ss.str() == "error: unknown error");
+}