print errno

Log.cpp

@@ -2,6 +2,7 @@
 
 #include <array>
 #include <chrono>   // date/time
+#include <cstring>
 #include <fstream>  // ofstream (logging to file)
 #include <iostream> // std::ostream, std::cout, std::cin
 #include <memory>   // std::unique_ptr
@@ -284,6 +285,12 @@ void setColoredOutput(bool enabled) {
 	dynamic_cast<ConsoleOutput&>(*outputs.at(0)).setColoredOutput(enabled); // has to exist
 }
 
+const PrintErrno err;
+
+std::ostream& operator<<(std::ostream& str, const PrintErrno&) {
+	return str << strerror(errno) << " (" << errno << ')';
+}
+
 std::ostream& operator<<(std::ostream& str, const FileSize& fs) {
 	static const char PREFIX[] {' ', 'K', 'M', 'G', 'T', 'P', 'E'};
 	static const uint_fast8_t PREFIXCOUNT = 7;

Log.h

@@ -127,6 +127,12 @@ private:
 	friend std::ostream& operator<<(std::ostream& str, const FileSize& fs);
 };
 
+struct PrintErrno {};
+
+extern const PrintErrno err;
+
+std::ostream& operator<<(std::ostream& str, const PrintErrno&);
+
 std::ostream& operator<<(std::ostream& str, const FileSize& fs);
 
 // copy Level values in Log namespace