add FileSize::operator std::string() const

Log.cpp

@@ -297,6 +297,12 @@ std::ostream& operator<<(std::ostream& str, const PrintErrno&) {
 	return str << strerror(errno) << " (" << errno << ')';
 }
 
+FileSize::operator std::string() const {
+	std::ostringstream str;
+	str << *this;
+	return str.str();
+}
+
 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

@@ -122,6 +122,7 @@ public:
 	explicit FileSize(uint64_t fs) : fs(fs) {}
 	FileSize(const FileSize&) = delete;
 	FileSize& operator=(const FileSize&) = delete;
+	operator std::string() const;
 private:
 	uint64_t fs;
 	friend std::ostream& operator<<(std::ostream& str, const FileSize& fs);