chore: String toHex should return a string not do inplace conversion

src/lib/base/String.cpp

@@ -176,7 +176,7 @@ std::string removeFileExt(std::string filename)
   return filename.substr(0, dot);
 }
 
-void toHex(std::string &subject, int width, const char fill)
+std::string toHex(const std::string &subject, int width, const char fill)
 {
   std::stringstream ss;
   ss << std::hex;
@@ -184,7 +184,7 @@ void toHex(std::string &subject, int width, const char fill)
     ss << std::setw(width) << std::setfill(fill) << (int)(unsigned char)subject[i];
   }
 
-  subject = ss.str();
+  return ss.str();
 }
 
 void uppercase(std::string &subject)

src/lib/base/String.h

@@ -71,8 +71,9 @@ std::string removeFileExt(std::string filename);
 //! Convert into hexdecimal
 /*!
 Convert each character in \c subject into hexdecimal form with \c width
+Return a new hexString
 */
-void toHex(std::string &subject, int width, const char fill = '0');
+std::string toHex(const std::string &subject, int width, const char fill = '0');
 
 //! Convert to all uppercase
 /*!

src/lib/net/InverseSockets/SslApi.cpp

@@ -221,7 +221,7 @@ int SslApi::getErrorCode(int status) const
 void SslApi::formatFingerprint(std::string &fingerprint) const
 {
   // to hexidecimal
-  deskflow::string::toHex(fingerprint, 2);
+  fingerprint = deskflow::string::toHex(fingerprint, 2);
   // all uppercase
   deskflow::string::uppercase(fingerprint);
   // add colon to separate each 2 charactors

src/lib/net/SecureSocket.cpp

@@ -625,7 +625,7 @@ void SecureSocket::formatFingerprint(std::string &fingerprint, bool hex, bool se
 {
   if (hex) {
     // to hexidecimal
-    deskflow::string::toHex(fingerprint, 2);
+    fingerprint = deskflow::string::toHex(fingerprint, 2);
   }
 
   // all uppercase

src/test/unittests/base/StringTests.cpp

@@ -56,12 +56,7 @@ TEST(StringTests, sprintf_formatWithArgument_formatedString)
 
 TEST(StringTests, toHex_plaintext_hexString)
 {
-  std::string subject = "foobar";
+  EXPECT_EQ("666f6f626172", string::toHex("foobar", 2));
-  int width = 2;
-
-  string::toHex(subject, width);
-
-  EXPECT_EQ("666f6f626172", subject);
 }
 
 TEST(StringTests, uppercase_lowercaseInput_uppercaseOutput)