[C++ runtime] Optimizations of Vocabulary::fromTokenNames

This commit doesn't change the behavior, it contains few small improvements:
- prevent useless copy when creating the variable tokenName
- avoid to check if tokenName is empty twice
- use std::string::empty() instead of creating an empty string to compare with
- use std::string::clear() instead of assigning an empty C string to clear a string

contributors.txt

@@ -242,4 +242,5 @@ YYYY/MM/DD, github id, Full name, email
 2020/02/21, StochasticTinkr, Daniel Pitts, github@coloraura.com
 2020/03/17, XsongyangX, Song Yang, songyang1218@gmail.com
 2020/04/07, deniskyashif, Denis Kyashif, denis.kyashif@gmail.com
-2020/04/30, TristonianJones, Tristan Swadell, tswadell@google.com
+2020/04/30, TristonianJones, Tristan Swadell, tswadell@google.com
+2020/06/04, IohannRabeson, Iohann Rabeson, iotaka6@gmail.com

runtime/Cpp/runtime/src/Vocabulary.cpp

@@ -22,8 +22,7 @@ Vocabulary::Vocabulary(const std::vector<std::string> &literalNames,
   // See note here on -1 part: https://github.com/antlr/antlr4/pull/1146
 }
 
-Vocabulary::~Vocabulary() {
-}
+Vocabulary::~Vocabulary() = default;
 
 Vocabulary Vocabulary::fromTokenNames(const std::vector<std::string> &tokenNames) {
   if (tokenNames.empty()) {
@@ -34,25 +33,18 @@ Vocabulary Vocabulary::fromTokenNames(const std::vector<std::string> &tokenNames
   std::vector<std::string> symbolicNames = tokenNames;
   std::locale locale;
   for (size_t i = 0; i < tokenNames.size(); i++) {
-    std::string tokenName = tokenNames[i];
-    if (tokenName == "") {
+    const std::string& tokenName = tokenNames[i];
+    if (tokenName.empty()) {
       continue;
+    } else if (tokenName.front() == '\'') {
+      symbolicNames[i].clear();
+    } else if (std::isupper(tokenName.front(), locale)) {
+      literalNames[i].clear();
+    } else {
+      // wasn't a literal or symbolic name
+      literalNames[i].clear();
+      symbolicNames[i].clear();
     }
-
-    if (!tokenName.empty()) {
-      char firstChar = tokenName[0];
-      if (firstChar == '\'') {
-        symbolicNames[i] = "";
-        continue;
-      } else if (std::isupper(firstChar, locale)) {
-        literalNames[i] = "";
-        continue;
-      }
-    }
-
-    // wasn't a literal or symbolic name
-    literalNames[i] = "";
-    symbolicNames[i] = "";
   }
 
   return Vocabulary(literalNames, symbolicNames, tokenNames);

runtime/Cpp/runtime/src/Vocabulary.h

@@ -14,9 +14,6 @@ namespace dfa {
   /// interface.
   class ANTLR4CPP_PUBLIC Vocabulary {
   public:
-    Vocabulary(Vocabulary const&) = default;
-    virtual ~Vocabulary();
-
     /// Gets an empty <seealso cref="Vocabulary"/> instance.
     ///
     /// <para>
@@ -25,7 +22,9 @@ namespace dfa {
     /// except <seealso cref="Token#EOF"/>.</para>
     static const Vocabulary EMPTY_VOCABULARY;
 
-    Vocabulary() {}
+    Vocabulary() = default;
+    Vocabulary(Vocabulary const&) = default;
+    virtual ~Vocabulary();
 
     /// <summary>
     /// Constructs a new instance of <seealso cref="Vocabulary"/> from the specified