improvements for StringUtils::StringUTF8 (cocos2d#18356)

* 1. add 'const'
2. add range support for 'getAsCharSequence'

* remove default arguments

* use another variable to make it more clear

* rename isAnsi-> isASCII

Author: summerinsects

cocos/base/ccUTF8.cpp

@@ -27,6 +27,7 @@
 #include "platform/CCCommon.h"
 #include "base/CCConsole.h"
 #include "ConvertUTF.h"
+#include <limits>
 
 NS_CC_BEGIN
 
@@ -379,12 +380,28 @@ void StringUTF8::replace(const std::string& newStr)
 }
 
 std::string StringUTF8::getAsCharSequence() const
+{
+    return getAsCharSequence(0, std::numeric_limits<std::size_t>::max());
+}
+
+std::string StringUTF8::getAsCharSequence(std::size_t pos) const
+{
+    return getAsCharSequence(pos, std::numeric_limits<std::size_t>::max());
+}
+
+std::string StringUTF8::getAsCharSequence(std::size_t pos, std::size_t len) const
 {
     std::string charSequence;
+    std::size_t maxLen = _str.size() - pos;
+    if (len > maxLen)
+    {
+        len = maxLen;
+    }
 
-    for (auto& charUtf8 : _str)
+    std::size_t endPos = len + pos;
+    while (pos < endPos)
     {
-        charSequence.append(charUtf8._char);
+        charSequence.append(_str[pos++]._char);
     }
 
     return charSequence;

cocos/base/ccUTF8.h

@@ -196,7 +196,7 @@ class CC_DLL StringUTF8
     struct CharUTF8
     {
         std::string _char;
-        bool isAnsi() { return _char.size() == 1; }
+        bool isASCII() const { return _char.size() == 1; }
     };
     typedef std::vector<CharUTF8> CharUTF8Store;
 
@@ -208,12 +208,15 @@ class CC_DLL StringUTF8
     void replace(const std::string& newStr);
 
     std::string getAsCharSequence() const;
+    std::string getAsCharSequence(std::size_t pos) const;
+    std::string getAsCharSequence(std::size_t pos, std::size_t len) const;
 
     bool deleteChar(std::size_t pos);
     bool insert(std::size_t pos, const std::string& insertStr);
     bool insert(std::size_t pos, const StringUTF8& insertStr);
 
     CharUTF8Store& getString() { return _str; }
+    const CharUTF8Store& getString() const { return _str; }
 
 private:
     CharUTF8Store _str;