JsonCpp源碼閱讀（一）——Reader類

 class JSON_API Reader
   {
   public:
      typedef char Char;
      typedef const Char *Location;

      /** \brief Constructs a Reader allowing all features
       * for parsing.
       */
      Reader();

      /** \brief Constructs a Reader allowing the specified feature set
       * for parsing.
       */
      Reader( const Features &features );

      /** \brief Read a Value from a <a HREF="http://www.json.org">JSON</a> document.
       * \param document UTF-8 encoded string containing the document to read.
       * \param root [out] Contains the root value of the document if it was
       *             successfully parsed.
       * \param collectComments \c true to collect comment and allow writing them back during
       *                        serialization, \c false to discard comments.
       *                        This parameter is ignored if Features::allowComments_
       *                        is \c false.
       * \return \c true if the document was successfully parsed, \c false if an error occurred.
       */
      bool parse( const std::string &document, 
                  Value &root,
                  bool collectComments = true );

      /** \brief Read a Value from a <a HREF="http://www.json.org">JSON</a> document.
       * \param document UTF-8 encoded string containing the document to read.
       * \param root [out] Contains the root value of the document if it was
       *             successfully parsed.
       * \param collectComments \c true to collect comment and allow writing them back during
       *                        serialization, \c false to discard comments.
       *                        This parameter is ignored if Features::allowComments_
       *                        is \c false.
       * \return \c true if the document was successfully parsed, \c false if an error occurred.
       */
      bool parse( const char *beginDoc, const char *endDoc, 
                  Value &root,
                  bool collectComments = true );

      /// \brief Parse from input stream.
      /// \see Json::operator>>(std::istream&, Json::Value&).
      bool parse( std::istream &is,
                  Value &root,
                  bool collectComments = true );

      /** \brief Returns a user friendly string that list errors in the parsed document.
       * \return Formatted error message with the list of errors with their location in 
       *         the parsed document. An empty string is returned if no error occurred
       *         during parsing.
       */
      std::string getFormatedErrorMessages() const;

   private:
      enum TokenType
      {
         tokenEndOfStream = 0,
         tokenObjectBegin,
         tokenObjectEnd,
         tokenArrayBegin,
         tokenArrayEnd,
         tokenString,
         tokenNumber,
         tokenTrue,
         tokenFalse,
         tokenNull,
         tokenArraySeparator,
         tokenMemberSeparator,
         tokenComment,
         tokenError
      };

      class Token
      {
      public:
         TokenType type_;
         Location start_;
         Location end_;
      };

      class ErrorInfo
      {
      public:
         Token token_;
         std::string message_;
         Location extra_;
      };

      typedef std::deque<ErrorInfo> Errors;

      bool expectToken( TokenType type, Token &token, const char *message );
      bool readToken( Token &token );
      void skipSpaces();
      bool match( Location pattern, 
                  int patternLength );
      bool readComment();
      bool readCStyleComment();
      bool readCppStyleComment();
      bool readString();
      void readNumber();
      bool readValue();
      bool readObject( Token &token );
      bool readArray( Token &token );
      bool decodeNumber( Token &token );
      bool decodeString( Token &token );
      bool decodeString( Token &token, std::string &decoded );
      bool decodeDouble( Token &token );
      bool decodeUnicodeCodePoint( Token &token, 
                                   Location &current, 
                                   Location end, 
                                   unsigned int &unicode );
      bool decodeUnicodeEscapeSequence( Token &token, 
                                        Location &current, 
                                        Location end, 
                                        unsigned int &unicode );
      bool addError( const std::string &message, 
                     Token &token,
                     Location extra = 0 );
      bool recoverFromError( TokenType skipUntilToken );
      bool addErrorAndRecover( const std::string &message, 
                               Token &token,
                               TokenType skipUntilToken );
      void skipUntilSpace();
      Value &currentValue();
      Char getNextChar();
      void getLocationLineAndColumn( Location location,
                                     int &line,
                                     int &column ) const;
      std::string getLocationLineAndColumn( Location location ) const;
      void addComment( Location begin, 
                       Location end, 
                       CommentPlacement placement );
      void skipCommentTokens( Token &token );
   
      typedef std::stack<Value *> Nodes;
      Nodes nodes_;
      Errors errors_;
      std::string document_;
      Location begin_;
      Location end_;
      Location current_;
      Location lastValueEnd_;
      Value *lastValue_;
      std::string commentsBefore_;
      Features features_;
      bool collectComments_;
   };

簡而言之：

Reader類的對外接口只有兩個構造函數：

Reader();
Reader( const Features &features );

第二個構造函數，咱們在後面再進行分析。

三個解析重載的解析函數：

 bool parse( const std::string &document, Value &root, bool collectComments = true );
 bool parse( const char *beginDoc, const char *endDoc, Value &root, bool collectComments = true );
 bool parse( std::istream &is, Value &root, bool collectComments = true );

一個獲取錯誤信息函數：

  std::string getFormatedErrorMessages() const;

這些是Reader類的對外接口。接着咱們看是如何實現的，從它的簡單的私有函數開始：

void Reader::skipSpaces()
{
   while ( current_ != end_ )
   {
      Char c = *current_;
      if ( c == ' '  ||  c == '\t'  ||  c == '\r'  ||  c == '\n' )
         ++current_;
      else
         break;
   }
}

void skipSpaces(); 該函數是跳過開通全部空格,'\t'，'\r'和'\n'字符。使用的是私有成員變量：

Location begin_;
Location end_;
Location current_;
Location lastValueEnd_;

其中的Location類型是const char *類型的指針，從typedef char Char;和typedef const Char *Location;這知道。

Reader::Char Reader::getNextChar()
{
   if ( current_ == end_ )
      return 0;
   return *current_++;
}

這裏的getNextChar()，是獲取到當前字符，而後 current_指針日後移動一次。這裏感受這個函數名和函數的行爲不符合啊，你們有什麼看法能夠提出了，有多是個人疏漏。

未完待續...