A Custom Writer Class for JsonCpp

JsonCpp provides a simple API for reading and writing JSON in C++. It has two classes for serialising JSON output: FastWriter produces minified output on a single line with no whitespace, and StyledWriter produces output padded with plenty of extra whitespace and a maximum line length.

I wanted more fine-grained control over the output, so I put together this simple CustomWriter class that allows a custom indent, custom maximum line length, and custom whitespace around brackets, colons and commas.

customwriter.h

// Copyright (c) 2013 Matt Swain. Released under the MIT license.
#include <string>
namespace Json {
   /** \brief Writes a Value in JSON format with custom formatting.
    *
    * The JSON document is written according to the rules specified 
    * in the constructor. Objects and arrays are printed on a single 
    * line if they are below a certain length, otherwise they are 
    * indented. It is possible to output invalid json if the 
    * customizable parameters are specified incorrectly. Set maxWidth
    * to 0 to print output on a single line. 
    *
    * \sa Reader, Value
    */
   class JSON_API CustomWriter : public Writer
   {
   public:
      CustomWriter( std::string opencurly = "{",
                    std::string closecurly = "}",
                    std::string opensquare = "[",
                    std::string closesquare = "]",
                    std::string colon = ":",
                    std::string comma = ",",
                    std::string indent = "  ",
                    int maxWidth = 74);
      virtual ~CustomWriter(){}
   public: // overridden from Writer
      virtual std::string write( const Value &root );
   private:
      void writeValue( const Value &value, std::string &doc, bool forceSingleLine );
      bool isMultiline( const Value &value );
      void indent();
      void unindent();
      std::string document_;
      std::string indentString_;
      std::string opencurly_;
      std::string closecurly_;
      std::string opensquare_;
      std::string closesquare_;
      std::string colon_;
      std::string comma_;
      std::string indent_;
      int maxWidth_;
   };
   
}

customwriter.cpp

// Copyright (c) 2013 Matt Swain. Released under the MIT license.
#include <json/json.h>
#include <json/customwriter.h>
namespace Json {
CustomWriter::CustomWriter( std::string opencurly,
                            std::string closecurly,
                            std::string opensquare,
                            std::string closesquare,
                            std::string colon,
                            std::string comma,
                            std::string indent,
                            int maxWidth)
   : opencurly_( opencurly )
   , closecurly_( closecurly )
   , opensquare_( opensquare )
   , closesquare_( closesquare )
   , colon_( colon )
   , comma_( comma )
   , indent_( indent )
   , maxWidth_( maxWidth )
{
}
std::string 
CustomWriter::write( const Value &root )
{
   document_ = "";
   indentString_ = "";
   writeValue( root, document_, false );
   document_ += "\n";
   return document_;
}
void 
CustomWriter::writeValue( const Value &value, std::string &doc, bool forceSingleLine )
{
   switch ( value.type() )
   {
   case nullValue:
      doc += "null";
      break;
   case intValue:
      doc += valueToString( value.asLargestInt() );
      break;
   case uintValue:
      doc += valueToString( value.asLargestUInt() );
      break;
   case realValue:
      doc += valueToString( value.asDouble() );
      break;
   case stringValue:
      doc += valueToQuotedString( value.asCString() );
      break;
   case booleanValue:
      doc += valueToString( value.asBool() );
      break;
   case arrayValue:
      {
         bool isMulti = false;
         if (!forceSingleLine)
         {
            std::string valLine = "";
            writeValue( value, valLine, true);
            if (valLine.length() > maxWidth_)
            {
               isMulti = true;
            }
            else
            {
               doc += valLine;
               break;
            }
         }
         doc += opensquare_;
         if (isMulti)
            indent();
         for ( int index =0; index < value.size(); ++index )
         {
            if (isMulti)
            {
               doc += "\n";
               doc += indentString_;
            }
            writeValue( value[index], doc, false );
            if ( index < value.size()-1 )
               doc += comma_;
         }
         if (isMulti)
         {
            unindent();
            doc += "\n";
            doc += indentString_;
         }
         doc += closesquare_;
      }
      break;
   case objectValue:
      {
         bool isMulti = false;
         if (!forceSingleLine)
         {
            std::string valLine = "";
            writeValue( value, valLine, true);
            if (valLine.length() > maxWidth_)
            {
               isMulti = true;
            }
            else
            {
               doc += valLine;
               break;
            }
         }
         Value::Members members( value.getMemberNames() );
         doc += opencurly_;
         if (isMulti)
            indent();
         for ( Value::Members::iterator it = members.begin(); 
               it != members.end(); 
               ++it )
         {
            if (isMulti)
            {
               doc += "\n";
               doc += indentString_;
               
            }
            const std::string &name = *it;
            doc += valueToQuotedString( name.c_str() );
            doc += colon_;
            writeValue( value[name], doc, forceSingleLine );
            if ( !(it + 1 == members.end()) )
               doc += comma_;
         }
         if (isMulti)
         {
            unindent();
            doc += "\n";
            doc += indentString_;
         }
         doc += closecurly_;
      }
      break;
   }
}
void 
CustomWriter::indent()
{
   indentString_ += indent_;
}
void 
CustomWriter::unindent()
{
   int idSize = int(indent_.size());
   int idsSize = int(indentString_.size());
   if (idsSize >= idSize)
      indentString_.resize (idsSize - idSize);
}
}