[Jsoncpp-devel] FW: Powerful Json::Path

[王远天 <xia...@ho...>]

From: xia...@ho...
To: jso...@li...
Subject: Powerful Json::Path
Date: Tue, 1 Apr 2014 09:54:45 +0000




Json::Value v;v["a"]["b"][0u] = 1;
Json::Path p("a.b.c");const Json::Value &ref = p.resolve(v);// assert
The above code with course an assert when call "resolve".That is because "resolve" do not process except conditions.
const Value &Path::resolve( const Value &root ) const{   const Value *node = &root;   for ( Args::const_iterator it = args_.begin(); it != args_.end(); ++it )   {      const PathArgument &arg = *it;      if ( arg.kind_ == PathArgument::kindIndex )      {         if ( !node->isArray()  ||  node->isValidIndex( arg.index_ ) )         {            // Error: unable to resolve path (array value expected at position...         }         node = &((*node)[arg.index_]);      }      else if ( arg.kind_ == PathArgument::kindKey )      {         if ( !node->isObject() )         {            // Error: unable to resolve path (object value expected at position...)         }         node = &((*node)[arg.key_]);         if ( node == &Value::null )         {            // Error: unable to resolve path (object has no member named '' at position...)         }      }   }   return *node;}

Users call "resolve" means they must take the risk of fatal error.so "Path" is useless. But Json::Path should be more powerfull.Suppose we have a function named "bad" in class Value, which specified the value itself is bad. User shall check it like below:
Json::Value v;v["a"]["b"][0u] = 1;
Json::Path p("a.b.c");const Json::Value &ref = p.resolve(v);// will not assert
if (ref.bad()){    std::cout << "ref is bad." << std::endl;    ref = 2; // any operation to bad value will cause assert}
all we need to do is two step:1,declare a static member in Json::Value:	static Value bad;	bool bad() const	{	  return &bad == this;	}2,replace each "// Error: *" line to 	return Value::bad;

further morewe could use Json::Path to resolve the exact type value, Like this:
Json::Value v;v["a"]["b"][0u] = 1;
Json::Path p("a.b[0]");Json::Integer i = p.resolveInt(v);
if (i.bad()){	//do something}
Wish this features will be added to new version.