Json made easy

In my last blog ‘An ode to the cast operator’ I talked about using a combination of 3 simple C++ syntactic sugar constructs in order to make a library API simple to use:

  • Cast operators
  • Subscript operators
  • Value semantic smart pointer wrappers.
I’ve now spent some more hours with the problem library that made me write that last blog and have come to a useful wrapper library that I would like to share with you.
I’ve called this library ‘JSON made easy for C++’ or JsonMe++ for short, and its now available on github. The result isn’t a full bidirectional JSON lib, at least noy at this moment. It just provides the facility for accesing the data inside a JSON document or string, the functionality I myself needed. Given that this library, that is a wrapper for the Glib json library is now complete up to a point that it should be usable to anyone, I wanted to revisit this subject to show how the resulting API is indeed trivial in use by the merit of the above 3 flavours of syntactic sugar.
So lets look at a piece of code using this library:
jsonme::JsonMeLib jsonlib;
try {
   jsonme::Node topnode=jsonlib.parseFile(pathToFile);
} catch (jsonme::ParseError &e) {
  ..
}
This simple piece of code is actualy the hardest part of the code. If the file path provided is invalid, or the file isn’t valid JSON,
an exception will get thrown. But lets not look at that. The interesting part is in the fact that there is no ‘new’ or ‘*’ anywhere in the code.
The user of the library doesn’t need to juggle with pointers, wrap them in smart pointers in order to take care of resource management,
mix pointer semantics and value semantics in the code, etc.
The library takes care of all of this, and all the user is exposed to are JsonMeLib and Node objects that can be used using value semantics,
internally taking care of effective resource management and efficient and relatively light object copy behavior. This is where we see
value semantic smart pointer wrappers in action. Both JsonMeLib and Node are value semantic smart pointer wrappers. So when the result of
parseFile is assigned to topnode, under the hood smart pointers are taking care of proper resource management, without the need of the user of the library being exposed to resource management. A subject that for many C++ developers is a difficult subject and large source of subtle bugs. The library user actually needs no knowledge about RAII, smart pointers or resource management in C++ in order to use the library without fear of creating resource management bugs, That is, if there are resource management bugs it would be the fault of the library author.
JSON knows basicaly 3 types of node’s:
  • Objects
  • Arrays
  • Scalars
The first two is where the second piece of syntactic sugar comes in: the subscript operator. The library uses size_t indexed subscript operators for arrays
and std::string indexed subscript operators for objects.
  size_t index=0;
  jsonme::Node firsfoobar=topnode[“foolist”][index][“bar”];
The overloaded subscript operators together with our value semantics smartpointer wrapper give the JSON object an interface that is friendly to the user of the library in that its intuitive and does not require the library user to get into the library type internals.
Combining these two with cast operators,  we make it the library user even easier:
   long long magicvalue=topnode[“magic”];
   std::string  owner= topnode[“foolist”][index][“bar”];
The library takes a step back from this to allow for validation. Node has a nodetype() method that can return  jsonme::INVALID. Next to this between Node and the primitive scalar types the library has a class Scalar that the user may use when working with JSON data structure not hard coded into the C++ code.  That is, the library user can choose to look into some more of the API for finer control, but the bottem line is that the provided syntactic sugar makes the amount of API internals a pretty small surface to work with.
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