JSON Parser – library MetaTrader 4

Author:

Andrew Lord

This parses mql4 strings that contain JSON code. It creates a JSONValue that can be used to retrieve the fields within the JSON structure. It also provides a JSONIterator class to loop over object keys.

It parses the JSON using a simple a recursive descent parser. It requires the Hash class.

I still have to implement the parsing of unicode escaped characters.

The Codebase version might not be the most recent. The latest version can be found here.

Documentation:

JSONParser class

JSONValue class

Example:

#include "hash.mqh"
#include "json.mqh"
    
string s = "{ \"firstName\": \"John\", \"lastName\": \"Smith\", \"age\": 25, "+
"\"address\": { \"streetAddress\": \"21 2nd Street\", \"city\": \"New York\", \"state\": \"NY\", \"postalCode\": \"10021\" },"+
" \"phoneNumber\": [ { \"type\": \"home\", \"number\": \"212 555-1234\" }, { \"type\": \"fax\", \"number\": \"646 555-4567\" } ],"+
" \"gender\":{ \"type\":\"male\" }  }";

    JSONParser *parser = new JSONParser();

    JSONValue *jv = parser.parse(s);

    if (jv == NULL) {
        Print("error:"+(string)parser.getErrorCode()+parser.getErrorMessage());
    } else {


        Print("PARSED:"+jv.toString());
        
        if (jv.isObject()) { // check root value is an object. (it can be an array)

            JSONObject *jo = jv;

            // Direct access - will throw null pointer if wrong getter used.
            Print("firstName:" + jo.getString("firstName"));
            Print("city:" + jo.getObject("address").getString("city"));
            Print("phone:" + jo.getArray("phoneNumber").getObject(0).getString("number"));

            // Safe access in case JSON data is missing or different.
            if (jo.getString("firstName",s) ) Print("firstName = "+s);

            // Loop over object keys
            JSONIterator *it = new JSONIterator(jo);
            for( ; it.hasNext() ; it.next()) {
                Print("loop:"+it.key()+" = "+it.val().toString());
            }
            delete it;
        }
        delete jv;
    }
    delete parser;

The latest version can also be found here.