JsonContainer

The JsonContainer class provides a simplified abstraction around complex JSON C++ libraries. It has the following constructors:

JsonContainer() // Creates an empty container
JsonContainer(std::string json_txt) // creates a JsonContainer from a JSON string

Consider the following JSON string wrapped in a JsonContainer object, data.

    {
      "module" : "puppet",
      "action" : "run",
      "params" : {
        "first" : "--module-path=/home/alice/modules"
      }
    }

You can construct a JsonContainer as follows:

    JsonContainer data { jsons_string };

The JsonContainer's constructor can throw the following exception:

data_parse_error - This error is thrown when invalid JSON is passed to the constructor.

Note that you can instantiate JsonContainer by passing a non-empty std::string representing a JSON object, array, or value. In case you want to pass a JSON string, you must include escaped double quotes. For example:

    JsonContainer { "4" };  // a number
    JsonContainer { "\"fast'n bulbous\"" };  // a string
    JsonContainer { "null" };  // the null value
    JsonContainer { "" };  // JsonContainer will throw a data_parse_error!

The following calls to the get method will retrieve values from the JsonContainer.

    data.get<std::string>("module"); // == "puppet"
    data.get<std::string>({ "params", "first" }); // == "--module-path=/home/alice/modules"

Note that when the get method is invoked with an initialiser list it will use each argument to descend a level into the object tree. Also, when an unsigned integer is provided as index, get will verify that the specified in an array and return its requested element.

The get method can throw the following exception:

data_key_error - Thrown when the specified entry does not exist.
data_type_error - Thrown when an index is provided but the parent element is not an array.
data_index_error - Thrown when the provided index is out of bounds.

The supported scalar types are: int, double, bool, std::string, and JsonContainer. Elements of such types can be grouped in an array, represented by a std::vector instance.

You can also set the value of fields and create new fields with the set method.

    data.set<int>("foo", 42);
    data.set<bool>({ "params", "second" }, false);

This will change the internal JSON representation to

    {
      "module" : "puppet",
      "action" : "run",
      "params" : {
        "first" : "--module-path=/home/alice/modules",
        "second" : false
      },
      "foo" : 42
    }

Note that the set method uses the initialiser list in the same way as the get method. Each argument to the list is one level to descend.

The set method can throw the following exception:

data_key_error - thrown when a nested message key is invalid (i.e. the associated value is not a valid JSON object, so that is not possible to iterate the remaining nested keys) or when the root element is not a valid JSON object, so that is not possible to set the specified key-value entry.

You can use the type method to retrieve the type of a given value. As done for get and set, you can specify the value's key with an initialiser list, in order to navigate multiple levels within a JSON object.

The type method returns a value of the DataType enumeration, defined as:

    enum DataType { Object, Array, String, Int, Bool, Double, Null };

As for the get method, type can throw the following exceptions:

data_key_error - Thrown when the specified entry does not exist.
data_type_error - Thrown when an index is provided but the parent element is not an array.
data_index_error - Thrown when the provided index is out of bounds.

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