JSON is a common data interchange format used on the Web. Phobos has a std.json
module that can be used to read and write JSON data. The std.json
module is an old module that doesn't take advantage of many of D's advanced features, making it somewhat difficult to use. While newer JSON libraries exist for D, including ones that can be used with syntax and convenience, which is extremely similar to JavaScript itself, Phobos has not yet adopted any of them. Here, we'll read a JSON string, print the current contents, add a new field, and then print out the new JSON.
Suppose we're consuming a web API that returns an array of person objects with a name
and ID
field. We get the following JSON string from the API:
[{"name":"Alice","id":1},{"name":"Bob","id":2}]
Let's execute the following steps to use the std.json
module:
The code is as follows:
void main() { import std.json, std.algorithm, std.stdio; auto list = parseJSON(`[{"name":"Alice","id":1},{"name":"Bob","id":2}]); foreach(person; list.array.map!(a => a.object)) { writeln("ID #", person["id"].integer, " is ", person["name"].str); } JSONValue newPerson; JSONValue[string] obj; obj["name"].str = "Charlie"; obj["id"].integer = 3; newPerson.object = obj; list.array = list.array ~ newPerson;; // append it to the list writeln(toJSON(&list)); // print out the new json }
It will print the following output:
[{"name":"Alice","id":1},{"name":"Bob","id":2},{"name":"Charlie","id":3}]
Phobos' std.json
module provides a tagged union to represent a dynamic JSONValue
. To use it, you can explicitly check the type with the type
property, or simply access the type you need through its str
, object
, array
, integer
, and other properties. It will throw an exception if you try to access a property of the wrong type. You can query the current type with the type
property.
Array elements in std.json
are also JSONValues
, which means they need to be accessed through the type
properties. That is why we called map
on the array. We can't work directly with JSONValue
, so mapping it to an object gives something we can immediately use. The map
function is a function from std.algorithm
that calls the given predicate on each element in the array.
JSONValue
properties are either read or write. We cannot modify them in-place. Instead, we read the data or create a temporary file to do our edits then set the finished data back to it.
Finally, toJSON
takes a pointer to a root element and returns the JSON string it represents.
std.json
module.Variant
. Unlike JSONValue
, Variant
can store any type, but it does not do serialization to and from string.var
struct with convenient syntax and a similar type model to JavaScript. We'll also learn some of the techniques that are used in that module later in this book.3.21.244.217