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RapidJSON Tutorial
This tutorial introduces Document Object Model(DOM) API of RapidJSON.
As shown in Usage at a glance, a JSON text can be parsed into DOM, and then be quried and modfied easily, and finally convert back to JSON text.
Value
Each JSON value is stored in a type called Value. A Document, representing the DOM, contains the root of Value.
Querying Value
In this section, we will use excerpt of example/tutorial/tutorial.cpp.
Assumes we have a JSON text stored in a C string (const char* json):
{
"hello": "world",
"t": true ,
"f": false,
"n": null,
"i": 123,
"pi": 3.1416,
"a": [1, 2, 3, 4]
}
Parse it into a Document
#include "rapidjson/document.h"
using namespace rapidjson;
// ...
Document document;
document.Parse(json);
The JSON text is now parsed into document as a DOM tree.
The root of a conforming JSON should be either an object or an array. In this case, the root is an object with 7 members.
assert(document.IsObject());
Query whether a "hello" member exists in the root object. Since a Value can contain different types of value, we may need to verify its type and use suitable API to obtain the value. In this example, "hello" member associates with a JSON string.
assert(document.HasMember("hello"));
assert(document["hello"].IsString());
printf("hello = %s\n", document["hello"].GetString());
JSON true/false values are represented as bool.
assert(document["t"].IsBool());
printf("t = %s\n", document["t"].GetBool() ? "true" : "false");
JSON null can be queryed by IsNull().
printf("n = %s\n", document["n"].IsNull() ? "null" : "?");
JSON number type represents all numeric values. However, C++ needs more specific type for manipulation.
assert(document["i"].IsNumber());
// In this case, IsUint()/IsInt64()/IsUInt64() also return true.
assert(document["i"].IsInt());
printf("i = %d\n", document["i"].GetInt());
// Alternative (int)document["i"]
assert(document["pi"].IsNumber());
assert(document["pi"].IsDouble());
printf("pi = %g\n", document["pi"].GetDouble());
JSON array contains a number of elements
// Using a reference for consecutive access is handy and faster.
const Value& a = document["a"];
assert(a.IsArray());
for (SizeType i = 0; i < a.Size(); i++) // Uses SizeType instead of size_t
printf("a[%d] = %d\n", i, a[i].GetInt());
Note that, RapidJSON do not automatically converting between JSON types. if a value is a string, it is invalid to call GetInt(). In debug mode it will assert. In release mode, the behavior is undefined.