* Similar to [SAX](http://en.wikipedia.org/wiki/Simple_API_for_XML), RapidJSON provides a event sequential access parser API (`GenericReader`). It also provides a generator API (`GenericWriter`) which consumes the same set of events.
* Similar to [SAX](http://en.wikipedia.org/wiki/Simple_API_for_XML), RapidJSON provides a event sequential access parser API (`rapidjson::GenericReader`). It also provides a generator API (`rapidjson::Writer`) which consumes the same set of events.
* DOM (Document Object Model) style API
* Similar to [DOM](http://en.wikipedia.org/wiki/Document_Object_Model) for HTML/XML, RapidJSON can parse JSON into a DOM representation (`GenericDocument`), for easy manipulation, and finally stringify back to JSON if needed.
* The DOM style API (`GenericDocument`) is actually implemented with SAX style API (`GenericReader`). SAX is faster but sometimes DOM is easier. Users can pick their choices according to scenarios.
* Similar to [DOM](http://en.wikipedia.org/wiki/Document_Object_Model) for HTML/XML, RapidJSON can parse JSON into a DOM representation (`rapidjson::GenericDocument`), for easy manipulation, and finally stringify back to JSON if needed.
* The DOM style API (`rapidjson::GenericDocument`) is actually implemented with SAX style API (`rapidjson::GenericReader`). SAX is faster but sometimes DOM is easier. Users can pick their choices according to scenarios.
## DOM (Document)
...
...
@@ -59,13 +59,13 @@
## SAX (Writer)
* Support PrettyWriter for adding newlines and indentations.
* Support `rapidjson::PrettyWriter` for adding newlines and indentations.
* Support custom precision for floating point values.
## Stream
* Support `GenericStringBuffer` for storing the output JSON as string.
* Support `FileReadStream`/`FileWriteStream` for input/output `FILE` object.
* Support `rapidjson::GenericStringBuffer` for storing the output JSON as string.
* Support `rapidjson::FileReadStream`/`rapidjson::FileWriteStream` for input/output `FILE` object.
In RapidJSON, `Stream` is a concept for reading/writing JSON. Here we first show how to use streams provided. And then see how to create a custom streams.
In RapidJSON, `rapidjson::Stream` is a concept for reading/writing JSON. Here we first show how to use streams provided. And then see how to create a custom streams.
## Memory Streams
...
...
@@ -10,7 +10,7 @@ Memory streams store JSON in memory.
`StringStream` is the most basic input stream. It represents a complete, read-only JSON stored in memory. It is defined in `rapidjson/rapidjson.h`.
```cpp
~~~~~~~~~~cpp
#include "rapidjson/document.h" // will include "rapidjson/rapidjson.h"
using namespace rapidjson;
...
...
@@ -21,16 +21,16 @@ StringStream s(json);
Document d;
d.ParseStream(s);
```
~~~~~~~~~~
Since this is very common usage, `Document::Parse(const char*)` is provided to do exactly the same as above:
```cpp
~~~~~~~~~~cpp
// ...
const char json[] = "[1, 2, 3, 4]";
Document d;
d.Parse(json);
```
~~~~~~~~~~
Note that, `StringStream` is a typedef of `GenericStringStream<UTF8<> >`, user may use another encodings to represent the character set of the stream.
...
...
@@ -38,7 +38,7 @@ Note that, `StringStream` is a typedef of `GenericStringStream<UTF8<> >`, user m
`StringBuffer` is a simple output stream. It allocates a memory buffer for writing the whole JSON. Use `GetString()` to obtain the buffer.
When the buffer is full, it will increases the capacity automatically. The default capacity is 256 characters (256 bytes for UTF8, 512 bytes for UTF16, etc.). User can provide an allocator and a initial capacity.
```cpp
~~~~~~~~~~cpp
StringBuffer buffer1(0, 1024); // Use its allocator, initial size = 1024
StringBuffer buffer2(allocator, 1024);
```
~~~~~~~~~~
By default, `StringBuffer` will instantiate an internal allocator.
...
...
@@ -69,10 +69,12 @@ However, if the JSON is big, or memory is limited, you can use `FileReadStream`.
`FileReadStream` reads the file via a `FILE` pointer. And user need to provide a buffer.
```cpp
~~~~~~~~~~cpp
#include "rapidjson/filereadstream.h"
#include <cstdio>
using namespace rapidjson;
FILE* fp = fopen("big.json", "rb"); // non-Windows use "r"
char readBuffer[65536];
...
...
@@ -82,7 +84,7 @@ Document d;
d.ParseStream(is);
fclose(fp);
```
~~~~~~~~~~
Different from string streams, `FileReadStream` is byte stream. It does not handle encodings. If the file is not UTF-8, the byte stream can be wrapped in a `EncodedInputStream`. It will be discussed very soon.
...
...
@@ -92,10 +94,12 @@ Apart from reading file, user can also use `FileReadStream` to read `stdin`.
`FileWriteStream` is buffered output stream. Its usage is very similar to `FileReadStream`.
@@ -129,7 +133,7 @@ Note that, these encoded streams can be applied to streams other than file. For
`EncodedInputStream` has two template parameters. The first one is a `Encoding` class, such as `UTF8`, `UTF16LE`, defined in `rapidjson/encodings.h`. The second one is the class of stream to be wrapped.
d.Accept(writer); // This generates UTF32-LE file from UTF-8 in memory
fclose(fp);
```
~~~~~~~~~~
### AutoUTFInputStream
...
...
@@ -182,7 +186,7 @@ Sometimes an application may want to handle all supported JSON encoding. `AutoUT
Since the characters (code units) may be 8-bit, 16-bit or 32-bit. `AutoUTFInputStream` requires a character type which can hold at least 32-bit. We may use `unsigned`, as in the template parameter:
d.ParseStream<0, AutoUTF<unsigned> >(eis); // This parses any UTF file into UTF-8 in memory
fclose(fp);
```
~~~~~~~~~~
When specifying the encoding of stream, uses `AutoUTF<CharType>` as in `ParseStream()` above.
...
...
@@ -211,7 +215,9 @@ You can obtain the type of UTF via `UTFType GetType()`. And check whether a BOM
Similarly, to choose encoding for output during runtime, we can use `AutoUTFOutputStream`. This class is not automatic *per se*. You need to specify the UTF type and whether to write BOM in runtime.
`AutoUTFInputStream` and `AutoUTFOutputStream` is more convenient than `EncodedInputStream` and `EncodedOutputStream`. They just incur a little bit runtime overheads.
...
...
@@ -232,7 +238,7 @@ In addition to memory/file streams, user can create their own stream classes whi
RapidJSON combines different types using templates. A class containing all required interface can be a stream. The Stream interface is defined in comments of `rapidjson/rapidjson.h`:
```cpp
~~~~~~~~~~cpp
concept Stream {
typename Ch; //!< Character type of the stream.
...
...
@@ -261,7 +267,7 @@ concept Stream {
//! \return Number of characters written.
size_t PutEnd(Ch* begin);
}
```
~~~~~~~~~~
For input stream, they must implement `Peek()`, `Take()` and `Tell()`.
For output stream, they must implement `Put()` and `Flush()`.
...
...
@@ -271,7 +277,7 @@ There are two special interface, `PutBegin()` and `PutEnd()`, which are only for
The following example is a wrapper of `std::istream`, which only implements 3 functions.
```cpp
~~~~~~~~~~cpp
class IStreamWrapper {
public:
typedef char Ch;
...
...
@@ -302,18 +308,18 @@ private:
std::istream& is_;
};
```
~~~~~~~~~~
User can use it to wrap instances of `std::stringstream`, `std::ifstream`.
```cpp
~~~~~~~~~~cpp
const char* json = "[1,2,3,4]";
std::stringstream ss(json);
IStreamWrapper is(ss);
Document d;
d.Parse(is);
```
~~~~~~~~~~
Note that, this implementation may not be as efficient as RapidJSON's memory or file streams, due to internal overheads of the standard library.
...
...
@@ -321,7 +327,7 @@ Note that, this implementation may not be as efficient as RapidJSON's memory or
The following example is a wrapper of `std::istream`, which only implements 2 functions.
```cpp
~~~~~~~~~~cpp
class OStreamWrapper {
public:
typedef char Ch;
...
...
@@ -344,11 +350,11 @@ private:
std::ostream& os_;
};
```
~~~~~~~~~~
User can use it to wrap instances of `std::stringstream`, `std::ofstream`.
```cpp
~~~~~~~~~~cpp
Document d;
// ...
...
...
@@ -357,7 +363,7 @@ OSStreamWrapper os(ss);
Writer<OStreamWrapper> writer(os);
d.Accept(writer);
```
~~~~~~~~~~
Note that, this implementation may not be as efficient as RapidJSON's memory or file streams, due to internal overheads of the standard library.
@@ -13,7 +13,7 @@ Each JSON value is stored in a type called `Value`. A `Document`, representing t
In this section, we will use excerpt of [`example/tutorial/tutorial.cpp`](../example/tutorial/tutorial.cpp).
Assumes we have a JSON text stored in a C string (`const char* json`):
```js
~~~~~~~~~~js
{
"hello": "world",
"t": true ,
...
...
@@ -23,10 +23,10 @@ Assumes we have a JSON text stored in a C string (`const char* json`):
"pi": 3.1416,
"a": [1, 2, 3, 4]
}
```
~~~~~~~~~~
Parse it into a `Document`
```cpp
~~~~~~~~~~cpp
#include "rapidjson/document.h"
using namespace rapidjson;
...
...
@@ -34,50 +34,50 @@ 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.
```cpp
~~~~~~~~~~cpp
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.
// 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());
```
~~~~~~~~~~
```
~~~~~~~~~~
a[0] = 1
a[1] = 2
a[2] = 3
a[3] = 4
```
~~~~~~~~~~
Note that, RapidJSON does not automatically convert values between JSON types. If a value is a string, it is invalid to call `GetInt()`, for example. In debug mode it will fail an assertion. In release mode, the behavior is undefined.
...
...
@@ -124,10 +124,10 @@ You may access the elements in array by integer literal, for example, `a[1]`, `a
*`a[0u]`
Array is similar to `std::vector`, instead of using indices, you may also use iterator to access all the elements.
```cpp
~~~~~~~~~~cpp
for (Value::ConstValueIterator itr = a.Begin(); itr != a.End(); ++itr)
printf("%d ", itr->GetInt());
```
~~~~~~~~~~
And other familiar query functions:
*`SizeType Capacity() const`
...
...
@@ -137,7 +137,7 @@ And other familiar query functions:
Similar to array, we can iterate object members by iterator:
Note that, when `operator[](const char*)` cannot find the member, it will fail an assertion.
If we are unsure whether a member exists, we need to call `HasMember()` before calling `operator[](const char*)`. However, this incurs two lookup. A better way is to call `FindMember()`, which can check the existence of member and obtain its value at once:
@@ -210,18 +210,18 @@ To conform RFC 4627, RapidJSON supports string containing `U+0000`. If you need
For example, after parsing a the following JSON string to `Document d`.
```js
~~~~~~~~~~js
{ "s" : "a\u0000b" }
```
~~~~~~~~~~
The correct length of the value `"a\u0000b"` is 3. But `strlen()` returns 1.
`GetStringLength()` can also improve performance, as user may often need to call `strlen()` for allocating buffer.
Besides, `std::string` also support a constructor:
```cpp
~~~~~~~~~~cpp
string( const char* s, size_type count);
```
~~~~~~~~~~
which accepts the length of string as parameter. This constructor supports storing null character within the string, and should also provide better performance.
...
...
@@ -232,43 +232,43 @@ There are several ways to create values. After a DOM tree is created and/or modi
### Changing Value Type
When creating a Value or Document by default constructor, its type is Null. To change its type, call `SetXXX()` or assignment operator, for example:
```cpp
~~~~~~~~~~cpp
Document d; // Null
d.SetObject();
Value v; // Null
v.SetInt(10);
v = 10; // Shortcut, same as above
```
~~~~~~~~~~
### Overloaded Constructors
There are also overloaded constructors for several types:
```cpp
~~~~~~~~~~cpp
Value b(true); // calls Value(bool)
Value i(-123); // calls Value(int)
Value u(123u); // calls Value(unsigned)
Value d(1.5); // calls Value(double)
```
~~~~~~~~~~
To create empty object or array, you may use `SetObject()`/`SetArray()` after default constructor, or using the `Value(Type)` in one shot:
```cpp
~~~~~~~~~~cpp
Value o(kObjectType);
Value a(kArrayType);
```
~~~~~~~~~~
### Move Semantics
A very special decision during design of RapidJSON is that, assignment of value does not copy the source value to destination value. Instead, the value from source is moved to the destination. For example,
```cpp
~~~~~~~~~~cpp
Value a(123);
Value b(456);
b = a; // a becomes a Null value, b becomes number 123.
```
~~~~~~~~~~
Why? What is the advantage of this semantics?
...
...
@@ -276,7 +276,7 @@ The simple answer is performance. For fixed size JSON types (Number, True, False
For example, if normal *copy* semantics was used:
```cpp
~~~~~~~~~~cpp
Value o(kObjectType);
{
Value contacts(kArrayType);
...
...
@@ -285,9 +285,9 @@ Value o(kObjectType);
o.AddMember("contacts", contacts); // deep clone contacts (may be with lots of allocations)
// destruct contacts.
}
```
~~~~~~~~~~
The object `o` needs to allocate a buffer of same size as contacts, makes a deep clone of it, and then finally contacts is destructed. This will incur a lot of unnecessary allocations/deallocations and memory copying.
...
...
@@ -297,7 +297,7 @@ To make RapidJSON simple and fast, we chose to use *move* semantics for assignme
So, with move semantics, the above example becomes:
```cpp
~~~~~~~~~~cpp
Value o(kObjectType);
{
Value contacts(kArrayType);
...
...
@@ -305,9 +305,9 @@ Value o(kObjectType);
o.AddMember("contacts", contacts); // just memcpy() of contacts itself to the value of new member (16 bytes)
// contacts became Null here. Its destruction is trivial.
}
```
~~~~~~~~~~
This is called move assignment operator in C++11. As RapidJSON supports C++03, it adopts move semantics using assignment operator, and all other modifying function like `AddMember()`, `PushBack()`.
...
...
@@ -323,7 +323,7 @@ To make memory allocation customizable, RapidJSON requires user to pass an insta
Therefore, when we assign a copy-string, we call this overloaded `SetString()` with allocator:
```cpp
~~~~~~~~~~cpp
Document document;
Value author;
char buffer[10];
...
...
@@ -331,7 +331,7 @@ int len = sprintf(buffer, "%s %s", "Milo", "Yip"); // dynamically created string
// author.GetString() still contains "Milo Yip" after buffer is destroyed
```
~~~~~~~~~~
In this example, we get the allocator from a `Document` instance. This is a common idiom when using RapidJSON. But you may use other instances of allocator.
...
...
@@ -339,12 +339,12 @@ Besides, the above `SetString()` requires length. This can handle null character
Finally, for literal string or string with safe life-cycle can use const-string version of `SetString()`, which lacks allocator parameter:
```cpp
~~~~~~~~~~cpp
Value s;
s.SetString("rapidjson", 9); // faster, can contain null character
\tparam Encoding encoding for both parsing and string storage.
\tparam Allocator allocator for allocating memory for the DOM, and the stack during parsing.
\warning Although GenericDocument inherits from GenericValue, the API does \b not provide any virtual functions, especially no virtual destructors. To avoid memory leaks, do not \c delete a GenericDocument object via a pointer to a GenericValue.
