This tutorial introduces the basics of the Document Object Model(DOM) API.
As shown in [Usage at a glance](../readme.md#usage-at-a-glance), a JSON can be parsed into DOM, and then the DOM can be queried and modified easily, and finally be converted back to JSON.
As shown in [Usage at a glance](@ref index), JSON can be parsed into a DOM, and then the DOM can be queried and modified easily, and finally be converted back to JSON.
[TOC]
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@@ -14,7 +14,7 @@ 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`.
Assumes we have a JSON stored in a C string (`const char* json`):
Assume we have the following JSON stored in a C string (`const char* json`):
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.
In the following, details about querying individual types are discussed.
In the following sections we discuss details about querying individual types.
## Query Array {#QueryArray}
By default, `SizeType` is typedef of `unsigned`. In most systems, array is limited to store up to 2^32-1 elements.
By default, `SizeType` is typedef of `unsigned`. In most systems, an array is limited to store up to 2^32-1 elements.
You may access the elements in array by integer literal, for example, `a[0]`, `a[1]`, `a[2]`.
You may access the elements in an array by integer literal, for example, `a[0]`, `a[1]`, `a[2]`.
Array is similar to `std::vector`, instead of using indices, you may also use iterator to access all the elements.
Array is similar to `std::vector`: instead of using indices, you may also use iterator to access all the elements.
~~~~~~~~~~cpp
for (Value::ConstValueIterator itr = a.Begin(); itr != a.End(); ++itr)
printf("%d ", itr->GetInt());
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@@ -144,7 +144,7 @@ for (auto& v : a.GetArray())
## Query Object {#QueryObject}
Similar to array, we can access all object members by iterator:
Similar to Array, we can access all object members by iterator:
~~~~~~~~~~cpp
static const char* kTypeNames[] =
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@@ -190,11 +190,11 @@ for (auto& m : document.GetObject())
## Querying Number {#QueryNumber}
JSON provide a single numerical type called Number. Number can be integer or real numbers. RFC 4627 says the range of Number is specified by parser.
JSON provides a single numerical type called Number. Number can be an integer or a real number. RFC 4627 says the range of Number is specified by the parser implementation.
As C++ provides several integer and floating point number types, the DOM tries to handle these with widest possible range and good performance.
As C++ provides several integer and floating point number types, the DOM tries to handle these with the widest possible range and good performance.
When a Number is parsed, it is stored in the DOM as either one of the following type:
When a Number is parsed, it is stored in the DOM as one of the following types:
When querying a number, you can check whether the number can be obtained as target type:
When querying a number, you can check whether the number can be obtained as the target type:
Checking | Obtaining
------------------|---------------------
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@@ -215,9 +215,9 @@ Checking | Obtaining
`bool IsInt64()` | `int64_t GetInt64()`
`bool IsDouble()` | `double GetDouble()`
Note that, an integer value may be obtained in various ways without conversion. For example, A value `x` containing 123 will make `x.IsInt() == x.IsUint() == x.IsInt64() == x.IsUint64() == true`. But a value `y` containing -3000000000 will only makes`x.IsInt64() == true`.
Note that, an integer value may be obtained in various ways without conversion. For example, A value `x` containing 123 will make `x.IsInt() == x.IsUint() == x.IsInt64() == x.IsUint64() == true`. But a value `y` containing -3000000000 will only make `x.IsInt64() == true`.
When obtaining the numeric values, `GetDouble()` will convert internal integer representation to a `double`. Note that, `int` and `unsigned` can be safely convert to `double`, but `int64_t` and `uint64_t` may lose precision (since mantissa of `double` is only 52-bits).
When obtaining the numeric values, `GetDouble()` will convert internal integer representation to a `double`. Note that, `int` and `unsigned` can be safely converted to `double`, but `int64_t` and `uint64_t` may lose precision (since mantissa of `double` is only 52-bits).
## Query String {#QueryString}
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@@ -225,7 +225,7 @@ In addition to `GetString()`, the `Value` class also contains `GetStringLength()
According to RFC 4627, JSON strings can contain Unicode character `U+0000`, which must be escaped as `"\u0000"`. The problem is that, C/C++ often uses null-terminated string, which treats ``\0'` as the terminator symbol.
To conform RFC 4627, RapidJSON supports string containing `U+0000`. If you need to handle this, you can use `GetStringLength()`API to obtain the correct length of string.
To conform RFC 4627, RapidJSON supports string containing `U+0000`. If you need to handle this, you can use `GetStringLength()`to obtain the correct string length.
For example, after parsing a the following JSON to `Document d`:
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~~~~~~~~~~
## Create String {#CreateString}
RapidJSON provide two strategies for storing string.
RapidJSON provides two strategies for storing string.
1. copy-string: allocates a buffer, and then copy the source data into it.
2. const-string: simply store a pointer of string.
Copy-string is always safe because it owns a copy of the data. Const-string can be used for storing string literal, and in-situ parsing which we will mentioned in Document section.
Copy-string is always safe because it owns a copy of the data. Const-string can be used for storing a string literal, and for in-situ parsing which will be mentioned in the DOM section.
To make memory allocation customizable, RapidJSON requires user to pass an instance of allocator, whenever an operation may require allocation. This design is needed to prevent storing a allocator (or Document) pointer per Value.
To make memory allocation customizable, RapidJSON requires users to pass an instance of allocator, whenever an operation may require allocation. This design is needed to prevent storing a allocator (or Document) pointer per Value.
Therefore, when we assign a copy-string, we call this overloaded `SetString()` with allocator:
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@@ -385,7 +385,7 @@ In this example, we get the allocator from a `Document` instance. This is a comm
Besides, the above `SetString()` requires length. This can handle null characters within a string. There is another `SetString()` overloaded function without the length parameter. And it assumes the input is null-terminated and calls a `strlen()`-like function to obtain the length.
Finally, for string literal or string with safe life-cycle can use const-string version of `SetString()`, which lacks allocator parameter. For string literals (or constant character arrays), simply passing the literal as parameter is safe and efficient:
Finally, for a string literal or string with a safe life-cycle one can use the const-string version of `SetString()`, which lacks an allocator parameter. For string literals (or constant character arrays), simply passing the literal as parameter is safe and efficient:
~~~~~~~~~~cpp
Value s;
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s = "rapidjson"; // shortcut, same as above
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For character pointer, the RapidJSON requires to mark it as safe before using it without copying. This can be achieved by using the `StringRef` function:
For a character pointer, RapidJSON requires it to be marked as safe before using it without copying. This can be achieved by using the `StringRef` function:
~~~~~~~~~cpp
const char * cstr = getenv("USER");
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@@ -408,7 +408,7 @@ s = StringRef(cstr,cstr_len); // shortcut, same as above
~~~~~~~~~
## Modify Array {#ModifyArray}
Value with array type provides similar APIs as`std::vector`.
Value with array type provides an API similar to`std::vector`.
*`Clear()`
*`Reserve(SizeType, Allocator&)`
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@@ -418,7 +418,7 @@ Value with array type provides similar APIs as `std::vector`.
Differs from STL, `PushBack()`/`PopBack()` returns the array reference itself. This is called _fluent interface_.
This API differs from STL in that `PushBack()`/`PopBack()` return the array reference itself. This is called _fluent interface_.
If you want to add a non-constant string or a string without sufficient lifetime (see [Create String](#CreateString)) to the array, you need to create a string Value by using the copy-string API. To avoid the need for an intermediate variable, you can use a [temporary value](#TemporaryValues) in place:
Object is a collection of key-value pairs (members). Each key must be a string value. To modify an object, either add or remove members. THe following APIs are for adding members:
The Object class is a collection of key-value pairs (members). Each key must be a string value. To modify an object, either add or remove members. The following API is for adding members:
The name parameter with `StringRefType` is similar to the interface of `SetString` function for string values. These overloads are used to avoid the need for copying the `name` string, as constant key names are very common in JSON objects.
The name parameter with `StringRefType` is similar to the interface of the `SetString` function for string values. These overloads are used to avoid the need for copying the `name` string, since constant key names are very common in JSON objects.
If you need to create a name from a non-constant string or a string without sufficient lifetime (see [Create String](#CreateString)), you need to create a string Value by using the copy-string API. To avoid the need for an intermediate variable, you can use a [temporary value](#TemporaryValues) in place:
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@@ -526,11 +526,11 @@ Swapping two DOM trees is fast (constant time), despite the complexity of the tr
This tutorial shows the basics of DOM tree query and manipulation. There are several important concepts in RapidJSON:
1.[Streams](stream.md) are channels for reading/writing JSON, which can be a in-memory string, or file stream, etc. User can also create their streams.
2.[Encoding](encoding.md) defines which character encoding is used in streams and memory. RapidJSON also provide Unicode conversion/validation internally.
3.[DOM](dom.md)'s basics are already covered in this tutorial. Uncover more advanced features such as *in situ* parsing, other parsing options and advanced usages.
4.[SAX](sax.md) is the foundation of parsing/generating facility in RapidJSON. Learn how to use `Reader`/`Writer` to implement even faster applications. Also try `PrettyWriter` to format the JSON.
5.[Performance](performance.md) shows some in-house and third-party benchmarks.
6.[Internals](internals.md) describes some internal designs and techniques of RapidJSON.
1.[Streams](doc/stream.md) are channels for reading/writing JSON, which can be a in-memory string, or file stream, etc. User can also create their streams.
2.[Encoding](doc/encoding.md) defines which character encoding is used in streams and memory. RapidJSON also provide Unicode conversion/validation internally.
3.[DOM](doc/dom.md)'s basics are already covered in this tutorial. Uncover more advanced features such as *in situ* parsing, other parsing options and advanced usages.
4.[SAX](doc/sax.md) is the foundation of parsing/generating facility in RapidJSON. Learn how to use `Reader`/`Writer` to implement even faster applications. Also try `PrettyWriter` to format the JSON.
5.[Performance](doc/performance.md) shows some in-house and third-party benchmarks.
6.[Internals](doc/internals.md) describes some internal designs and techniques of RapidJSON.
You may also refer to the [FAQ](faq.md), API documentation, examples and unit tests.
You may also refer to the [FAQ](doc/faq.md), API documentation, examples and unit tests.
