rapidjson操作指南

本文将介绍如何用rapidjson的DOM方式进行json的常见操作：

一、读取json数据

上代码：

const char json[] = " { "hello" : "world","t" : true,"f" : false,"n": null,"i":123,"pi": 3.1416,"a":[1,2,3,4] } ";

	Document document;	// Default template parameter uses UTF8 and MemoryPoolAllocator.
	char buffer[sizeof(json)];
	memcpy(buffer,json,sizeof(json));
	if (document.ParseInsitu<0>(buffer).HasParseError()) {
		return 1;
	}

	printf("nParsing to document succeeded.n");

	////////////////////////////////////////////////////////////////////////////
	// 2. Access values in document. 

	printf("nAccess values in document:n");
	assert(document.IsObject());	// Document is a JSON value represents the root of DOM. Root can be either an object or array.

	assert(document.HasMember("hello"));
	assert(document["hello"].IsString());
	printf("hello = %sn",document["hello"].GetString());

	assert(document["t"].IsBool());		// JSON true/false are bool. Can also uses more specific function IsTrue().
	printf("t = %sn",document["t"].GetBool() ? "true" : "false");

	assert(document["f"].IsBool());
	printf("f = %sn",document["f"].GetBool() ? "true" : "false");

	printf("n = %sn",document["n"].IsNull() ? "null" : "?");

	assert(document["i"].IsNumber());	// Number is a JSON type,but C++ needs more specific type.
	assert(document["i"].IsInt());		// In this case,IsUint()/IsInt64()/IsUInt64() also return true.
	printf("i = %dn",document["i"].GetInt());	// Alternative (int)document["i"]

	assert(document["pi"].IsNumber());
	assert(document["pi"].IsDouble());
	printf("pi = %gn",document["pi"].GetDouble());

	{
		const Value& a = document["a"];	// Using a reference for consecutive access is handy and faster.
		assert(a.IsArray());
		for (SizeType i = 0; i < a.Size(); i++)	// rapidjson uses SizeType instead of size_t.
			printf("a[%d] = %dn",i,a[i].GetInt());
		
		// Note:
		//int x = a[0].GetInt();					// Error: operator[ is ambiguous,as 0 also mean a null pointer of const char* type.
		int y = a[SizeType(0)].GetInt();			// Cast to SizeType will work.
		int z = a[0u].GetInt();						// This works too.
	}

	system("pause");

Parsing to document succeeded.


Access values in document:
hello = world
t = true
f = false
n = null
i = 123
pi = 3.1416
a[0] = 1
a[1] = 2
a[2] = 3
a[3] = 4
请按任意键继续. . .

二，构造json数据

Document变量被声明之后，要先调用SetObject方法

如果Document中没有某个字段，那么要先调用addmember增加这个字段，才可以往里面赋值

向array中增加字段时，要传入allocator

给某个字段赋予字符串时，有两种方式，一、只传入字符串的指针，二、将整个字符串拷贝一份进去

const char json[] = " { "hello" : "world",sizeof(json));
	if (document.ParseInsitu<0>(buffer).HasParseError()) {
		return 1;
	}

	// Adding values to array.
	{
		Value& a = document["a"];	// This time we uses non-const reference.
		Document::AllocatorType& allocator = document.GetAllocator();
		for (int i = 5; i <= 10; i++)
			a.PushBack(i,allocator);	// May look a bit strange,allocator is needed for potentially realloc. We normally uses the document's.

		// Fluent API
		a.PushBack("Lua",allocator).PushBack("Mio",allocator);
	}

	// Making string values.

	// This version of SetString() just store the pointer to the string.
	// So it is for literal and string that exists within value's life-cycle.
	{
		document["hello"] = "rapidjson";	// This will invoke strlen()
		// Faster version:
		// document["hello"].SetString("rapidjson",9);
	}

	// This version of SetString() needs an allocator,which means it will allocate a new buffer and copy the the string into the buffer.
	Value author;
	{
		char buffer[10];
		int len = sprintf(buffer,"%s %s","Milo","Yip");	// synthetic example of dynamically created string.

		author.SetString(buffer,len,document.GetAllocator());
		// Shorter but slower version:
		// document["hello"].SetString(buffer,document.GetAllocator());

		// Constructor version: 
		// Value author(buffer,document.GetAllocator());
		// Value author(buffer,document.GetAllocator());
		memset(buffer,sizeof(buffer)); // For demonstration purpose.
	}
	// Variable 'buffer' is unusable now but 'author' has already made a copy.
	document.AddMember("author",author,document.GetAllocator());

	assert(author.IsNull());		// Move semantic for assignment. After this variable is assigned as a member,the variable becomes null.

	////////////////////////////////////////////////////////////////////////////
	// 4. Stringify JSON

	printf("nModified JSON with reformatting:n");
	FileStream f(stdout);
	PrettyWriter<FileStream> writer(f);
	document.Accept(writer);	// Accept() traverses the DOM and generates Handler events.

	system("pause");

需要调用document的Accept方法，传入一个Writer，Writer可以用文件流或者字符串来构造

导出json数据到文件，这里以导出到stdout为例

FileStream f(stdout);
	PrettyWriter<FileStream> writer(f);
	document.Accept(writer);	// Accept() traverses the DOM and generates Handler events.

StringBuffer strbuf;
	Writer<StringBuffer> writer(strbuf);
	document.Accept(writer);

完毕。。

（编辑：李大同）

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